Assessment of soil degradation in the southern states of Nigeria

* b N V THE WORLD BANK SECTOR POLICY AND RESEARCH STAFF Environment Department Assessment of Soil Degradation in the Southern States of Nigeria Rattan Lal and Bede Okigbo September 1990 Environment Working Paper No. 39 This paper has been prepamd for internal use. The views and interpretations herein are those of the author(s) and should not be attributed to the World Bank, to its affiliated organizations or to any individual acting on their behalf. - 1 - This paper has been prepared by Drs. Rattan Lal and Bede Okigbo, both latterly of the International Institute for Tropical Agriculture (IITA), Ibadan, Nigeria, acting as consultants to the Environmeni Department of the Bank. Dr. Lal is currently Professor of Agronomy at Ohio State University and Dr. Okigbo, Visiting Fellow at the Department of Agronomy, Michigan State University. This study is part of a program of work undertaken by the Environmental Policy and Research Division. The authors would like to thank all those who assisted Dr. Okigbo during his visit to Nigeria in early 1989, and are grateful for comments received on earlier drafts from Steven Carr, Lewis Campbell, Bill Magrath, Ridley Nelson and Ashok Seth of the World Bank and for assistance received in preparing the report from John English. Departmental Working Papers are not formal pablications of the World Bank. They present preliminary and unpolished results of country analysis or research that is circulated to encourage discussion and comment; citation and the use of such a paper should take account of its provisional character. The findings, interpretations, and conclusions expressed in this paper are entirely those of the author and should not be attributed in any manner to the World Bank, to its affiliated organizations, or to members of its Board of Executive Directors or the countries they represent. Because of the informality and to present the results of research with the latest possible delay, the typescript has not been prepared in accordance with the procedures appropriate to formal printed texts, and the World Bank accepts no responsibility for errors. ABSTRACT This study was carriad out as part of a program of work on soil degradation in Nigeria undertaken in support of agricultural sector studies on the country. A companion report on soil dagradation in the northern states has also been published (Mortimore, 1989). The objective of the study was to assess on the basis of secondary data and a quick diagnostic field survey conducted in 1989 the extent and characteristics of soil degradation in the southern states, the causes of this degradation and measures available to counteract it. Southern Nigeria has a generally humid climate with rainfall above 1,500 mn with a dry season. Physically, most upland soils in the region are easily compacted, have low available water holding capacity, and are susceptible to accelerated erosion. Structural deterioration sets in as these soils are cultivated intensively. Chemically, the soils have low nutrient reserves and although initial levels of organic matter are generally high, they decline rapidly with deforestation intensive cultivation and removal of crops. Nutrient losses are also caused by increased leaching, volatalization, especially during bruning, and runoff and erosion. These processes are most rapid on areas cultivated under shifting cultivation or with extensive mechanical methods. On farm studies have shown that yields in intensively cultivated "gardens" (comnounds) close to homes were not related to inherent soil fertility. These areas are heavily manured and the soils' fertility status is generally maintained at a satisfactory level. However, in less intensively cultivated, more distant fields, most commonly used for staple crops, fertility and yields are significantly lower. Possibilities exist to introduce some of the attributes of the compounds into the utilization of the more distant crops. These include active management of "fallows", and increased use of perennial species either as crops, in contouring, provision of cover and bionass for mulch, etc., and with legumes in nitrogen fixation. One variant of these has been dubbed "alley-cr<pping" but others are possible. The attractiveness of these possibilities to farmers is increasing but dependent on local circumstances. Because of increased population the pressure to intensify will continue. Research is needed to assist in the development of viable alternative technologies and systems. In particular greater, knowledge is required of the characteristics and degradation processes of soils in the region and work is also required on indigenous plants of the region to determine those which may have characteristics and uses which will enable them to be utilized as components of sustainable agricultureal systems. ASSESSMENT OF SOIL DEGRADATION IN THE SOUTHERN STATES OF NIGERIA Table of Contents .PaRe I. INTRODUCTION................................................ 1 II. GEOGRAPHICAL BACKGROUND..................................... 2 III. CHARACTERISTICS OF UNDEGRADED SOILS IN SOUTHERN NIGERIA..... 5 IV. EVIDENCE OF SOIL DEGRADATION................................ 6 Acidificationf........... 7 Depletion of Soil 0.7ganic Matter and Principal Nutrients.... 7 Deforestation of Soil Structure............................. 8 Soil Fauna and Flora........................................ 10 V. EFFECTS OF SOIL DEGRADATION ON CROP YIELDS.................. 11 VI. CAUSES OF SOIL DEGRADATION IN SOUTHERN NIGERIA.............. 15 Fire and Burning of Vegetation.............................. 15 Deforestation and Land Clearing............................. 16 Increased Intensity of Farming and Shortening the Period of Fallow........................................... 16 Low Input Agriculture....................................... 17 Accelerated Erosion......................................... 18 Roads, Buildings and Industrial Construction........... .... 18 VII. MEASURES USED TO PREVENT OR COMBAT SOIL DEGRADATION......... 19 Manuring and Mulching....................................... 19 Planted Fallows and Cover Crops............................. 19 Sustainable Farming Systems................................. 20 Chemical Fertilizers........................................ 22 VIII. IS CONTINUOUS CROPPING ECOLOGICALLY AND ECONOMICALLY SUSTAINABLE?.................................. 24 Interagency Collaboration................................... 28 Research.................................................... 29 Extension and Public Awareness.............................. 31 IX. RESEARCH AND DEVELOPMENT PRIORITIES......................... 29 BIBLIOGRAPHY................................................ 36 I. INTRODUCTION 1. Soil is the natural resource that provides the basis of the food chain that ends with human consumption and waste disposal. There is no other practical physical and chemical source of supply of human needs and, in fact, needs of all living things, that can replace soil as the most basic component of the life support system. Soil, under the normal conditions of equilibrium, where the rate of soil loss equals that of soil formation, is regarded as a renewable resource. But, it can be rendered non-renewable by several human activities that characterize present civilizations. Soil degradation is defined as the decline in soil quality caused through misuse by humans. It results in deterioration of soil's life support processes and decline in its capacity to produce food, feed, fiber and fuel. Specifically, soil degradation is the diminution of the soil's current and/or potential capability to produce quantitative or qualitative goods or services as a result of one or more degradative processes (UNEP, 1982; 1984). 2. There is a need to distinguish between agents, processes and factors of soil degradation. Agents of soil degradation are the frrces involved, e.g-, the population pressure, development of infrastructure including roads and buildings, socioeconomic conditions, etc. Processes of soil degradation are the physical, chemical or biological mechanisms that lead to decline in soil productivity. The most common processes of soil degradation include: depletion in soil fertility, deterioration of soil structure leading to compaction and accelerated erosion, nutrient imbalance caused by salt accumulation or leaching, and buildup of organic or inorganic pollutants. Factors of soil degradation are the natural or man- made parameters that determine the magnitude and relative predominance of the process of degradation, e.g., deforestation, burning, continuous monocropping, cultivation of marginal lands, and indiscriminate use of agrochemicals. 3. The term degradation is, however, vaguely used leading to ambiquity and confusion. It is important, therefore, that soil degradation be expressed quantitatively by delineating critical limits of properties beyond which soil's life support processes are drastically affected. These limits may differ among soils, crops, landuse, and agroecological environment. There is a paucity of basic research information on crop growth in relation to degree of soil degradation caused by fertility depletion and reduction in soil organic matter content, soil compaction, reduction in rooting depth due to erosion and decrease in plant-available water reserves, and buildup of toxic levels of Al and Mn in acid soils. The quantity and quality of organic matter necessary to maintain an adequate structural condition differ for different soils and environments and are not known. 4. Soil degradation is one of the major concerns of modern times, because it has global implications even when it occurs in geographically isolated conditions. Soil-related processes are a major cause of the so- called "greenhouse effect". Deforestation leads to a sizeable emission of C02, widespread fire and burning of biomass eject many radiatively active gases into the atmosphere, continuous cropping leads to decomposition of soil organic matter and release of 002 into the atmosphere, accelerated - 2 - soil erosion caused by deforestation and cultivation of marginal lands is a major cause of fertility depletion, transport of sediment-laden pollutants, eutraphication of water and pollution of environment. Because of its local, regional and global implications, we can no longer afford to loose our basic resources to soil degradation. 5. Obiective of this Report: The main objective of this study is to determine in the basis of (a) available secondary data or already published materials, (b) responses to a questionnaire sent to individuals or agencies likely to be concerned with soil degradation in southern Nigeria, and; (c) a quick diagnostic field survey conducted in late February 1989, the extent of soil degradation in the southern states of Nigeria, the causes of this degradation and measures being taken to combat it. An attempt is also made to provide guidelines for preventive and restorative measures and resource management strategies. Recommendations are made on some measures against soil degradation. 6. Procedure and Data Collection Methods: As much relevant information on soil degradation in 10 southern states of Nigeria was collected by (1) review of available literature published in Nigeria and elsewhere, (') discussions with scientists in Nigerian universities and at the Internat:Lonal Institute of Tropical Agriculture (IITA) and Internationa:. Livestock Center for Africa (ILCA) at Ibadan, working on or concerned with soil degradation, (3) discussions with scientists in the agricultural research institutes in southern Nigeria, and with relevant personnel in Federal Ministry of Science and Technology in Lagos, and (4) discussions with staff of River Bisin Development Authorities and other government and private agencies. II. GEOGRAPHICAL BACKGROUND 7. Southern Nigeria extends from latitude 4015. to 90 north of the equator and from longitude 30251 to 9024D west of the %reenwich line. It has an area of about 193,000km2 and is bordered by the Atlantic Ocean of the Bight of Benin in the south and by the Middle Belt states of Gongola, Benue and Kwara in the north. The 10 states of region consist of Akwa- Ibom, Anambra, Bendel, Cross River, Imo, Ogun, Ondo, Oyo and Rivers. Population densities in this region are among the highest in sub-Saharan Africa. The 10 states have a total population of 42.7 million, average density of 211/km2 and a range of from 138/km2 in Bendel to 618/km in Imo and 1,239/km2 in Lagos state. While settlement in southeastern states is dispersed, it is highly urbanized in the w,stern states with over 10 large cities having populations of over 100,000 people each. 8. Tbooraphy: The relative locations of the states and relief are shown in Figure 1. Topographically, much of the area is lowland below 300 m but there are isolated hills or highland areas consisting of the Oban and Obudu hills in the eastern border which are 1,000-1,500 m in elevation, and the 800-920 m high Ishan and Idanre hills in the west. Geologically, much of the area is dominated by sedimentary rocks, shales, sand, silt and clay of Cretaceous and Tertiary ages, Pre-Cambrian Basement Camplex granitic gneises, diorites and basic rocks in the eastern part and Pre-Cambrian Basement Complex rocks consisting mainly of gneises; quarzites, schists and granitic gneises in the western states. -3- 0 0o w Å Land above 310m (1000 ft) Land above 610m Land below 310m Escarpment FIG. 1. Relief aud drainage of the southern states of Nigeria (Adapted from Udoh , 1978). 9. CliMate: The area has a tropical climate with mean annual temperatures ranging from 21-350C in coastal areas to 21-400C further inland. Minimum temperatures of slightly below 180C occur in December and January when northeastern cold, dry and dusty harmattan winds blow from the Sahara. Hilly areas have lower mean annual temperatures of 21-240C. Relative humidity is usually high except during the harmattan in the northernmost areas. The mean annual rainfall of a largely bimodal pattern with an 'August break' ranges from above 4,000 mm in the extreme southeast to less than 1,300 mm in the northwestern margin of Oyo state. The duration of the rains range from 330 days in the southeast to less than 240 days in the northeastern parts of Oyo state during each year. Considerable variation in rainfall occur from year to and from one location to another. 10. Vegetation: This consists of parallel east to west zones of varying width and from the coastal areas inland is made up of (1) coastal vegetation of mangrove, aquatic grassland and herbaceous swamp, (2) freshwater swamp forest, (3) lowland or moist tropical forest and (4) derived savanna and forest/savanna mosaic in the northernmost areas. However, most of the natural climatic climax vegetation is now of anthropic origin having been greatly modified by increasing intensity of cultivation, deforestation, annual fires, construction and human settlements under increasing population pressure. As a result, with the exception of the Oban Hills in Cross River State, much of the eastern states is now oil palm bush north of the swamp vegetation. 11. Farming Systems: The predominant farming system in the region is slash and burn, bush or rotational bush fallow with fallow periods of 0-5 years in most of the southeastern states, and less than 10-12 years in the western states, alternating with two or more years of cropping. Dominant staples consist of cassava, yams, maize, cocoyams, rice, some sweet potatoes, bananas and plantains. Major tree crops include oil palm, cocoa, kola, rubber, coconut, mango, avocado, guava, citrus fruits and cashew. The areas in utilization are small (less than 5 ha on over 70% of farms), fragmented into 5 or more plots per farmer with staple food crops grown mainly on mounds and ridges with predominantly manual labor, very little mechanization and increasing but limited use of fertilizers. In the southeastern states, there are elaborate home gardens with high crop diversity including exotic tree crops, some indigerous tree crops (African mango, breadfruit and pear), staple foods crops and vegetables forming a multistoried structure with fertility maintained with animal manure and household refuse. Fields adjacent to the Lome gardens are also intensively cropped. There are no elaborate home gardens in highly organized western states where small livestock are kept on free range unlike in the eastern states where they are restricted. Cash crops in the southeastern states consist of oil palms in addition to rubber (Cross River and Rivers), cocoa in parts of Imo and Cross River but rice is increasing in importance. In the western states, main cash crops are rubber, oil palms, yams and some cocoa in Bendel state and cocoa, kola and some rice in Ogun, Ondo and Oyo states. 12. Overall Changes in the Cultural Landscape: During the last 20 years there has been significant change in the cultural landscape related to changes in vegetation, land use, human settlements, roads and farming systems which either influence land degradation or are manifestations of it. Some of the changes in the cultural landscape include: - 5 - . reduction of area and species complexity of forests secondary bush and relic vegetation; . increasing sparsity of vegetation and marked increase in areas colonized by the Siam weed (Chromolaena odorata) including roadsides (Figures 3-5); . marked increase in the network of roads tarred or utarred with hamlets, villages or markets springing up along them (Figure 3); . increased number and proportion of houses covered with corrugated iron and aluminum sheets and large areas of compounds surfaced with concrete; . many streams partly silted up or laden with sediment; . some indications of environmental degradation is evidenced by the number of gullies along roads in outskirt of cities. 13. Significant changes in farming systems and associated product mixes have also occurred including: . increase in area of cassava and area under continuous cropping with cassava; . decline in area under yams especially in southeastern Nigeria; , increasing area under more than one year continuous sole crop cassava or maize, especially in western Nigeria; . increasing area of mechanized large scale farms of staple food crops and plantains, especially in Rivers and Bendel States in addition to oil palm plantations; and . increased production of vegetables and ornamental plants for propagation or sale near to urban centers. III. CHARACTERISTICS OF UNDEGRADED SOILS IN SOUTHERN NIGERIA 14. When in equilibrium with its climax vegetation, soils have the most favorable characteristics achieveable within the ecological limits governed by the parent material and the climate. With the exception of soils of the delta region, those derived from coastal sediments and recent alluvium, most soils of south and southeastern Nigeria are very old. Consequently weathering has occurred to greater depths in the bed rock, the clay has eluviated to deeper horizons and is strongly aggregated. Furthermore, strong leaching has caused depletion of bases and acidification of the surface horizons. 15. Soils in the region consist mainly of Ultisols, Oxisols, Alfisols, Iceptisols and Entisols in hydromorphic and valley bottom areas. In - 6 - general, with the exception of the Entisols and soils of younger volcanic origin in Cross River State, most of these soils are highly weathered, acid and of low inherent fertility. 16. The soils of the Western states (Ogun, Odo, Oyo and small areas of Bendel) are formed from parent materials consisting mainly of Pre-Cambrian basement Complex rocks (gneises associated with quartzites, quartz, schists and granitic gneises). They are mainly high base status soils consisting of Alfisols, Mollisols and Inceptisols ui ler semi-deciduous vegetation with pH values of above 6.0 and high base saturation of up to 90%. On the other hand, most of the soils of southeastern Nigeria with the exception of those of the younger volcanic origin are formed from sedimentary rocks, shales, sand, salts and some clays of Cretaceous and Tertiary ages, underlain by Pre-Cambrian Basement Complex granitic gneises, diorites and basic rocks. They are mainly soils of low base status (Ultisols and Oxisols) with PH 4.5 to 5.0 and base saturation of less than 50% even when undegraded. There are also Ultisols and Oxisols derived from beach sands, mangrove and coastal alluvium in flood plains and other soils derived from shales and basalt which are also highly acid and of low inherent fertility. 17. Texturally, most of the highly leached Ultisols, Oxisols and Alfisols vary from sandy to loamy sand, while the valley bottoms and depressions with Inceptisols and Entisols are high in silt and clay. Many of the soils have 60-90% sand, generally with gravel content ranging from 50-70% in the subsoil (B2t) horizon in Shepetiri sequence in Oyo and Ondo states, very low level of silt (in most cases below 12%), and low in clay content which is often below 20%. Soils east of the Niger and in Bendel state are much coarser than those in the West and very highly weathered with deteriorated lattice structure dominated by low activity clays (kaolinite) and iron and aluminum oxides. Bulk densities of most soils are low and may be less than 1.2 g/cm3 and increase with depth. These are low undegraded soils where there is high activity of soil fauna (earthworms and termites). As indicated above, soils in the southeastern states are lower in exchangeable bases (Ca,+2 Mg+2 and K+) and available phosphorus, but are higher in aluminum and total soil acidity as compared to those in the western states. With the exception of soils derived from beach sands and coastal sediments, soil organic carbon and total t.itrogen contents of undergraded soils are usually high while phosphorus and effective cation exchange capacity are generally low. Considerable variation in many of these characteristics occur from location to location. IV. EVIDENCE OF SOIL DEGRADATION 18. With the exception of isolated islands of protected forest reserves, the primary rainforest which prevailed several centuries ago has long been degraded to secondary regrowth and thicket. Consequently, the true shifting cultivation, characterized by short duration of cultivation phase followed by long forest fallow, is nowhere to be observed. Due to high population pressure, especially in eastern parts of Nigeria, shifting cultivation has been replaced by rotational bush fallow in relatively poor soils and continuous farming on fertile Inceptisols and flood plains. There is a minimum of unused lands in the region and the soils are rapidly being degraded. Severe soil degradation is evidenced in farms where the fallow period is minimal or none at all. Most of the evidence of soil -7- degradation presented below is from analyses of soil sampled from experiments conducted at research stations. There is little, if any, research done on farmers' field to provide concrete and reliable quantitative information on the rate, extent and distribution of soil degradation. 19. Symptoms of soil degradation include: crusting and formation of surface seal, compaction of the soil surface horizon, reduction of pore space and infiltration rate (Lal, 1986), low available-water holding capacity, increased susceptibility to soil splash and rain drop impact, accelerated runoff and erosion which are most evident on undulating ground, decline in soil organic water content, decrease in pH, and nutrient imbalance. Many of these symptoms are manifested in the cultivated soil of southern Nigeria and especially in the southeastern states. In soils of relatively flat terrain and gentler slops, the leaching out of the clay and colloidal fraction of the soil and their deposition in the subsoil may be the cause of textural and structural -changes. On the steep slopes and undilating lands accelerated runoff and erosion are most pronounced. Gully erosion is very severe in eastern states. 20. Acidification: Continuous cultivation leads to depletion of bares resulting in decrease in pH and increase in exchangeable aluminum and hydrogen. The rate of decline in pH is more rapid in coarse-textured soils of low cation exchange capaciv! (CEC) and low organic carbon content than in heavy-textured soils of high buffer capacity. An experiment conducted on an Entisol, of relatively high inherent fertility, showed that pH declined with continuous cultivation. Soil pH declined from an initial value of 6.1 to 5.65, 5.50, 5.45 and 5.42 with first, second third and fourth cultivation, respectively. A similar evidence of decline in pH with cropping is observed from an Ultisol derived from coastal sediment at Onne, in eastern Nigeria (Maurya and Lal, 1979). A rapid decline in pH with cropping is also observed for relatively fertile Alfisols of the subhumid regions of southwestern Nigeria. Lal (1985) observed that soil pH declined from an initial value of 6.1 to 4.4 within 6 years of continuous cultivation. The rate of decline in pH was more in the case of plow-based system than with no-till farming. 21. Decline in soil pH is associated with depletion of bases. Once again depletion of bases is expectedly more rapid in Ultisols and Oxisols than in Alfisols and Inceptisols. The data in Table 1 indicate that 5-years of continuous cropping lead to a decrease from 4.8 to 4.0 meq/100 g for calcium, 1.3 to 0.8 meq/100 g for magnesium, and 0.6 to 0.4 meq/100 g for potassium. Similarly, the data for a plow-based system indicate even more drastic decline in exchangeable bases. Six years of continuous cropping led to a decrease from 7.9 to 1.9 meq/100 g of calcium, 2.0 to 0.13 meq/100 g of magnesium and 0.8 to 0.18 meq/100 g of potassium. In contrast, there was an increase of 0.05 to 0.3 meq/100 g of manganese, and from traces to 0.95 meq/100 g of total acidity. These apparently drastic reductions in exchangeable bases and sharp increases in total acidity were brought about by maize grain production of only 2 to 4 t/ha/yr. Acidification is a severe problem especially with intensive use of chemical fertilizers such as Ammonium Sulphate. 22. Depletion of soil organic matter ad principal nutrients: Soil degradation is also evidenced by reductions in soil organic matter, and - 8 - total and available fractions of nitrogen and phosphorus. Low levels of soil organic matter content is an important indication of soil degradation. Continuous cropping for production of food crop annuals leads to decline in soil organic matter content due to a multitude of interacting factors. High soil temperatures, accelerated erosion, and removal of stover and other biomass are some reasons of decline in soil organic carbon during the cultivation phase. Some of these factors will be discussed in the forthcoming section. There are some examples of data from research stations indicating decline in soil organic matter with cropping. Data from an acid tropical soil of eastern Nigeria indicate that organic carbon content declined from an initial value of 0.9% to 0.62% in the first year, 0.5% in the second year, 0.45% in the third year, and 0.42% in the fourth year of continuous cultivation. Drastic reductions in organic carbon content are also reported for Alfisols in Western Nigeria from an initial value of 1.7% under forest to 1.2% with 5-years of continuous cropping. The data in Table 1 also show decline in organic carbon content from an initial value of 1.7% to 1.2% following 6-years of cropping with a plow- based system. 23. Reduction in soil organic carbon content also leads to decline in total nitrogen and available phosphorus. The data in Table 3 show reduction of total nitrogen from 0.16 to 0.12 percent with 6-years of continuous cropping with a plow-based system. These reductions were observed despite regular additions of 100 to 150 kg/ha/yr of nitrogen applied as urea. These research plots also received 40 to 60 kg/ha/yr of P as single or triple superphosphate. Consequently, soil levels of available P increased with the cropping duration (Tables 1 and 2). Nutrient depletions are expected to be severe in farmers' field where chemical fertilizers are either not applied or are applied at extremely low rates. 24. Deterioration of soil structure: In comparison with soils under forest or bush fallow, cultivated soils in south and southeastern Nigeria are characterized by deterioration of soil structure and impairment in their ability to regulate water and air movement through the soil profile. Soils with degraded structure exhibit symptoms of crusting and formation of surface seal, compaction of the surface horizon, reduction in infiltration capacity, increased susceptibility to soil splash by raindrop impact, and accelerated runoff and erosion. The latter is evident only in sloping lands. In soils of relatively flat terrain and gentler slopes, however, the clay and colloidal fraction is leached or eluviated into the subsoil. 25. Soil compaction is a widespread problem on all farms. Increase in soil bulk density occurs due to cultivation regardless of the methods used. However, the problem is comparatively more severe in mechanized than nonmechanized farms. Experiments conducted on Alfisols at the research farm of IITA have shown that the deterioration in soil structure, as evidenced by increase in bulk density and decrease in infiltration capacity, is less with traditional farming and slash and burn methods than with intensive mechanized agriculture. (See Tables 3 and 4.) Similarly, decline in equilibrium infiltration rate was also less with traditional than with intensive mechanized farming (Figure 2). Furthermore, the infiltration rate showed an improvement even during the first year of the fallow phase. An universal evidence of compaction on farmers' fields is the accelerated runoff and erosion leading to sheet wash and rill eresion. Uncontrolled rills turn into guilies within a short period. -9- aoo o.- oTRAOTIONAL FARMING 8... -x JNTENSIVE MECHANSEo FARMING I (X) STANOARO OVIATN tIoo - PHV4IoW MASZ I rALLow mSE TIME (æm) Fig. 2 Reduction in infiltration rate of an Alfisol in western Nigeria due to continuous cropping (Lal, 1986). - 10 - 26. Soil fauna and flora: The clearing and burning of vegetation and subsequent cropping usually results in changes in the number and composition of soil fauna and flora (Nye and Greenland 1960). There is an overall decline in soil fauna population following cultivation of forest soils. Seasonal migrations from the top soil also occur in cultivated land due to drying up of soil and high insolation. After an initial decrease, the population of microflora usually redevelops to a level greater than before thereby causing increases in nitrogen mineralization. After clearing, burning and cropping there is a tendency for pests and diseases to increase especially where rotation of crops is not practiced. Weed growth also increases where cultivation is continued without fallowing. 27. Lal (1987) reviewed the occurrence, activities and effects on soil of a diversity of soil organisms and effects of deforestation clearing and cropping on them. Deforestation results in significant reduction of the invertebrate fauna consisting of crickets, spiders and millipedes. Critchley (1979) reported from Western Nigeria that catches of microfaunna in cultivated plots were 40-60 percent lower than in uncleared plots. With the exception of Prostigmata, which later became the dominant microarthropods in the cleared plots, there was a reduction in population densities of all groups of animals. Caveness (1982) observed that population densities of plant parasitic nematodes increased after land clearing. Deforestation apparently has both direct adverse effects on soil fauna through disturbance of the soil and scraping, which damages the habitats and lowers faunal populations, and indirect effects through changes in microclimate, decreased food diversity, and exposure to predators and parasites. 28. Various cultural practices including burning and the use of herbicides and insecticides adversely affect soil fauna on cropped land, but very scanty information about this phenomenon in the tropics is available (Lal, 1977). Studies by Crichley et al. (1979, 1980) in Western Nigeria indicated that DDT treatment reduced activities of adult and immature Gryllids (crickets), lycosid spiders, subterranean mesofauna, ants and some beatles and millipedes which ingested contaminated food. Perfect et al. (1981) reported DDT to reduce populations of Collembolla and Acari in treated plots to 50 percent of the untreated ones. Cultural practices also significantly affect microbial activities in soil and, while initially such treatments as burning may drastically reduce their abundance and activity, subsequent cropping practices and crop species may produce a range of differential effects. 29. As regards earthworm activities in the soil, deforestation reduces activity of Hyperiodrilus africanus but not as much of that of Eudrilus eugeniae. No-till practices which retain plant residue on the surface tend to encourage more earthworm activity than plowing. Mulching either with dead plant residue or live mulch has beneficial effects on earthworms as compared to bare plots which are detrimental to them. Fallowing with grasses or leguminous covers enhances earthworm activity to the extent dependent on crop species. Burning, and the subsequent compaction and erosion which may result, also causes reduction in earthworm and other faunal activities. Various other treatments such as fertilizers and pesticides may adversely or beneficially affect earthworm activity depending on the circumstances (Lal, 1987). - 11 - V. EFFECTS OF SOIL DEGRADATION ON CROP YIELDS 30. Reduction in crop yield is a serious economic consequence of soil degradation. Crop yield is an integrated response of many factors including climate, pests and management. However, adverse effects of soil degradation are accentuated due to suboptimal status of other environmental factors. For example, crops grown on degraded soils are highly susceptible to drought and infestation by pests. 31. There is considerable evidence of decline in crop yield due to soil degradation caused by continuous cropping. In the subhumid region of western Nigeria, Couper et al. (1979) observed drastic reductions in maize yield due to continuous cropping. Land was cleared from a secondary forest in 1974, and yields of the first-year unfertilized crops were low. In comparison with the best yields obtained for the plow-based system in the second year (1976), maize grain yields declined by 2.5, 0.0, 27.5 and 75.0 percent in the third, fourth, fifth and sixth year respectively. Yield decline also occurred in the no-till treatment but only during the fifth and sixth years of cropping. Furthermore, the relative decline was less in no-till compared with the plowed system. For example, in comparison with maize yield in the second year, there was a reduction of 15.6 and 33.3 percent in the fifth and sixth year, respectively. Severity of yield reduction in the plow-based system was related to rapid depletion of soil fertility and degradation of soil structure. Another evidence of yield decline of maize and cowpea due to soil degradation is shown by the data in Fig. 4 (Lal, 1989). Despite intensive input (of fertilizers and pesticides) and use of Leucaena and Gliricidia, there was a reduction in yield of maize and cowpea. For example, yield declined at the rate of 0.34 t/ha/yr for maize and 96.3 kg/ha/yr for cowpea (Fig. 3). Similar to the data in Table 5, yield decline in this experiment was also attributed to degradation of soil physical and chemical properties due to continuous cropping. 32. Degradation-induced yield reductions are usually more severe in acid infertile soils of eastern Nigeria than in relatively fertile Alfisols of western Nigeria. A principal cause of rapid and severe yield decline is the depletion of soil fertility and deficiency of principal plant nutrients. The results of a long-term study conducted on a sandy soil in eastern Nigeria relating the effects of a cover crop and green manuring on maize grain yield show a drastic decline in maize yield with cropping duration. For example, grain yield of 1800 kg/ha in 1934 declined to 1050 kg/ha in 1935, 1200 kg/ha in 1936, 250 kg/ha in 1937 and 200 kg/ha in 1938. Grain yields in the fourth and fifth years of cultivation were about 10% of the initial yields. Odurukwe and Orji (1981) evaluated yields of yam, cassava and maize under continuous cropping on acid soils in eastern Nigeria (Fig. 4). The data show that yield reduction was more severe in maize compared with cassava or yam. In the 4th year of continuous cropping, the relative decline in yield was 65% in maize, 38% in yam and only 25% in cassava. Nitrophilic grain crops such as maize are more susceptible to fertility depletion than long-duration root crops such as cassava. 33. Lagemann (1977) examined crop yields in relation to land use systems of small holders in high population density of eastern Nigeria. He - 12 - 5.0- 4 Cy 68 - 0.340x e Ra =45.2%6 0 ac 1.0 ( ' 10001- y=933 - 96.3x -~~R 80 -74411'* 8 00- R2749 400C - ! eoo 01 200- 1982 1983 1984 1985 1986 1987 Fig. 3. Yield decline of maize and cowpea with continuous cultivaLion using alley cropping (Lal, 1989). - 13 - CROP 4976 YIELD i YAM 23*6 T/HA 2 CASSAVA 16*4 1/WA 3 MAIZE 4.6 T/14A 4, CALOIES 17.3 X 107 CALS/MA 60 0 1404 20 of 1976 1977 1978 1979 YEARS OF CONTINUOUS CROPPING FIC 17.0tCLINE IN YIELD OF YAM, MAIZE AND CASSAVA WITH 4 YEARS OF CONTINUOUS CROPPING YIELDS AE AVERAGED OVER ALL PHASES CARRYING THE RESPECTIVE CROPS.(SOURCE 00UAUXWE AND ORJI 1981) - 14 - related crop yields on farmers' fields to soil chemical and nutritional properties. The results of regression analyses relating yield to soil properties are shown in Table 6. Soil organic carbon, and the level of phosphorus in soil were positively correlated with yields. The correlation coefficient, however, was more; significant for organic carbon than phosphorus. Lagemann (1977) also related grain yield of sole-cropped maize from 69 farms to organic carbon and Bray-1 P levels of soil. He found that yield of maize was very sensitive to level of organic carbon in the surface soil. Grain yield of maize increased by 1.23 percent with every 1 percent increase in level of soil organic carbon. However, the range of soil organic carbon content for 69 farms studied was rather narrow, 2.0 to 2.9 percent. In comparison, there were wide ranges for soil phosphorus (1.36 to 20.73 ppm) and base saturation percentage (18.1 to 66.0 percent). Maize grain yield increased by 0.18 percent with every one percent increase in level of soil test phosphorus. Maize grain yield also increased by 0.83 percent for every one percent increase in base saturation. An important conclusion of Lagemann study from 69 farms in 3 villages in eastern Nigeria was that as much as 73 percent of the observed differences in maize yield among farms could be explained in terms of differences in soil fertility and management. 34. A similar study based on evaluation of soil properties and management characteristics for several hundred farms in eastern Nigeria was conducted by Armon (1984). He conducted a survey of soil degradation on 6 locations in eastern Nigeria. Soil characteristics and crop yields were evaluated on farms in the vicinity of Calabar, Onitsha, Owerri, Ogoja, Umudike and Enugu for both compound and distant farms. Yields of yam, cassava and cocoyam were related to the length of the fallow period and soil properties. On distant fields yields were significantly higher on fields with fallow period exceeding 4 years than on fields with a fallow period of less than 2 years. Furthermore, yields on distant fields were significantly correlated with total nitrogen content of the soil (Eqs. 1 and 2) and with length of the fallow period (Eqs. 3 and 4). Y - 147.2 + 334.6 N, r - 0.41 ** Eq. 1 Y - 253.6 - 761.7 N + 1050.3 N,2 R = 0.60** Eq. 2 Y - 131.9 + 21.8 F, r - 0.23* Eq. 3 Y - 129.6 + 23.6 F - 0.26 F,2 R - 0.34* Eq. 4 where Y is yield expressed as MJ/ha, N is soil nitrogen content (%), and F is length of the fallow period in years. 35. The analyses of yield in relation to soil properties for distant fields indicated that about 40% variability in crop yields among fields was attributed to differences in soil test value for total nitrogen, and about 12% to differences in the length of fallow period. Apparently, nitrogen content of the soil was an important determinant of yields of root crops on farmers fields in eastern Nigeria. 36. Armon's study led to two important conclusions. Firstly, crop yields in compounds were not related to inherent soil fertility. These areas are heavily manured by the household waste and animal dung, and the soil's fertility status for nitrogen and other nutrients is generally favorable. Secondly, crop yields on distant fields were significantly affected by the soil test for total nitrogen. - 15 - 37. Studies by Lagemann and Armon have important management and policy implications. Their data clearly indicated that in areas of high population densities in eastern Nigeria, soil degradation sets in due to reduction in length of the fallow period. Furthermore, degradation-caused yield decline is related to reductions in soil tests for organic carbon, total nitrogen, available phosphorus, and base saturation percentage. These conclusions are further supported by the high fertility status of compound vis-a-vis distant fields. The data of soil chemical analyses for three sites studied by Lagemann is shown in Table 7. The data indicate significant differences among compounds and distant fields in acidification, and soil test for phosphorus and exchangeable cations. Soil pH in compounds had not deteriorated when compared with near and distant fields. Exchangeable cations and phosphorus levels were also significantly higher in the compound than in the outer fields. 38. The survey conducted by Armon showed that fertility status of .ompounds, heavily manured by traditional methods of organic farming, is significantly superior to those of unmanured distant fields (Table 8). The high fertility status of compounds is despite the intensive cultivation without fallowing. Taking the mean soil properties for all sites in compound versus distant farms, respectively, showed that soil test values were: 5.84 vs 5.23 for pH, 1.62% vs 1.20% for organic carbon, 0.15% vs 0.17% for total N, 96.6 ppm vs. 44.8 ppm for P, 3.71 meq/100 g vs 1.50 meq/100 g for Ca, 0.63 meq/100 for g vs. 0.31 meq/100 g for Mg, and 0.24 meq/100 g vs 0.09 meq/100 g for K. Statistical analyses of the data indicated that soil test values in compounds were significantly higher than in distant fields for pH, organic carbon, available phosphorus and exchangeable calcium, magnesium and potassium. Addition of plant nutrients through mulch, household and animal wastes, and by recycling due to deep- rooted perennials maintain high level of soil fertility in compounds despite intensive land use and continuous cropping. This type of intensive management, however, is impossible to be followed on large areas of regularly cultivated distant fields. VI. CAUSES OF SOIL DEGRADATION IN SOUTHERN NIGERIA 39. Causes of soil degradation encountered either in the literature or during the field survey and discussions in Nigeria consist of (1) fire and burning of vegetatinn, (2) deforestation, (3) increasing intensity of farming and cultivation, and tillage and related practices, (4) low input agriculture, (5) accelerated erosion, (6) road building and other construction. Fire and Burning of Vegetation 40. There has recently been increasing frequency and intensity of fires and burning of vegetation, especially in the dry season. Fires may be (1) purposely used for clearing of vegetation in traditional farming systems, (2) accidaitial, (3) used to drive out game and wild animals and (4) used to eliminate undesirable refuse or some inflammable waste. During the field survey in February and March 1989 many fires were observed burning uncontrolled along roadsides in all of the 8 states visited to the extent that 5 to above 30% of roadsides were burnt during the dry season (see Table 9). Despite laws against burning of vegetation many economic - 16 - trees (oil palm, rubber, cocoa, bananas and plantains), food crops (cassava, cocoyam, pineapples, pepper, vegetables), forest plantations, ornamentals and vegetation were burnt. Many fires start from dry Siam weed growing along roadsides and penetrate far into nearby bushes except where thick vegetation and planted fallows of Acioa and Anthonotha blocked their path. 41. Uncontrolled fire causes soil degradation through short-term and long-term effects. A short-term effect is that related to high temperatures generated during the fire. Temperature at the fire base, at the soil-air interphase, may exceed 2500C (Lal, 1987). Although, the heatwave may not penetrate deep, the top 5-cm of soil may be completely sterilized. In addition to alteration of soil chemical properties, excessive heat changes soil structure and its wettability. Fire-burned soil is hydrophobic and becomes prone to excessive runoff and accelerated erosion. Ecological instability caused by repeated cycles of fire have resulted in denudation of vast areas of its protective vegetation cover, soil compaction, and unprecedented runoff and accelerated rates of erosion. Furthermore, economic crops damaged by fire are prone to infestation by pathogens or insects. Deforestation and land clearing 42. Deforestation, which increases with intensity of cultivation, and harvesting of timber and removal of vegetation along roads and on construction and mining sites, expose the soil to the elements with increased runoff, erosion and soil degradation. Mechanized clearing which is replacing manual clearing and logging operations cause scraping away of rich top soil and compaction of subsoil which restricts water infiltration and root penetration and increases runoff and erosion. Experiments conducted in the subhumid regions of western Nigeria showed that runoff and erosion from mechanically cleared lands were several orders of magnitude greater than those from traditional or manually cleared land (Table 10). The data in Table 11 from western Nigeria indicate drastic reductions in fertility status of the surface soil horizon of mechanically cleared land in comparison with manual clearing or slash-and-burn traditional farming. Increased Intensity of Farming and Shortening the Period of Fallow 43. Increasing population pressure has resulted in decreased periods of fallow and intensification of cultivation. A survey by Goldman (1988) in Imo state of Nigeria has indicated that during the last 20 years fallow periods have declined from 1-9 years to 0-6 years--a period much shorter than the 5-7 years required to restore soil fertility. The decline of fallow periods had rendered fallows incapable of supplying stakes, fuelwood, fodder or browse, edible mushrooms, drugs, fiber, fruits and vegetables to rural communities. This either causes the farmer to spend money to purchase these things or deforest other areas to obtain them. Some highly degraded areas are colonized by spear grass. 44. Similar to the data on degradation of soil physical properties, there is little additional information available from farmers' fields on depletion of soil fertility with continuous cropping. Although it is highly suggestive of its impact, the data by Lagemann and Armon do not provide sufficient information for establishment of the direct cause-effect - 17 - relationships. Several studies conducted on research farms, however, indicate drastic decline in soil fertility due to continuous cropping. Experiments conducted at the research farm of IITA showed rapid acidification, ano reduction in organic carbon, total nitrogen, effective CEC and exchangeable cations due to continuous cropping (Table 2). Fertility depletion occurred despite addition of the recommended rates of fertilizers and return of crop residue as mulch. In another experiment also conducted at IITA, Lal (1985) showed drastic changes in soil chemical properties over a short period of 6-years of continuous cropping. In a plow-based system soil pH decreased by almost 2 units, organic carbon by 0.2 to 0.5%, total N by 0.1 to 0.2%, and ECEC by 5 to 6 meq/100 g. The rate of fertility depletion, however, can be somewhat less with no-till, mulch farming and alley cropping systems than with conventional plow-based systems. The data in Table 12 is indicative of beneficial effects of the no-till system on soil fertility. Nonetheless, fertility decline is inevitable with intensive continuous cropping regardless of the management system. At the research farm of the University of Ife, Aina (1979) observed severe decline in soil chemical and physical, properties. Decline in soil pH, exchangeable cations and CEC were severe with continuous cropping both with and without fertilizer application. Expectedly, however, fertility decline was more drastic in relatively less fertile Oba than fertile Iwo soil series. In addition to decline in soil chemical properties, continuous cropping also results in degradation of soil physical properties. A principal victim of continuous cropping especially by mechanized means, is soil structure. Structural degradation is evidenced by rapid increase in bulk density, decrease in total and macro- porosity, reduction in infiltration capacity, and decrease in percent aggregation. Some of these effects were discussed in several reviews by Lal (1986; 1987). 45. If fertility depletion is rapid and drastic on research farms even with intensive input of chemical amendments, the problem is likely to be at least as severe on farmers' fields. Major differences, however, are in intensity of cropping and in cropping systems used. Farmers rarely use monocropping commonly used by research farms. Furthermore, farmers deliberately use low plant stand and grow crops in association with trees and woody perennials. The rate of fertility decline under farmers' conditions may, therefore, be less thar that observed on research farms. Low Input Agriculture 46. Intensive farming without restorative fallowing or exogenous inputs is wasteful and degradative. It is a system of mining the inherent fertility that leads to degradation of the entire ecosystem. Fertilizer use on traditional farms is still negligible. While the excessive and indiscriminate use of fertilizers and other chemicals must be avoided, exploitation of native fertility through continuous mining is an ecologically incompatible system. Increasing cropping intensity without external input is to aggravate the degradative trend. In the studies cited above Armon (1984) and Lagemann (1977) reported severe degradation on distant farms because these fields do not receive mulch, manure or fertilizers. Poor crop stand and low plant vigor results in scanty canopy cover, especially at the onset of heavy rains. Native trees are severely damaged by repeated cycles of uncontrolled fire. All biomass is purposely removed away or burnt. Soil degradation, especially on distant farms, is . 18 - caused by low-input, exploitive agriculture driven by high population density and shortage of good arable land. Accelerated Erosion 47. Erosion, which is the wearing away and removal of weathered rock material or soil from the surface of the land, varies in intensity from one state to another in southern Nigeria. It is a selective process which removes the topmost fertile 0-5 cm of soil and its ability to grow crops. While many human activities may cause erosion, it has been observed that the extent of erosion often depends more on relief, surface materials, climate and vegetation interacting with each other and human activities. The coarser soil texture in southeastern Nigeria and presence of cuestas running north to south has made the area more subject to erosion than in the western states of Nigeria. Erosion is more of an urban phenomenon in the West as compared to the East where it is rural. Erosion may be manifested as (1) sheet erosion which is the removal of a very thin layer uniformly over the surface which is most serious in Ogun, Oyo, Imo and Anambra states; (2) XJl_ erosion which removes materials from relatively shallow and narrow channels, especially in Anambra, Imo and Lagos states; and (3) gully erosion which removes materials in deep channels and is most pronounced in Imo, Anambra, Cross River, Akwa-Ibom and Rivers States; (4) coastal erosion causing wearing out of ocean shoreline in Rivers, Cross River, Akwa Ibom, Lagos and Bendel states. About 80,000 km2 or 45% of southeastern Nigeria is affected by rill and sheet erosion and about 2-3% by gully erosion (Figure 19). Roads. Buldings and Industrial Construction 48. The oil boom in Nigeria resulted in building of a network of express roads and ordinary tarred roads in many states of Southern Nigeria. Coupled with these are untarred roads and footpaths to all nooks and corners. Recently the Directorate of Food, Roads and Rural Infrastructure (DFRRI) has increased the network of untarred rural roads, that are not all properly provided with drainage. The Enugu-Onitsha, Enugu-Nsukka and the Port Harcourt-Aba-Enugu roads have been reported by Fubara (1988) to be treated or cut by gulley erosion. Many untarred roads are damaged or are turning into gullies because of regular scraping to level them after damage by torrential rainstorms. Roadsides covered by Siam weed, which is not regularly mowed constitute fire hazards in the dry season. 49. Rain water rushing down corrugated iron roofed buildings in rural and urban areas especially when surrounded by concreted compounds without adequate drainage cause channels of runoff and erosion. Various industrial buildings, airports, playing fields, car parks, and markets without adequate drainage are associated with a lot of soil degradation. Grazing along roadsides also contributes to exposure to soil to erosion. VII. MEASURES USED TO PREVENT OR COMBAT SOIL DEGRADATION 50. Measures used to combat soil degradation may be preventive or remedial. Preventive measures are practices used to minimize the chances of soil degradation occurring or the magnitude or severity of the damage thereof when they do occur. The preventive measures consisting of soil conservation practices or aspects of agricultural production are reviewed - 19 - briefly under different subheads below followed by a brief survey of sustainable agricultural production systems which have the potential foi minimizing soil degradation while achieving economically viable yield levels. Remedial measures taken by government are considered later in -:he next section. Manuring and Mulching 51. Manuring involves the application of organic residues consisting of rotted or decomposed organic materials, household refuse, compost, kitchen refuse including ashes and animal manures to the land on which crops are grown. These manures are used to restore soil fertility and productivity. Very often pits are dug in which manure is accumulated. Animals are kept in pens where their droppings or dung are made into farmyard manure (FYM). This kind of manure is used to maintain fertility in home gardens, adjacent fields and sometimes market gardens. A lot of soil waste in urban areas can be compacted but is not a widespread practice. 52. Mulches usually consist of plant residues; industrial wastes such as sawdust; crop residues such as straw and rice husks and even polythene films which are used to cover the soil in mounds, beds, ridges and flat, where crops are growing. Mulching protects the soil from the beating action of the rain, increases water infiltration rate and prevents compaction of the soil thereby reducing runoff and erosion, supplies organic matter and nutrients to the soil, reduces the amplitude of temperature fluctuations and increases yields. Both manures and mulches are only economical to use on very small farms and high labor costs minimize extent of their use. Living mulches such as Pueraria and Flemingia have been found useful in tree crop plantations and in the growing of plantains, respectively. Planted Fallows and Cover Crops 53. Properly managed planted fallows and cover crops are more efficient and require less time to restore soil properties than natural fallows. Because of high rate of biomass production, soil organic matter can be rapidly built up and th, soil structure improved even on severely eroded and degraded lands by growing appropriate planted fallows for 3 to 5 years (Jaiyebo and Moore, 1964; Juo and Lal, 1977; Wilson et al., 1982). There is a considerable amount of research information available from western Nigeria regarding the beneficial effects of cover crops in restoring soil properties. Studies by Jaiyebo and Moore (1964) indicate drastic increases by Pueraria in soil organic matter, total and N03-N, total P, CEC and exchangeable bases. Short-term fallowing by Pueraria was as effective as long-term bush fallowing. Lal et. al., (1979) reported significant improvement in soil organic matter by 3-years of fallowing by several grasses and leguminous cover crops. Stylosanthes and Cynodon were more effective than Paspalum and weed fallow control. Similar improvements in soil fertility were reported by Wilson et al. (1982) Improvements in soil organic matter content and fertility status are also accompanied by restoration of soil structure. In western Nigeria, Lal et al. (1978) observed that planted fallows enhanced biotic activity of earthworms. Stable and continuous macropores and biochannels thus created decreased bulk density and increased infiltration rate. Some important cover crops - 20 - adaptable for southern Nigeria are Calopogoni muconoides, Centrosema pubescens, indigfera spictata, Pueraria phaseoloides Stylosanthes guianensis, Dolicos hosel, Glycine Javanica, Muouna utilis, Brachiaria ruziziensis, and Panicum maximum. 54. Improvements in soil fertility are reflected in higher yields of crops grown after the cover crops (Lal, 1986). However, growing cover crops for a short period without chemical fertilizers and manure is not adequate to enable high yields on a continuous basis. Data from eastern Nigeria show rapid yieli decline of maize grown after Mucuna and Dolichos after the second consecutive crop. Grain yield for the sixth consecutive maize crop was 21% of the first crop in case of Mucuna and only 14% in case of Dolichos. Nutrient removed by nitrophilic crops such as maize cannot be replenished by short fallowing alone without supplemental applications of chemical fertilizers or organic amendments. Sustainable Farming Systems 55. Soil degradation could be prevented or minimized where various crops are grown on soils in parts of the landscape where they are most adapted and have both economic and ecological advantages. The production systems which are most adapted to the humid and subhumid areas of Nigeria include: (1) tree crop plantations, (2) home gardens or compound farms, (3) alley cropping and related agroforestry systems, and (4) wetland rice. 56. Tree crops such as rubber, cocoa, oil palm and coffee when grown in plantations, and especially with leguminous cover crops, effectively protect the soil. Such tree crops give good yields 'or 20-25 years or more. Tree crops enterprises vary in size from small holder plantations of under 5 ha to large scale farms of more than 100 ha. Small holder tree crops plantations have naot been economically viable due to size limitations, and the tendency of farmers not to replant uneconomic plantations with improved cultivars when yields start to decline after 20 years. The result is reduced yields and ineffective soil cover. 57. Home Gardens or Compound Farms: This is one of the most widespread sustainable agricultural production systems in the humid and subhumid tropics and, for Southeastern Nigeria, has been described in some detail by Okafor and Fernandes (1987). Compound farms are a type of agroforestry system involving the deliberate management of multipurpose trees and shrubs in a multi-storied association with agricultural crops and small livestock within the compounds of individual houses. These intensively cultivated areas are usually operated in conjunction with a more distant, less intensively cultivated area, used for staple crop production (e.g., of yams, cassava, rice, maize, etc.). The stability of the system is, no doubt, related to the species diversity, production of a wide range of food and useful products, effective integration of crop and animal production with fertility maintained under continuous cultivation by the use of household refuse, animal manure, farmyard manure and nutrient cycling by trees and shrubs, with some of the leguminous crops also fixing nitrogen. The animals yield meat, manure and cash. Ornamentals in home gardens also perform an aesthetic function. Increasing population pressure in rural areas, soil degradation and the building of houses on ancestral lands are causing some breakdown of the system. There is a need for a - 21 - special research program on home garden improvement if the system is to survive. 58. Alley Cropping and Related Agroforestry Systems: Alley cropping is a cropping system in which arable crops are grown in between two hedgerows that are regularly pruned in order to obtain cuttings or twigs for mulching and for soil fertility maintenance. With the development of alley cropping at IITA there has also been developed alley farming which is a silvopastoral production system in which fallow species grown in the hedgerows are used as browse or sources of fodder for small ruminants. Advantages of alley cropping include: . supply of mulch for covering and protecting the soil thereby reducing runoff and erosion . increased supply of organic matter and nutrients for maintaining soil fertility and increasing yields . increased infiltration . supply of stakes and miscellaneous useful products . supply of fuelwood, and . supply of fodder or browse 59. Despite all these advantages, there has not been widespread adoption of this practice due to high labor requirements for pruning, very narrow species of crops that can be grown in small plots under alley cropping and hence high risk of crop failure. Alley cropping without herbicides was not found to be as attractive as traditional practices. Alley cropping with Gliricidia and Leucaena was found to be promising in the keeping of sheep and goats, but its acceptance remains limited even in southeastern Nigeria where it has been demonstrated to be viable. This is mainly because of high soil acidity and low fertility, incompatibility of woody species, division of labor and decision-making process within households and tree tenure rules, need for more sustained extension effort and longer period of orientation of the farmer to ensure better understanding and appreciation of interaction of various factors (Francis and Atta-Kra (1988), Ehui (1988), Kang et al., (1989) and Osembo (1987). Alley farming was more attractive in Western Nigeria where animals are on free range than in Eastern Nigeria where animals are usually confined. 60. Alley cropping is a low-input system designed to bring about improvements in yield when the fallow period is reduced. Similar to the use of cover crops, this system has also inherent limitations. There is a limit to the amount of bases that can be recycled from the deeper layers of impoverished acid sands. With continuous cropping, therefore, decline in soil fertility and reduction in crop yields are inevitable unless the nutrient capital is augmented by external input or supplemented by fallowing. Data from a relatively fertile Alfisol show that continuous cultivation of maize-cowpea with alley cropping led to a serious decline in total nitrogen (from a range of 0.214 to 0.397 in 1982 to 0.038 to 0.105% in 1986). This drastic reduction was observed despite the addition of at least 120 kg/ha of N (Lal, 1989). Although N contents decreased, the soil - 22 - test value for exchangeable K showed some improvements. Such a level of improvement , however, was not observed for calcium and magnesium. The overall fertility decline in these plots was also reflected in yield decline. 61. It is important to realize that restorative effects of forests and agroforestry systems are usually over-exaggerated. With improper management and intensive landuse without adequate input, severe land degradation and fertility depletion can occur even with tree-based systems. Undoubtedly with good management, however, the chances of attaining economical success and ecologically compatible systems are better with tree-based systems than with monoculture of seasonal crops. However, without special conservation measures and supplemental applications of nutrients, intensive landuse even with tree-based systems will inevitably result in soil degradation in the form of decreased soil organic matter content and nutrient levels and loss of top soil structure and porosity (Lundgren, 1978; Lal, 1989). 62. Wetland Rice: Of most of the arable crops production systems in the tropics, the Asian wetland rice appears to be the most sustainable since it is adapted to flooded areas where most other crops are not adapted. Moreover, although lowland rice gives high yields when alternated with upland crops, rice can be grown year in year out without significant reductions in yields. While rice is becoming a very important cash crop. especially with the fanning of rice imports in Nigeria, it has not become as widely grown as expected in many valley bottoms and hydromorphic soils where it has the highest potential. This is partly due to cultural reasons and partly because of health hazards in hydromorphic areas. Chemical Fertilizers 63. It is apparent from the foregoing analyses that economic levels of intensive cultivation cannot be maintained with traditional methods of restoring soil fertility. Furthermore, excessive soil degradation now prevalent in the region is driven by low-input, subsistence, and fertility- mining farming practices. Although low-input systems are to be preferred, it does not imply that use of synthetic fertilizers and soil amendments is prohibited. 64. Many experiments have been done since the early 1920's to evaluate the agronomic and economic benefits of chemical fertilizers. Experiments conducted during 1930s in acid soils of eastern Nigeria showed increase in yields of maize by application of lime. Research data from the Agricultural Research Station at Umudike showed significant increase in yield of cassava tubers by the application of NPK fertilizers. Yield of cassava tubers increased between the application rate of 0 to 630 kg/ha. Another set of experiments conducted in 1960s demonstrated drastic increases in yields of yam, maize, cowpea and cassava due to application of fertilizers. Fertilizer application increased yields by 7 to 41% for yam, 90 to 150% for maize, 4 to 354% for cowpea, and 71 to 150% for cassava. These improvements were obtained despite the use of Stylosanthes in rotation. 65. Similar beneficial effects of fertilizers on crop yields have been reported from relatively fertile Alfisols of the subhumid regions of - 23 - western Nigeria. Research conducted at the University of Ibadan by Agboola (1978) showed significant interaction between the level of soil organic matter content and the rate of N application on grain yield of cowpea. Expectedly, fertilizer response was evident only on degraded soils of low organic matter content. That is why application of fertilizers is not recommended for newly cleared lands. The data in Table 13 show that the effect of 1% to 2% of soil organic matter contents were compensated by addition of 50 to 100 kg/ha of nitrogen in soil with 0.5% of organic matter. For coarse-textured sandy soils in the subhumid region, Lal and Kang (1981) observed that use of crop residue mulch can partially compensate the need for additional fertilizer. Retaining crop residue as mulch increased yield of maize by 1700 kg/kg. 66. It is apparent, therefore, that the use of fertilizers is inevitable for increasing yields from impoverished and degraded soils in southern states of Nigeria. However, more appropriate soil and crop management systems need to be developed so as to reduce the amount and frequency of their application while also increasing efficiency of their utilization by crops. The nutrient requirements for high yields can be met partly be decreasing losses and increasing efficiency, partly by symbiotic nitrogen fixation through cover crops and alley cropping, partly by re- cycling organic wastes and crop residue mulches. Nonetheless, the vital difference in nutrients needed for high yields on continuous basis will have to be met by adding synthetic fertilizers. Judicious management can decrease the need for chemical fertilizers but cannot eliminate their use. VIII. IS CONTINUOUS CROPPING ECOLOGICALLY AND ECONOMICALLY SUSTAINABLE? 67. Presently, yields are limited by soil, environmental, and socio- political constraints. Physically, most upland soils are easily compacted, have low available water holding capacity, and are susceptible to accelerated erosion. Structural deterioration sets in as soon as these soils are cultivated intensively. Chemically, soils of southern Nigeria have low nutrient reserves, especially of nitrogen, phosphorus and some trace elements. Although the initial level of soil organic matter is generally high, it declines rapidly with deforestation and intensive cultivation. In addition to harvest, nutrient losses are also caused by leaching, volatilization, especially during burning, and runoff and erosion. 68. Compound farming is an example of an intensive and economically viable farming system at a small scale of farming. This technology, however, cannot easily be extended to more distant plots. Subsistence and degradative traditional farming systems, as practiced on most more remote farm plots,can be replaced by conservative and economically productive system provided that the climate/vegetation/soil balance is kept at an ecologically favorable level; an adequate and regular supply of organic matter to the soil surface is maintained; the activity and diversity of soil fauna is enhanced to enable favorable soil turnover; - 24 - . structural conditions of the soil are improved to minimize compaction and crusting; . a nutrient recycling mechanism is built into the system to minimize nutrient losses due to leaching; . a desirable nutrient balance is maintained and risks of acidification are minimized; and . nutrients harvested by plants and animals are replenished by judicious use of chemical fertilizers. 69. The success of compound farming is attributed to all the above mentioned factors. However, nutrient replenishment through organic manure at the level needed to restore a high level of productivity is not feasible for distant plots. Improved systems must, therefore, be soil restorative, ecologically compatible, and commercially or economically viable. 70. A self-sustaining closed system, as the traditional true shifting cultivation was, is outdated and a non-solution in the context of present day realities. The system, at its best, was ecologically stable but at a subsistence level of farming. While the desirable aspects of the system can be preserved, the new system must enable intensive farming at a commercial or semi-commercial level of production. For a soil low in inherent fertility and depleted of its nutrient reserves due to natural soil forming factors involving intense weathering, commercial farming cannot be achieved without boosting its nutrient dowry. The nutrient status can be improved by adoption of conservation-effective farming systems, regular returns of some organic matter and biomass to the soil, and supplemental applications of judicious levels and appropriate types of inorganic fertilizers. 71. Adoption of Improved Technologies: Based on the field surveys undertaken in 1989, and a review of various technologies and associated production systems that reduce soil degradation, an attempt has been made to summarize the extent to which the different more sustainable production systems and associated technologies are being adopted or used in Southern Nigeria. (See Figure 5.) Reference to this table shows that most of the sustainable production systems such as tree crop plantations are being adopted widely. Tree crops are the most adapted and sustainable production systems in the humid tropics. In most of southeastern Nigeria population densities are very high and large scale plantations are out of the question. But at the same time not much progress is being made in rehabilitation of small-holder production systems by growing them with arable crops in more permanent agroforestry systems. In areas of low population density, where large scale plantations are appropriate, there is an increasing tendency for widespread adoption of inappropriate mechanical clearing techniques which usually result in irreversible soil degradation, increased runoff, and low productivity. Unless tree crops or adapted arable crops such as cassava and yams are promoted in humid areas, soil degradation problems will continue to mount. 72. Wetland rice, especially with the banning of rice importation and high returns from lowland rice, is becoming widely adopted in low lying - 25 - Figure 5. Extent of used or adoption of certain technologies in various sustainable production systems that can potentially reduce soil degradation in the southern states of Nigeria Systems and Component Technologies Extent of Use Remarks A.FARMING OR PRODUCTION SYSTEMS 1. Tropical Tree Crops a. Small holder +++ Declining yield, not being improved b. Large scale ++ Increasing and destructive 2. Wetland Rice ++r Only in suitable niches 3. Traditional Agroforestry a. Home Gardens/Compound Farms +++- Mainly in rural southeastern Nigeria b. Arable Crop/Tree Crop Farms In rural areas c. Rural Hedgerow Cropping ++ Rural areas especially southeastern Nigeria 4. 'Modern' Agroforestry a. Taungya system Causes genetic erosion, declining b. Alley cropping NT Promising still in on-farm trials c. Alley farming NT Promising still in on-farm trials d. Permanent Tree Crop/ Arable Crop Farms + Not yet well developed and of reasonably low input S. Intensive Livestock Production a. Poultry ++ Especially in urban areas b. Mixed farming + Restricted to a few poultry farms 6. Commercial Crop Production a. Market gardens +++ Especially in urban areas b. Large scale farms ++ Increasing but destructive B. SUB-SYSTEMS OF PRODUCTION a. Mixed cropping +++- Mainly traditional, no improvement b. Relay cropping ++- Not being involved in rotations c. Crop rotations + More of continous cropping developing C. COMPONENT TECHNOLOGIES/PRACTICES a. Traditional Land Clearing +++- Increasingly being abandoned because of high cost - 26 - Figure 5. (Continued . . . Systems and Component Technologies Extent of Use b. Selective Mechaniza- tion N Techniques and benefits not yet widely known c. Reduced Tillage + Not yet well studied for various crops d. Manuring ++ Maintain home gardens and by fallowing e. Mulching and Good Plant Residue Management + More common to traditional systems than in modern ones f. Fertilizer application ++ Move to cash crops, not much in food crops g. Fertilizer/Manuring ++ In home gardens and market gardens h. Integrated Pest or Disease Management N Limited research to make this possible i. Use of Resistant High-yielding*Vrieties + Not yet possible for many crops j. Fallowing - long term tree/ shrub ++" Length of fallow declining - short term cover crop N Not being actively researched Key 4++ Widespread or widely practiced ++ Common + Limited Use NT Negligible and under trial or testing !+ Increasing in use Declining in use Restricted to specific situations - 27 - areas and inland swamps where cost of land development and water control is not too expensive. However, the priority being given to this in agricultural development is not commensurate with wetland rice potentialities. 73. Home gardens are fairly sustainable, but under increasing population density and urbanization which is extending to rural areas, they may be declining and with increased growing of arable crops, soil degradation is becoming a serious problem. There is still a lot of opportunity for development of arable crop/tree crop farms, but this has not received the attention it deserves in research and development. The hedgerow cropping system could be adopted in all rural areas along boundaries between farms, but there may be need for some land consolidation to ensure that areas occupied by hedgerows are not proportionately too large. 74. The taungya (forestry) system is of limited use since it is not only destructive but requires using the force of the law to extend the length of fallow in addition to strictly controlling farmer's execution of operations. Alley cropping and alley farming need to be given higher priority in research and more innovatively adapted to the farmer's needs and circumstances to ensure economic viability and cultural acceptability. 75. Poultry farming which is not operated as a mixed farming enterprise often constitutes a serious source of environmental pollution if the waste is not properly disposed of, or used in growing crops. Use of poultry waste in market gardens or use of compost in market gardening has already been found to be a reliable practice that ensures sustainability provided labor is available. The returns from the market gardening also facilitate purchase of some limited amounts of fertilizers. Large scale arable crop farms will not be sustainable until they are executed with careful mechanized use of reduced tillage systems, adequate plant residue management in addition to fertilizers, and suitable rotations. 76. Monocropping, of annual or short-duration crops is highly degradative and cannot be sustained even with high levels of fertilizer application, which are not cost effective, but also have low residual effect, without use of adequate levels of organic matter in a well proven rotational sequence of cropping. The use of well designed and well tested rotations of simple crop mixtures is imperative in the development of sustainable arable cropping systems in the humid tropics. This needs to receive higher priority, but because of the current emphasis on shorter term evaluation of cropping patterns, it may take some time before this can be done. 77. Fallowing with trees and shrubs while ecologically sound, is increasingly no longer economically viable and has to be replaced with more intensive systems such as alley cropping or actively managed fallows, e.g., shorter duration (possibly one year), fallows with herbaceous legumes and judicious application of fertilizers. Fertilizer use, adequate organic residue management, reduced tillage systems and integrated pest and disease management systems are ingredients that must be components of sustainable agricultural production systems. - 28 - IX. RESEARCH AND DEVELOPMENT PRIORITIES 78. This paper has indicated the complex nature both of the processes of land degradation itself and of the social and economic conditions which appear to facilitate its occurrence. No individual deliberately set out to destroy these resources, but this can be the cumulative effect of actions undertaken for other purposes. Given the rapid increase in population and consequent pressure on land resources, their conservation is a justifiable objective of national policy. 79. While individual actions are important in causing or alleviating land degradation, a sustainable pattern of land use, particularly in more fragile areas, cannot be achieved by the independent and random actions of individual land holders and users. It will require collaborative action at the local level. In order to achieve this a national effort will be required with actions in a number of areas: (a) increasing public awareness of the problem; this will include steps to raise the profile of related activities in the public sector at the national and state levelsand ensure adequate interagency coordination; (b) research to increase understanding of the nature of the problem and of the type of technologies and farming systems discussed above; (c) demonstration and extension efforts. In addition to individual actions, soil and water conservation efforts require collaboration among resource users at the local level. Measures to foster and support such collaboration will be required. A. Interagency Collaboration 80. A considerable number of institutions have been created over the past two decades to address different aspects of soil and water management and this structure is still evolving (FAO/CP, 1990). At present there is excessive overlap of theoretical responsibilities, which leads to inadequate and fragmented funding and, in consequence, poor liaison and very limited action. There is need or one top level body and there are indications that the Natural Resources Conservation Council (NRCC), announced in 1989, may play this role, especially if the President assumes its chairmanship. NRCC will be empowered to insist that State level agencies carry out conservation programs. It will be advised by the National Committee on the Conservation of Renewable Resources (NCCRR) which was created at the same time and includes Directors of the relevant federal departments, and representatives of other national agencies and the President's Office of Budget and Planning. 81. Several states have established Interministerial Committees or Task Forces to address resource conservation issues. If local initiatives are to be fostered it will be essential that the state bodies be given a high profile and the full backing of state leadership. - 29 - 82. As indicated below a broad program of investigation and research will be necessary to support action programs. In order to focus these efforts it would be desirable to establish a National Soil Degradation Study Group charged with the responsibility for (1) conducting adaptive research in selected areas aimed at utilizing results of research to design more practical methods of combating soil degradation in agriculture, and determining constraints to farmers adoption of the practices and what should be done to enhance adoption; and (2) monitoring of soil degradation processes in different situations based on standardized procedures and quantifying the extent and contributions from various processes. This body could function under the umbrella of the NCCRR and the membership should consist of university scientists in relevant disciplines in addition to staff of some Federal research institutes. The adaptive research could be conducted in the different sectoral activities in agriculture, forestry, fisheries, construction, mining, etc. There is need for careful detailed discussion and consultations to ensure that this body functions effectively with the Federal Department of Agricultural Land Resources without being stifled by bureaucratic civil service procedures and might have a semi- autonomous status under a director possibly from the university system. B. Research 83. A principal constraint in improving agriculture in southern Nigeria is the lack of reliable data on present status of the resource base. No one knows for sure the type, degree, and extent cf soil degradation. Most data, as reviewed in this report, are clincerned with principles and processes of soil d6gradation observed oi researcher managed on-station experimentation. There are few, if any, studies on evaluation of soil resources on farmers' fields. 84. Applied Research: It is difficult to improve soil characteristics through better management without better knowledge of the current status of soil properties and the rate of their change under different farming/land use systems. An organized effort must, therefore, be made to conduct farm surveys regarding the current status and future trends in the prevalent farming systems, crop rotations and combinations, cultural practices, the amount and type of inputs, crop yields and economic returns. In addition, detailed and reconnaissance surveys should be undertaken to characterize vegetation, water and soil resources. Vegetation surveys should establish the successional changes in predominant species following deforestation, uncontrolled burning, traditional farming and intensive land use. Water resources should be evaluated in terms of stream flow, ground water level, and their dissolved and suspended loads. Current status of the soil (Physical, chemical and biological properties) must be related to the past and prevalent farming systems, and compared with the initial level of soil characteristics in an undisturbed state. The latter may be hard to find except in forest reserves and communal protected lands. Traditionally, a wide range of plants have been used for food, agricultural or other purposes. However, in most cases little is known about their cultural characteristics, which may include useful attributes which can be utilized in a farming system. For example acia which has traditionally been used in the south-east grows more vigorously in the sandy, infertiles acid soils of the region than Leucaena or GliricidiA, which are generally recommended for alley type systems or contour hedging. A systematic review of useful species (see Okafor, 1987) for this purpose, should be a high priority. - 30 - 85. Standardization of methodologies used for field and laboratory evaluation is an important consideration. While traditional methods of soil survey can be upgraded, efforts must be made to use modern technologies to assess land and water resources. Remote sensing provides a unique opportunity to evaluate soil, water, and vegetation. Methodologies for using remote sensing are available to assess change in land use, vegetation cover, extent of the damage by fire, extent and rate of gully erosion, level of water in the reservoir and stream (Paul et al., 1989; Estes and Cosentino, 1989). Some soil properties and sediment load in reservoirs and rivers can also be assessed by methods based on remote sensing. 86. Establishing farm demonstrations is vital in showing farmers the use and benefits of new technologies. Farm demonstrations should be established in indicate the benefits of different components or sub-systems of improved agricultural practices. An example of the method of step-wise evaluation of sub-systems is shown below: A. Traditional system. B. A+ improved cultivar, cropping systems and agro-forestry. C. B+ improved tools and labor-saving devices. D. C+ chemical fertilizers and amendments. 87. Systems A through D involve incremental improvement of a sub- system at a time. The farmer can judge by him/herself, the return from each step. Local specific components for each of the steps listed above are already available for major soils and ecological regions of Nigeria, and may need only slight validation and adaptation. Benefits of chemical fertilizers may be more substantial with improved cultivars, better cropping system, and efficient tools and implement than with traditional farming. 88. Basic Research: Over and above the need for applied research discussed above, there is also a need to conduct some basic research. The term soil degradation is an emotional rhetoric and is used rather vaguely. To avoid ambiquity and confusion, it is important that predominant soil degradation processes in southern Nigeria be defined on a more quantitative basis. To do so will require basic research in delineating critical limits of soil properties beyond which crop growth and economic yields are seriously jeopardized. These critical limits, of the life-governing soil processes and properties that support them, are not known. Some of these processes are affected by interactions, such as those between organic matter and soil structure, soil structure and water availability, texture and nutrient status, etc. These interactions are specific among soils and are not known. 89. Furthermore, critical limits of soil properties must be defined to delineate different levels of soil degradation. These limits vary among soils, antecedent soil moisture regime, climatic conditions, land use, crops and agro-ecological regions. It is impossible to assess the type, extent, and rate of degradation without knowing these limits for predominant soils of south and southeastern Nigeria. For example, if the critical limits of organic matter content, water and nutrient status, porosity, compaction and effective rooting depth are not known for major 31 - soils and crops of the region, it is difficult to judge whether a soil is degraded and if so to what degree and by which process(es). Such information is currently not available. It is apparent from the literature presented that we do now know yields of major crops (yam, cassava, corn, etc.) in relation to soil organic matter content, top soil depth, porosity, plant-available water reserves, and soil test values for lime, N, P, and basic cation. Without the benefits of this basic information, we cannot judge the restorative effects of conservation-effective measures. Land evaluation criteria, developed through the basic research proposed above, should indicate: when a soil should be taken out of production and put under a restorative and ameliorative phase, or when a soil is sufficiently restored and should be put under cultivation. C. Extension and Eblic Awareness 90. It is recommended that initial efforts focus on two activities; preparation of materials for general dissemination to increase public awareness; and targeted demonstrations of improved technology. As an initial step a small public enlightenment and education bulletin could be prepared aimed at: . Indicating the nature and magnitude of soil degradation occurring in Nigeria. . The causes of soil degradation and processes involved. Actions being taken to understand and combat soil erosion and limitations of such actions. . What really needs to be done that is not being done. . Roles of governments, private sector, non-government organizations and private individuals in combating soil degradation. The bulletin should be non-technical, simple and well illustrated and aimed at the man in the street, farmers and policy-makers. 91. Demonstrations of various soil conservation and management practices that have been proved to work in different situations either to prevent soil degradation occurring or to check incipient manifestations of soil degradation from becoming more serious and expensive to remedy. These demonstrations should encompass agricultural soil conservation measures, forestry, road and building construction mining and other sectoral activities wherever they can be located and effectively demonstrated. The agricultural demonstrations should be looked after by state agricultural extension officers. Those dealing with road and building construction should be under the Ministry of Works, Housing and Environment. There should be arrangements for special field days to be held each year at certain times for the public to see them and for students to visit such sites as part of their educational training in agriculture, geography, science and civics. 92. In order to foster adoption of modified technologies to ensure conservation of soil resources, awareness campaigns and demonstrations will - 32 - have to be backed up by action programs which provide some form of incentives, and/or requirement, for group action by villagers. This might include the preparation of village conservation plans as a basis for provision of required inputs on a concessional basis or, perhaps, as a condition for participation in other public programs. Effective implementation of such a program would itself require a major effort to increase the numbers of appropriately trained extension personnel and possibly changes in the organization of extension. These issues, however, are outside the scope of this report. - 33 - Table 1. Changes in chemical properties of the surface 0-15 cm layer of an Alfisol near lba6an, western Nigeria due to cultivation (Wilson et al., 1982). Forest fallow Continuous cultivationl/ Planted fallow for 2 yrs Soil property (20-25 yrs) (5 yrs) Grasses Legumes pH (1:1, H20) 65 55 52 5.6 Organic carbon (%) 1.7 12 1.5 1.6 Total N (%) 0.16 0.12 0.18 020 Bray P (ppm) 15 37 60 33 Cat2 (meqI100g) 4.8 4.0 62 6.7 Mg+2 (meql100g) 1.3 08 1.3 1.6 K+ (meqll 00g) 0.6 0.4 0.9 0.9 M/With application of fertilizer at the rate of 100 kg N, 60 kg K and 50 kg P per ha per year for maize-cowpea rotation. . 34 � iаЫе 2. Decline in feпility of an Alfisol пеаг lbadan in Оуо state ипдег con�nuous cropping, (la1 t 985). Pгe-cultivation, yrs � 2 угs 4 угв 6 yrs Propetty NT Р NT Р NT Р NT рИ (з ;1 in H2f3) в.з t о.з s.з � о.а s,a t о.в 5.в t о.з s.в t о.за 5.в t о.зз s.a t о.э а.а t oz Organic сагbоп (�,) з.а � а.а з.т t о.э з.sг t o.s2 з.эг t о.зз з.ss t о.за з.зт t о.зз з.sо t о.зт з.гэ t о.оа Totat N(�о} а.за t о.зз о.ао � о.оа о.зв t о.оа о.г2 � о.оа о.г4 t о.зз оzз t о.з2 o.зss � а.оа o2os � о.оа егау•t Р(ррт� аtа аtз гs.зts.o зз.5ts.e э2tгв ззtв зstв sstзв E�cchangeablе cations (meq1100 g) � �а+г в.о ± з.т тs � з.в s.o t z.з т.з t з.а s.з � os sz t з.з з.о t з.а � s� о.а Mg+2 з.а t о.в г.о � о.т ts t а.т з.9 t os о.5а t о.от oss t о.за о.аз t оzз о.зз t о.оз � к¢ а.т ± о.г о.а t о.2 о.вт ± о.з o.9s t од о.аа � о.оа о.за t о.от о.зs ± o.os о.за ± o.os N8+ - - ОА9 * о.03 0.09 t 0.05 А.з з t о.о2 а.1 з t 0.02 а.г4 t аАг 0.21 t о.02 мп+г o.os ± о.о2 o.os t о.ог o.os ± о.оз о.оз t о.о2 -- - о.за ± o.os о.з±о.зз т'otal aad;ty - - о.ов t о.аа o.os ± о.оз - - о.зs ± о.2з o.ss ± o.as - 35 - Table 3. Changes in properties of the surface layer (0-15 cm) of an Alfisol near lbadan, western Nigeria, due to cultivation (Wilson et al., 1982). Forest fallow Continuous cultivation Planted fallow for 2 yrs Soil property (20-25 yrs) (5 yrs) Grasses Legumes Clay 184 212 - - Silt (%) 10.3 9.5 - - Sand 71.3 69.3 - - Gravel 5.0 20.0 - - Bulk density (g/cm3) 1.00 1.33 129 Infiltration rate (cm/h) 100 9 17 23 - 36 - Table 4. Effects of methods of deforestation aad cropping on bulk density and penetrometer resistance of 0.5 cm layer of an Alfisol in Oyo stato (Lal, 1984). Bulk density (glcm3) Penetrometer resistance (k/cn2j Pre- Pre- clearing clearing Treatment 1978 1979 1980 1981 1978 1979 1980 1981 Traditional farming 0.64 1.06 1.07 1.27 0.21 0.96 0.52 1.32 Manual clearing 0.68 1.17 1.17 1.39 0.20 1.4 0.75 1.19 Shear blade 0.70 1.19 1.37 1.38 0.26 1.0 1.84 2.19 Tree pusher/root 0.60 1.24 1.32 1.42 0.20 1.3 0.73 1.23 rake Each figure is a mean of 25 separate analyses. -37 - Table 5. Decline in grain yield of maize on newly cleared land with continuous cultivation for an Alfisol in Oyo State (Couper et al., 1979). GQrain.vield (t/havr) Year Plowed No-till 1975 2.7 2.8 1976 4.0 4.5 1977 3.9 4.8 1978 4.0 5.0 1979 2.9 3.8 1980 1.0 3.0 Yield is total of two crops per year. - 38 - Table 6i Soil and management effects on crop yields from distant farms in three villages in eastern Nigeria (Lagemann, 1977). R2 F (a) Linear function Y = 231.9 + 64.5 C + 13.2 P + 616.7 F - 52.7 M - 31.2 W + 0.19 T 0.49 41" (b) Cobb Douglas function Y = 1366.5 + 0.07 C + 0.14 P + 0.11 F - 0.06 M + 0.09 W - 0.05 T 0.41 30* (c) Quadradic function Y = 579.2 + 1980.08 C - 502.2 C2 + 3.6 P - 0.06 P2 - 477.0 F + 162.8 F2 0.64 38* - 194.6 M + 2.34 M2 + 76.02 W - 22.8 W2 - 0.35 T + 0.001 T2 Y = total yield (dry matter, in kg/ha) of all arable crops grown as mixture C 7 soil organic carbon (%) P= Bray-1 P (ppm) F= fallow period (years) M = planting month W= month to first weeding T= tree density - 39 - Table 7. Effects of farm types on soil fertility status of 3 sites in eastern Nigeria (Lagemann, 1977). Exchangeable cations (mea/100 a Organic Bray-P Total Site/Farm Carbon pH Ca*Mg K ppm Nitrogen (%) A. CoTound -farms Umuokile (25) 2.06 5.06 3.50 0.14 36.2 0.168 Owerre-Ebeiri (63) 1.06 5.06 2.77 0.17 19.2 0.085 B. Near farMs Okwe (35) 2.35 4.59 1.64 0.09 25.01 0.178 Umuokile (32) 2.30 4.71 1.90 0.06 10.58 0.170 Owerre-Ebeiri (63) 1.20 4.45 1.28 0.11 9.77 0.092 C. Distant farms Okwe (33) 2.37 4.47 0.89 0.08 9.78 0.169 Umuokile (44) 1.94 4.71 1.61 0.05" 8.03 0.151 Owerre-Ebeiri (63) 1.00 4.26 0.69 0.08 9.98 0.067 Figure in parenthesis is the number of soil samples analyzed. - 40 - Table 8. Fertility status (0-10 cm depth) of compound and distant farms for 25 farms at each of five locations in eastern Nigeria (Armon, 1984). Farm type Agula/ Calabar Isiekenesi Ohafia/ Uyo Nanka Abriba A. Compound farms pH 5.6 6.1 5.72 6.04 5.81 Organic carbon (%) 0.94 2.50 1.10 1.80 1.90 Total nitrogen (%) 0.1 0.15 0.14 0.16 0.18 Bray-P (ppm) 30.4 116.2 37.7 92.0 184.4 Exchangeable cations: (meq/100 g) Calcium 3.00 4.33 3.16 3.20 5.44 Magnesium 0.63 0.46 0.52 0.66 0.82 Potassium 0.24 0.21 0.23 0.13 0.25 B. Distant farms pH 5.11 5.28 5.28 5.50 5.03 Organic carbon (%) 0.75 0.83 1.67 1.22 1.30 Total nitrogen (%) 0.08 0.13 0.19 0.23 0.12 Bray-P (ppm) 10.3 13.40 22.4 52.1 37.3 Exchangeable cations: (meq/100 g) Calcium 1.24 1.40 1.54 2.0 1.30 Magnesium 0.31 0.20 0.34 0.40 0.25 Potassium 0.10 0.08 0.13 0.08 0.06 - 41 - TABLE 9 . Extent of fire damage and soil degradation manifested by cassava, other crops, plants and soils State Extent of fire damage Condition of crops and soil as observed along roads passed observed on roadsides Akwa Ibom Not visited Not visited Anambra 5-15% burn on roadside in Poor cassava crop in most areas between villages. Crop damage exhibiting symptoms of mealy bug slight. No serious damage in and spider mite damage. Soils planted fallow areas and home degraded colored light gray, low gardens. in organic matter. Spear grass, common on some degraded soils. Bendel 5-20% but along Benin Auchi No serious stunted growth observ- road up to 10-20% in some ed in cassava crop except on the places. Fire damage to Benin-Asaba roadsides where con-* economic trees and crops tinuous cassava production in (rubber, oil palms, cassava some areas has resulted in poor and cocoa) serious in some growth. areas. Cross River Not visited Not visited Imo 5-15% crop damage slight. Some areas with sandy soils de- No serious fire damage graded, soils low in organic observed in areas with matter, cassava in such areas Acioa/Anthonotha fallow on showing stunted growth as one roadside. goes northwards. Ogun State 10-25% and damage to economic No serious stunting of cassava trees (cocoa cola, oil palms) crop but there are areas with suffered slight to medium fire invasion by spear grass. damage. Ondo State 10-25% in many stretches along Cassava with stunted growth to- Akure-Benin roads many economic wards northern parts of the state. plants (cocoa, oil palms, Moisture stress serious in cassava and cola) damaged by bananas/plantains. fire. Many teak plantations were affected. - 42 - TABLE 9. (Continued . . State Extent of fire damage Condition of corps and soil as observed along roads passed observed on roadsides Oyo 15-30%, a lot of damage to Some areas toward northern economic plants (cocoa, oil areas of the state with palms, cola and cassava, cassava exhibiting stunted especially along Ibadan, Ife growth, some areas with spear and Ibadan-Oyo road. . grass invasion. Rivers 5-10% limited area visited. Not much stunting noticed in Only areas between Owerri cassava crop except in some - Port Harcourt and Port areas near boundary with Harcourt Aba roads observed. Imo state. Some moisture Slight fire damage to stress noticed in plantains. plantains, oil palms, cassava and teak plantations. - 43 - Table 10. Effects of methods of deforestation on runoff and erosion (Lal, 1981). Treatment Runoff (mmlyr) Soil erosion (t/ha/yr) Traditional farming 3 0.01 Manual clearing 35 2.5 Shear blade 86 3.8 Tree pusher/root rake 202 17.5 - 44 Table l1Effects of land clearing methods on degradation of soil fertility and chemical properties for 0-10 cm layer of an Alfisol in Oyo State (Lal, 1981). Land clearing method ECEC CA+2 Mg+2 K+ (meq/100 g) Mechanical 8.2 7.0 1.7 0.08 Slash and burn 14.0 11.3 2.4 0.10 Slash 9.5 7.6 1.6 0.08 LSD (0.05) 1.4 1.1 0.4 0.007 -45- Table 12.7Illage effects on soil fertility and chemical properties of the 0-S cm soil layer of an Alfisol at IITA, Ibadan, Nigeria (Lal, 1986). Property No-till Plowed PH (1: 1 in H20) 5.9 :t0. 1 5.3±*0.2 Organic carbon M% 1.69 ± 0.27 1.44 ± 0.07 Total nitrogen M% 0.179 ± 0.04 0.093 ± 0.007 CEC (meq/100g) 7.84 ± 1.4 3.99 ± 0.91 Exchangeable C12+ (meq/100g) 5.31 ± 0.84 2.87 ± 0.62 Exchangeable Me+~ (meq/100g) 0.97 ± 0.21 0.31 ± 0.08 Exchangeable K+ (meqtlOOg) 1.06 ± 0.46 0.32 ± 0.16 - 46 - Table 13.Average effect of N and soil organic matter on the yield of Ife - Brown Cowpea, 1973 - 1975. Teanent 0.5% 1% 2% 3%. 5% NO N 00 c 1,200 b 1,700a 1.750 a 1,800 a 10 kg N/ha 1,3006 1,650 a 1,800 a 1.700 a 1,750 a 20 kg N/ha 1,850 a 1,600 a 1,750 a 18008a 1,700A 50 kg N/ha 1,870 a 1,700a 1,800 a 1,850 a 1,800 a 100kg N/ha 1,750a 1,800 a 1,800 a 1,750 a 1,800 a Note: Numbers within columns followed by the same letter are not different (P = 0.05) according to Duncan's multiple Range Tes t + All treatments had basal dressing ofP and K based on soil test. *Source: Agboola (1978). - 47 - REFERENCES Adejuwon, J.O. (1976). 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