THE AFRICAN NETWORK FOR SOIL BIOLOGY AND FERTILITY: NEW CHALLENGES AND OPPORTUNITIES
Soil fertility degradation has been described as the single most important constraint on food security in SSA. Soil fertility decline is not just a problem of nutrient deficiency but also of: (i) inappropriate germplasm and cropping system design; (ii) interactions with pests and diseases; (iii) the linkage between poverty and land degradation; (iv) often perverse national and global policies with respect to incentives; and (v) institutional failures. Therefore, tackling soil fertility issues requires a long-term perspective and a holistic approach. The African Network for Soil Biology and Fertility (AfNet) of the Tropical Soil Biology and Fertility Institute of the CIAT is devoted to overcoming this challenge. AfNet’s ultimate goal is to strengthen and sustain stakeholder capacity to generate, share and apply soil fertility management knowledge and skills to contribute to the welfare of farming communities. This Africa-wide network has over 200 members from national agricultural research and extension services and universities from various disciplines, mainly soil sciences, social sciences and technology exchange. This paper highlights AfNet’s main activities, which include: network field research activities, information and documentation, training and capacity building.
Bationo, A., Kimetu, J., Ikerra, S., Kimani, S., Mugendi,
D., Odendo, M.,
Silver, M., Swift, M.J. & Sanginga,
N.
ASSESSMENT AND MONITORING OF NUTRIENT FLOWS AND STOCKS TO DETERMINE APPROPRIATE NUTRIENT MANAGEMENT STRATEGIES FOR ARID AND SEMI-ARID LANDS
The problem is the migration of population from high to low potential areas, which results in increased pressure on land, with erratic rainfall, continued cultivation/ land degradation, increasing soil fertility problems and depressed crop yields. The main objective of the project is to identify in close cooperation with the farmers the major constraints faced by small-scale farm households in the arid and semi-arid lands of Kenya, with special emphasis on nutrient balances. A second objective is to identify, through participatory design and testing, alternative production techniques that will alleviate constraints and contribute to the implementation of more sustainable agricultural production systems. The project is implemented in six villages with different farming systems. The NUTMON approach to inventory and monitoring is implemented twice per season (diagnostic phase). The results (usually nutrient balances, economic indicators, etc.) of this phase are taken back to the farmers for evaluation and designing (implemented using PLAR approach) options to solve the identified soil fertility constraints.
Gachimbi, L.N. & De Jager, A.
SOIL NUTRIENT BALANCES BY CLASS OF FARM IN SOUTHERN MALI
The mining of nutrients in SSA is one of the main causes of yield stagnation and decline. In order to improve the sustainability of agricultural production, several nutrient balances were performed in the southern Mali. These balances provide valuable information of a general order. Nevertheless, little information exists concerning the impact of the techniques used by different categories of farmers on the fertility of their soils. This study was carried out in two villages (Noyaradougou and M’Peresso) and for three classes (categories) of farms ranging from Class I (good management of soil fertility) to Class III (poor management). The study intends to analyse the management of nutrient flows by different categories in both villages and to capture the major evolutionary tendencies in a period of 3-4 years.
In both villages and for all classes, partial balances of NPK (more visible and easily measurable components of nutrient balances) are positive for cotton in contrast to millet and sorghum. The same balance for maize depends largely on production strategy and improves from Class I to Class III in both villages. Depending on the village, the partial farm balances for N oscillate between -2.5 and 12 kg/ha/year. The P balance is positive for all classes and both villages and varies between 2.9 and 6.6 kg/ha/year, while that for K is usually negative. The partial balances at farm level have improved over the years thanks to higher input levels in 1997 as compared to previous years, and to lower crop and residue yields in all classes, except for Class I in Noyaradougou. The complete nutrient balance takes ‘easy to measure’ (elements of partial balances) and ‘difficult to measure’ flows into account. The complete N and K balances are negative. The K deficit is less marked in Noyaradougou, where the cropping system is based largely on cotton-maize, than in M’Peresso, where millet and sorghum occupy 48 percent of the rotation systems with cotton. The complete P balance is positive in all cases.
Kanté, S., Smaling, E.M.A. & Van Keulen, H.
EXPERIENCES WORKING WITH RURAL COMMUNITIES TO ACHIEVE IMPROVED SOIL NUTRIENT BALANCES AND ORGANIC MATTER
Trials and demonstrations on farmers’ fields have shown that proper fertilization and other technologies such as conservation tillage can increase crop yields by large margins in much of SSA. The challenge is now to improve the availability and accessibility of fertilizers and other technologies to farming communities. Benefits from improvements in input distribution would be greater if coupled with crop management systems that promote fertilizer-use efficiency. Declines in soil organic matter lead to soil degradation, resulting in weak fertilizer responses and so eroding profitability. Over the years, the African Centre for Fertilizer Development has developed programmes that place special emphasis on: facilitation of fertilizer supply to communities; and promotion of ISFM involving the use of chemical fertilizers, organic materials and other yield. The results from work with rural communities indicate that the facilitation of input supply and proper farm management practices can improve nutrient balances and soil organic-matter levels considerably.
Muchena, S.C.
EFFECT OF COCOA PRODUCTION ON THE SOIL NUTRIENT BALANCE IN GHANA
In Ghana, cocoa is produced by small-scale farmers with a low level of management. These farmers do not use fertilizers. Thus, production has tended to decline over the years. The main gains in farm nutrients are through the process of cocoa-leaflitter decomposition, which adds 119 kg/ha N, 8 kg/ha P and 22 kg/ha K to the soil each year. The main losses of nutrients from the farm are through harvesting. Based on the average annual dry cocoa yield of 300 kg/ha, the annual amounts of major nutrients removed from the soil through harvesting of the pods are 7.5 kg/ ha N, 1.2 kg/ha P, 4.4 kg/ha K for the beans and 4.2 kg/ha N, 0.5 kg/ha P and 12.1 kg/ha K for the husks. This loss, when estimated for an annual marketable harvest of 400 000 tonnes of dry cocoa beans, is equivalent to 10 000, 1 640 and 5 840 tonnes of N, P and K respectively, which represents about 6 percent of the quantity of nutrients (NPK) returned to the soil through the litter. The non-use of fertilizer to replenish the nutrients removed through harvesting over long periods could be one of the causes of the declining cocoa production in Ghana.
Ofori-Frimpong, K., Afrifa, A.A. & Appiah, M.R.
A HIERARCHICAL METHOD FOR SOIL EROSION ASSESSMENT AND SPATIAL RISK MODELLING
The Ph.D. thesis titled “A hierarchical method for soil erosion assessment and spatial risk modelling: a case study of Kiambu District by Peter F. Okoth” has devised a new method for assessing soil erosion for three levels of a landscape system. The method used in the thesis identified farmers as the beneficiaries of the lowest field-plot level assessment, a group of farmers as the beneficiaries of the second level, and government agencies as the beneficiaries of the highest level, the landscape unit. Spatial attributes considered important for the farmer to manage include: crop selection, ground cover, and the placement of the crop in particular positions in the landscape. Footpaths and field-plot boundaries need to be managed in watersheds in order to curb or control the problem of soil erosion. Roads, built-up areas, school compounds, river valleys and stream banks need to be managed at the highest landscape unit level. The method proposes an interdisciplinary systems approach in soil erosion risk management involving interventions, policies, resources, landscape properties, processes, time, energy and space for each level to achieve sustainability.
Okoth, P.F.
WHY SOIL NUTRIENT BALANCES? SOME ALTERNATIVES
This paper challenges the objectives of nutrient balances, reviews their limitations, and suggests two alternative approaches for evaluating and measuring impacts of farming practices on soil productivity. The first approach distinguishes nutrient supply from: (i) current inputs (organic resources and soluble inorganic fertilizers), which depend on current management; and (ii) stored nutrients (slow turnover pool of soil organic matter and sorbed inorganic pools), which depend on past management. Current nutrient supply is a sensitive indicator of the effects of current management on soil productivity in the short term (1-2 seasons). Stored nutrient supply is an indicator of soil buffering and sustainability in nutrient supply over the long term (20 years).
The principal input data required to estimate these indices are: (i) dry matter and nutrient contents of organic-litter inputs to the soil; (ii) amounts of inorganic fertilizer inputs; and (iii) organic carbon content of the topsoil. The framework is suitable, and more informative than nutrient budgets, for gauging potential impacts of alternative farming practices on shortand long-term soil productivity. However, like nutrient budgets, the input data is difficult to collect in systematic surveys and over large areas, and the model has several assumptions that are difficult to validate.
The second approach recognizes that soil ability to supply nutrients is only one dimension of soil quality. It fills a need for integrated indicators of soil quality that capture other dimensions of productive capacity and ecosystem function, such as ability to infiltrate and store water and resist erosion. Such indicators need to be easily measurable to permit monitoring of actual impacts of alternative farming practices on soil quality. To this end, the paper proposes and demonstrates the use of visible near-infrared reflectance (NIR) spectroscopy for deriving integrated indicators of soil quality that relate directly to plant productivity and soil enrichment-depletion processes (e.g. organic inputs, erosion). These are slow, ecological indicators that are responsive to natural variation and long-term management effects but not noise from short-term effects of current management and environmental conditions. This non-destructive technique allows large numbers of soil samples to be characterized rapidly (2 000 samples/week). The paper demonstrates the use of the technique, in conjunction with stable isotope determinations, to assess effects of historic land use change and management effects on soil quality. It shows how the use of these spectral indicators in conjunction with statistical techniques allow: (i) confounding effects of spatial correlation and environmental noise to be separated from management effects; and (ii) uncertainties in soil quality indicators to be quantified at different levels of aggregation. Geo-referenced observations of the spectral quality index can also be spatially interpolated over large areas (> 103 km2) using satellite imagery. The paper also shows the use of NIR spectroscopy for rapid assessment of organic resource quality.
Shepherd, K.D. & Walsh, M.G.
FUNCTIONING OF SOIL FERTILITY GRADIENTS AT THE FARM LEVEL AND IMPLICATIONS FOR INTEGRATED SOIL FERTILITY MANAGEMENT
Most agricultural research and development organizations accept the ISFM paradigm as the paradigm for developing and disseminating improved soil management options. Technically, the ISFM paradigm stands for the combined utilization of organic and mineral inputs combined with resilient germplasm. Although nutrient balances in SSA have been shown to be negative at national and regional scales, when zooming in at the farm scale, large differences in nutrient balance can usually be observed between fields, some of the latter showing positive balances. This is driven mainly by concentration of available resources, both organic and mineral, on fields near the homestead. Such management practices, when implemented over long periods, result in substantial differences in soil fertility status between fields within a farm. Most of the time, these differences take the form of gradients, decreasing with distance from the homestead. While the existence of soil fertility gradients has been observed in most areas in SSA, little is known on how these gradients affect processes underlying ISFM practices, such as fertilizer-use efficiency or organic-resource decomposition. Several hypotheses have been formulated in efforts to unravel relationships between the soil fertility status of individual fields and the functioning of ISFM practices but little information is available to test these hypotheses. This presentation presents a strategy to determine soil fertility gradients, explore the links with farmers’ appreciation of soil fertility status, and present preliminary evidence in relation to above-mentioned hypotheses. Obtaining this information would enable judgements on whether recommendations for input use need to be refined by taking into account soil fertility gradients existing at the farm level.
Vanlauwe, B.
THE SOIL-MASS BASIS FOR ASSESSING CHANGES IN SOIL PROPERTIES
High-resolution data are necessary to assess accurately changes induced by integrated natural resource management (INRM) strategies, which are often 20 percent or less. These changes are detectable, given the correct statistical design. However, systematic errors (as opposed to random error) introduced in soil sampling methods and laboratory analysis generate data that are either always greater than or less than the actual sample mean. This has resulted in mis-assessment of trial results, including failure to detect actual changes and indications of non-existent changes. The erratic and often non-logical data caused by systematic error have led many soil scientists to believe that changes in soil properties cannot be detected reliably or quantified. As a result of both inaccurate methods and scientist disinterest, changes in soil properties resulting from INRM strategies are rarely assessed accurately. This slows the identification of successful interventions to farmers. This presentation examines the most widespread cause of systematic error in soil sampling: sampling a soil to a given depth increment and assessing changes in that increment. Even minor changes in bulk density, which commonly occur over the course of a trial owing to natural processes or to INRM interventions, change the mass of a soil being sampled in a given depth increment. Where the soil is compacted during the course of a trial, this always results in an overestimation of nutrient stocks in a given depth increment, whereas where the soil is de-compacted, an underestimation occurs. Errors of 10-15 percent are not uncommon. Similar systematic errors can be introduced in the laboratory analysis. When combined with soil sampling errors, such errors generate misleading data and erroneous conclusions. Soil-mass sampling eliminates sampling errors caused by depth sampling, but requires methods that are not commonly employed. Ellert and Bettany (1995) concluded that “recent publications indicate a serious and persistent lack of awareness about the influence of soil mass on estimates of nutrient storage” - a conclusion that applies equally to nutrient-availability indices or soil-quality measures. Adoption of soil-mass sampling procedures, along with minor modifications to routine laboratory procedures, can virtually eliminate systematic error, so accelerating the identification of promising INRM interventions.
Wendt, J.
