Sustaining soil productivity in intensive African agriculture
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CTA. 1994. Sustaining soil productivity in intensive African agriculture. Spore 49. CTA, Wageningen, The Netherlands.
Permanent link to this item: http://hdl.handle.net/10568/49299
In Africa, the agricultural sector is faced with many problems. Among them is the restricted access to good agricultural land in regions where land area per capita is continually decreasing as a result of the increasing population growth. Yet it is...
In Africa, the agricultural sector is faced with many problems. Among them is the restricted access to good agricultural land in regions where land area per capita is continually decreasing as a result of the increasing population growth. Yet it is in these regions where the demand for agricultural products is continually rising. Consequently there is a need to intensify land use. The strategy for achieving this should aim at generating higher yields per unit of land. However this may not be sustainable through production based on the natural fertility of the soil types found in Africa. Increased output of agricultural products can only be achieved if nutrients removed from the fields with the harvests are continuously replaced. Studies are needed that can provide adequate, reliable and up-to-date information which can be used to quantify nutrient removal and fertilizer requirements and hence improve our ability to understand and monitor the processes involved. Large gaps exist between developed and developing nations in terms of agricultural productivity and the quality of life enjoyed by their rural populations. Improvements in the quality of life in many developing nations may be viewed as a race between agricultural production and population growth. In sub-Saharan Africa in particular, this race is being steadily lost because all the modest gains in production are being more than offset by increased population. One of the major consequences of this race is the depletion of soil fertility. Soil depletion is not a simple parameter that can be rapidly evaluated and corrected. It is a situation where diminished availability of under-utilized lands, continuous soil erosion and nutrient removal, coupled with the scarcity of livestock feed, fuel and water resources results in a spiralling decay in productive capacity and a diminished resilience of the soil system to provide a suitable medium for crop growth. The immediate consequences of soil depletion are further deforestation, urban migration and increased unemployment. In short, farmers are increasingly less able to overcome the soil constraints to crop productivity as it becomes more crucial that they be able to do so. Understanding the resource base There is a pressing need to understand better the soil as a resource base and to relate these understandings to soil inventory and land evaluation methods. Soils serve as both a reservoir and source of plant nutrients. The inherent fertility of soils is associated with mineralisation of soil organic matter. Decline in total soil organic matter may often be a fundamental cause of nutrient exhaustion in farming systems that are provided with few external nutrient inputs. While the inherent capacity of a soil to provide plant nutrients may be supplemented through the application of chemical fertilizers, far less fertilizers are being applied per unit of land area in Africa than in other agricultural regions of the world. Intensified agricultural production is best achieved through the use of inorganic fertilizers and calls for the production and importation of the correct forms and quantities for timely distribution. These fertilizers must be supplied at reasonable cost and packaged in amounts suitable for use by small farmers. Fertilizer recommendations should be based on tested information which takes into account individual crop requirements and soil conditions. The overall economic return to the use of fertilizers can be improved by identifying the form, rate of application, timing and placement that offer the highest rate of economic return to farmers. Too often, the results of fertilizer-use trials are viewed in terms of their immediate crop return, rather than as an agroecosystem-wide resource input that interacts with, and may be optimized through, nutrient recycling processes. Another important means of improving nutrient cycling is through the use of applied organic inputs and the retention of crop residues. In many tropical cropping systems, few or no agricultural residues are returned to the soil and this leads to a decline in soil organic matter, lower crop yields and less plant biomass. Application of organic residues and the avoidance of burning are management practices that play an important role in reversing these trends, along with the adoption of reduced tillage systems and the establishment of live mulches. The level of organic matter within a soil is determined by its rates of formation and loss, yet the effect of applying many farmer-available organic resources and agro-industrial wastes remains poorly understood from the perspective of organic matter dynamics. The use of under-utilized, farmer-available organic resources as a means of providing nutrients to the crop and improving fertilizer use efficiency warrants greater attention from agricultural scientists Studies of organic matter dynamics over time must be initiated as a means to develop soil conservation measures that reduce carbon loss from soils. These studies must be further consolidated into improved management practices that are acceptable to farmers. Nutrient retention results from ionic attraction between plant nutrients in solution and the charged surfaces within the soil. These charged surfaces occur in both the mineral and organic fractions, although in highly weathered tropical soils there is a greater dependence on the role of organic matter because of a decreased charge density of mineral oxides. An important factor in the continuous productivity of tropical soils is the maintenance and improvement of soil physical characteristics. Once this is achieved, the production capacity of these soils can be further improved by the use of organic and inorganic fertilizers. Soil toxicity Toxic levels of aluminium and other cations, particularly iron and magnesium, occur in highly weathered, low pH soils that are dominated by oxide mineralogy. High concentrations of these cations interfere with nutrient uptake and, when entering the plant, interfere with sugar phosphorylation and DNA synthesis. One common symptom of cation toxicity is the development ofswollen, stubby roots resulting from an inhibition of root elongation. The situation can be improved by applying lime, although this is frequently unavailable to many sub sistence and local market farmers in the humid tropics. Soil organic matter interacts with toxic cations in two different ways, both of which are beneficial to plants. Humic substances absorb toxic cations resulting in their immobiligation and detoxification. Organic acid from decomposing residues in soils also interact with aluminium in solution, resulting in less toxic forms of aluminium in the soil solution, again without changing the soil pH. While management may be technically feasible at the small farm level, larg application rates of organic residues a required to detoxify acid soils. Unfortunately, the relative effectiveness of most available organic resources in Africa remains unknown. However, relatively simple plant bioassays are available to measure the toxicity of test soils by comparing the root elongation of recently germinated mung bean seedlings. Soil moisture Moisture retention is both an intrinsic property of soils and one that is subject to management practices. The pores of sandy soils are emptied of gravitational water at 0.1 bar, while silicate layered clayey soils retain this moisture until 0.5 bar. Other soils, including most of those in the tropics, fall somewhere inbetween. Farmers manage moisture retention in many ways, the most obvious and important one being the reduction of water run-off along the soil surface by terracing, contour ridges and other more elaborate water-capture strategies. Added benefits to run-off reduction are improved control of nutrient losses and lower soil erodibility. A key to reduced run-off and its consequent benefits is the protection of the soil surface with mulch. The depth of rooting is often an overlooked factor affecting the moisture availability and one that need not be universally associated with shallow soils. The ability of plant roots to extract moisture reserves from deeper soil layers may be inhibited by the inability of plant roots to penetrate to that depth by physical and chemical barriers. Little can be done to improve rooting depth in extremely rocky soils. Hardpans can develop in clayey soil immediately beneath the shallow tillage layer, due to compaction from mechanical tillage. The formation of hardpans may be disrupted by occasional deep tillage. Acidic subsoils may limit a rooting system's ability to recover moisture reserves. This is a common limitation in oxisols and ultisols, where moisture in the well-structured surface horizon is depleted but abundant moisture in the acidic subsoil remains unexploited. This problem has various solutions ranging from deep tillage and liming to the use of more acid-tolerant, tap-rooted crops and cultivars. In many cases agricultural researchers and farmers are unaware of rooting limitations. This is an area which requires further research. Irrigation projects Fresh-water resources are under-utilized in many areas where the length of the cropping system is limited by the precipitation pattern and in areas that suffer from periodic droughts but which are of otherwise excellent potential for intensified agriculture in terms of soil fertility and radiation receipts. Additional benefits to the development of irrigation schemes are the opportunities for energy generation and rural electrification. Two hazards to the development of large scale water conservation and irrigation projects are siltation of reservoirs and salinization of irrigated lands over time, often with horrific environmental effects. While national scientists and the private sector are seeking solutions to these problems, a possible alternative is the support of smaller-scale, farm community managed projects. Soil erosion Erosion results in the degradation of soil physical characteristics such as infiltration rate, soil structure and crusting. The physical removal of surface soil is serious for shallow, gravelly soils, but more serious are the resulting effects of poor soil physical conditions on crop production. Soil erosion also decreases the fertilizer use efficiency by increasing the nutrient losses. Soil erosion is seldom a direct constraint to an individual cropping cycle but rather a chronic depletion of the soil as a resource base. The principal causes of soil erosion are deforestation, overgrazing, shortened cycles of shifting cultivation and the cultivation of slopes without any form of conservation. The protection afforded by natural vegetation from erosion includes protection from direct impact of rainfall by the canopy, presence of a surface litter layer to further buffer droplet impact and to impede runoff and abundant rooting within the soil surface horizon which stablizes soils. The importance of carbon cycling within an ecosystem must never be overlooked. As surface and root litter decays, organic ma:erials assist in the formation and stabilization of soil aggregates which resist erosive forces. Soil structure and the distribution of macropores are further facilitated by increased rooting and macrofaunal activity. The resilience of natural ecosystems versus improperly managed agroecosystems shows the advantages of combining plant species of different architecture and chemical composition. Soil erosion by water poses a serious threat to the productive capacity of agriculture to keep pace with, and exceed, population growth. Steep, non-terraced hill-sides are the most susceptible to erosion but, given the frequency and intensity of precipitation in semi-arid and sub-humid areas, even moderate slopes are often in jeopardy. Too often bench terraces, grass strips, hedgerows, windbreaks, mulches and contour ditches are not only under-utilized but are even unknown within the cropping systems. One difficulty in getting farmers to appreciate the importance of soil conservation is that the return on labour and investments are not immediately evident and too often are beyond the planning horizon. It is only when land has become severely degraded that those farmers who did not practice soil conservation become aware of the consequences by comparison with those farmers who did. Agricultural resource scientists and planners must better familiarize themselves with the labour requirement and capital expense of candidate soil conservation practices, and must prepare farmer information packages which recommend socially acceptable and technically feasible options to the farmer. Soil survey and land use planning Susceptibility to deterioration is a very important soil quality and one that needs to be considered early in development planning. Erosion is the most obvious and serious hazard resulting from agricultural or other vegetation-based land uses. Susceptibility to erosion of soil types occurring in an area can be derived from soil survey information. Soil surveys also provide information on hazards restricted to specific soils, for example the presence of limiting layers for root development such as hardpans or salt and sodium accumulation in soils irrigated without adequate drainage, or subsidence of organic soils. From soil surveys interpretations can be made with regard to the suitability of soils for management operations such as tillage either by hand labour, animal traction or motorised machinery. Information used for such evaluations, which is collected during soil surveys, includes slope, soil depth, rock outcrops, surface stoniness, drainage condition and soil consistency. Soil surveys also make an essential contribution to the development of the interpretative classification of land suitability for irrigation. Knowledge of the characteristics of individual soils is essential for planning the economic use of water. It provides a basis for ensuring that development is concentrated on the most suitable soils available and for determining the method and quantity of water application that will achieve optimum efficiency. It is important also that water application should be planned in relation to data on soils, particularly subsoil characteristics, if the hazards of salinization and water-logging are to be avoided. Soil surveys also give information on major limitations of land use. Some of the limitations may be of a permanent nature: that is, they cannot be changed by management. Such factors include soil depths, texture and slope. Others may be of a temporary nature and can be corrected by management, for example soil fertility. To facilitate storage and retrieval of soil survey information for planning and resource management, the Kenya Soil Survey has established a Geographical Information System. This is also being used for digitizing soil maps and in the preparation of interpretation maps for various land use alternatives. Sustenance of land productivity calls for careful planning for the utilization and management of soil and water resources. To prepare the most rational plans, land evaluation must be called into play. It involves the execution and interpretation of basic surveys of soils, climate and vegetation and other aspects of land and its productivity in terms of the requirements of alternative forms of land use. The end result of land evaluation is the identification of a number of appropriate types of land use alternatives together with their consequences on implementation. Where the most appropriate options in Africa have been properly identified through land evaluation, there is no reason to fear that the consequences of intensification will cause irreversible damage to the soil resource. And for certain parts of Africa, the time to intensify agricultural production is now. For further reading: The importance and management of soil organic matter in the tropics by P L Woomer, A Martin, A Albrecht, D V S Resck and H W Scharpenseel, to be published in 1994 by J Wiley and Sons, Chichester UK.