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Maintenance of soil organic matter and N supply in the moist savanna zones of West Africa
... The enhanced friability also gave better seedbeds under direct drill. Using results from 34 farmer trials, Carsky et al. (1998) showed a diminishing yield response to fertilisers as soil organic carbon increased from 0.5% to 2.0%; this confirmed the capacity of soil organic matter to act as a nutrient supply. ...
This report examines sustainable land management (SLM) and its potential as an integrative strategy to address multiple environmental and sustainable development objectives. It highlights the linkages between SLM and soil health, land degradation, food security, climate changes mitigation and adaptation. The report is intended to provide information and guidance on fostering SLM, to a wide range of stakeholders involved in agriculture, environmental management and sustainable development. It aims to support investment in SLM by the Global Environment Facility (GEF), particularly investments in pursuit of Land Degradation Neutrality. The report explores the anthropogenic and natural drivers of land degradation, and the potential environmental and socioeconomic benefits of SLM; examines the role of SLM in addressing the critical challenge of global food security; describes the key processes of land degradation and their impacts, as the basis for developing good practice guidance on SLM that is scientifically sound and robust;
proposes principles for SLM that promote soil health, productivity and ecosystem services; presents a framework for identifying SLM practices suited to the context and objectives; provides guidance on identifying indicators for evaluation of a site in terms of land potential and soil condition, and indicators for monitoring outcomes of SLM investments; discusses the barriers to adoption of good practice for SLM; and provides recommendations for developing and implementing SLM programs in ways that optimise global environmental benefits.
Recommendations are provided to guide investment in support of SLM, and planning of SLM programs.
Also available at http://www.stapgef.org/sustainable-land-management-environmental-benefits-and-food-security-synthesis-report-gef
... Intensive work conducted over the past decade in smallholder agricultural systems in sub-Saharan Africa has demonstrated the existence of within-farm variability of soil fertility and related soil properties (Prudencio 1993;Carsky et al. 1998;Tittonell et al. 2005aTittonell et al. , b, 2013. A common feature of these farming systems is the existence of strong gradients of decreasing soil fertility with increasing distance from the homestead, which mainly occur as a result of differential resource allocation driven by the farmer. ...
This book provides standards and guidelines for quantifying greenhouse gas emissions and removals in smallholder agricultural systems and comparing options for climate change mitigation based on emission reductions and livelihood trade-offs. Globally, agriculture is directly responsible for about 11% of annual greenhouse gas (GHG) emissions and induces an additional 17% through land use change, mostly in developing countries. Farms in the developing countries of sub-Saharan Africa and Asia are predominately managed by smallholders, with 80% of land holdings smaller than ten hectares. However, little to no information exists on greenhouse gas emissions and mitigation potentials in smallholder agriculture. Greenhouse gas measurements in agriculture are expensive, time consuming, and error prone, challenges only exacerbated by the heterogeneity of smallholder systems and landscapes. Concerns over methodological rigor, measurement costs, and the diversity of approaches, coupled with the demand for robust information suggest it is germane for the scientific community to establish standards of measurements for quantifying GHG emissions from smallholder agriculture. Standard guidelines for use by scientists, development organizations will help generate reliable data on emissions baselines and allow rigorous comparisons of mitigation options. The guidelines described in this book, developed by the CGIAR Research Program on Climate Change, Agriculture, and Food Security (CCAFS) and partners, are intended to inform anyone conducting field measurements of agricultural greenhouse gas sources and sinks, especially to develop IPCC Tier 2 emission factors or to compare mitigation options in smallholder systems.
... Intensive work conducted over the past decade in smallholder agricultural systems in sub-Saharan Africa has demonstrated the existence of within-farm variability of soil fertility and related soil properties (Prudencio 1993;Carsky et al. 1998;Tittonell et al. 2005aTittonell et al. , b, 2013. A common feature of these farming systems is the existence of strong gradients of decreasing soil fertility with increasing distance from the homestead, which mainly occur as a result of differential resource allocation driven by the farmer. ...
Smallholder agricultural systems in tropical and subtropical regions may have significantly contributed to greenhouse gas (GHG) emissions over the past number of decades. As a result, these systems currently offer large GHG mitigation potentials (e.g., soil organic carbon (SOC) sequestration), which can be realized through the implementation of good management and sustainable agricultural practices. In this chapter we synthesize current available methodologies designed to assess SOC stocks and stock changes. From this analysis, it becomes apparent that the design and subsequent implementation of any quantification and monitoring scheme envisaged for studies focusing solely on the soil component greatly differs from those developed for whole ecosystem accounting, not just in its approach, but also in the amount of resources needed to implement it within a given degree of accuracy. We provide analyses and recommendations on methods specifically dealing with quantification and assessment of SOC at both the individual farm and the landscape scale in smallholder agricultural systems.
... Generally, on responsive soils, where the applied fertilizer nutrients overcome crop nutrient limitations, substantial responses to fertilizer can be expected . On soils where other constraints are limiting crop growth (lessresponsive soils), fertilizers alone in absence of other corrective measures results in relatively low agronomic effi ciencies and small improvement in crop yield (Carsky et al., 1998;Zingore et al., 2007). Also important is the heterogeneity that exists between households within a community, translated in diff ering production objectives and resource endowments (Tittonell et al., 2005;Giller et al., 2006). ...
... The consistent nutrient 'mining' of 'outfields' results in nutrient deficient soils and crops (such as maize and beans that are grown in these sites) that are characteristic of most small-scale farms in Central Kenya (Murage et al., 2000). Carsky et al. (1998) and Vanlauwe et al. (2002) found that fields close to the homestead had better soil fertility characteristics than fields that were remotely placed and far from the homestead. Additionally, Vanlauwe et al. (2002) found that maize yield was significantly related to initial total soil nitrogen in Western Kenya, showing the effect of soil fertility gradients on crop productivity. ...
The following study was conducted to determine smallholders' land use management practices and agricultural indicators of soil quality within farmers' fields in Chuka and Gachoka divisions in Kenya's Central Highlands. Data on cropping practices and soil indicators were collected from farmers through face-to-face interviews and field examinations. Farmers characterised their fields into high and low fertility plots, after which soils were geo-referenced and sampled at surface depth (0–20 cm) for subsequent physical and chemical analyses. Farmers' indicators for distinguishing productive and non-productive fields included crop yield, crop performance and weed species. Soils that were characterised as fertile, had significantly higher chemical characteristics than the fields that were of poor quality. Fertile soils had significantly higher pH, total organic carbon, exchangeable cations and available nitrogen. Factor analysis identified four main factors that explained 76 per cent of the total variance in soil quality. The factors were connected with farmers' soil assessment indicators and main soil processes that influenced soil quality in Central Kenya. Soil fertility and crop management practices that were investigated indicated that farmers understood and consequently utilised spatial heterogeneity and temporal variability in soil quality status within their farms to maintain and enhance agricultural productivity. Copyright © 2007 John Wiley & Sons, Ltd.
... As highlighted above, the soil fertility status of the different production units in one farm is not equal and this is likely going to influence crop production and use efficiency of applied N fertilizer. Carsky et al. (1998a,b) have reported a clear positive relationship between the soil organic C content and unfertilized maize yields for a number of sites in Northern Nigeria. An interesting research issue is whether the returns to N fertilizer application are higher on soils with a high soil fertility status, such as the fields near the homestead, compared with soils with lower soil fertility status. ...
The aim of this paper is to demystify some of the popular myths related to tropical soil fertility management that have gained hold in the development community and are often being promulgated by NGO's and development agencies in the tropics. Negative nutrient balances at farm scale or at larger scales are very often presented as proof that soil fertility is at stake in SSA. However, nutrient balances at plot and farm section scale are not always negative. In areas with large nutrient stocks, short-term nutrient mining is fully acceptable. Fertilizer use continues to face considerable controversy in SSA. In this paper, we demonstrate that fertilizers rarely damage the soil; that fertilizers are being used in SSA, often with favourable value-to-cost ratios; and that fertilizers do not cause eutrophication in SSA. Rock phosphates are abundantly present in SSA but most are poorly soluble. Adding these phosphates to compost heaps does not enhance the short-term availability of their P. Although organic inputs are essential soil amendments besides fertilizer, organic inputs alone cannot sustain crop production due to limitations in their quality and availability. Organic resources can also potentially stimulate harmful pests and diseases. Legumes are often advocated as important sources of organic matter but not all legumes fix nitrogen, require inoculation, or are a source of free nitrogen, as even green manures require land and labour. Certain grain legumes with high N harvest indices do not improve soil fertility, but remove net amounts of N from the soil. These myths need correction if we are to harness the role of science in the overall goal of assisting farmers to address the acute problems of poor soil fertility for smallholder farmers in SSA.
... Dembélé et al. (2000) for Mali; Krogh (1997) and Kiema (1999) for Burkina Faso). Long-standing differences in management also lead to contrasting soil nutrient and organic matter status and to corresponding differences in crop yields (Manlay et al., 2001; Hilhorst et al., 2000; Carsky et al., 1998). As recognised by Defoer et al. (2000), systems with concentric rings of fertility represent a conceptual ideal for geometrically minded planners and modellers rarely found in the field. ...
Nutrient balances aggregated at the continental, national, or regional levels for African farming systems are usually reported as strongly negative. At the landscape or farm scale, the most commonly reported variability is the gradient of decreasing soil fertility from intensively managed “home” fields to more extensively managed “bush” fields. Case study evidence from an agro-pastoral community of southern Mali’s cotton zone showed that “home” and “bush” fields differed significantly in nutrient balances and soil fertility status but that inter-household differences related to household practice and social factors were even more important.
Soil carbon in the form of organic matter is a key component of the soil ecosystem structure. In most parts of West Africa agro-ecosystems (except the forest zone), the soils are inherently low in SOC content due to low organic matter additions, and accelerated degradation. The rapid turnover rates of organic material is as a result of high soil temperatures and fauna activity particularly termites. The SOC levels rapidly decline with continuous cultivation. For the sandy soils, average annual losses may be as high as 4.7% whereas with sandy loam soils, losses are lower, with an average of 2.0%. To maintain food production for a rapidly growing population, application of mineral fertilizers and the effective recycling of organic amendments such as crop residues and manures are essential especially in the smallholder farming systems that rely predominantly on organic residues to maintain soil fertility. The efficiency of fertilizer use is likely to be high where the organic matter content of the soil is also high. In unhealthy or depleted soils, crops use fertilizer supplied nutrients inefficiently. Where soils are highly degraded, crops hardly respond to fertilizer applications. When SOM levels are restored, fertilizer can help maintain the revolving fund of nutrients in the soil by increasing crop yields and, consequently, the amount of residues returned to the soil. Crop yields can be increased by 20–70 kg ha⁻¹ for wheat, 10–50 kg ha⁻¹ for rice, and 30–300 kg ha⁻¹ for maize with every 1 Mg ha⁻¹ increase in soil organic carbon pool in the root zone. There is need to increase crop biomass at farm level and future research should therefore focus on improvement of nutrient use efficiency in order to increase crop biomass.
In sub-Saharan Africa (SSA), traditional farming methods have led to severe nutrient depletion, low crop yields, and poverty, leaving many farm families disappointed. The situation is aggravated by low use of fertilizers by farmers, lack of policy and institutional support, weak fertilizer markets, farmers’ lack of access to credit and inputs, inappropriate fertilizer packaging sizes, deteriorating soil science capacity, and weak agricultural extension. This leads to a huge gap between the actual and potential crop yields and widespread poverty. Opportunities, options, and innovations are available to turn around this situation and reduce poverty in Africa. We reviewed the status of agricultural soils of sub-Saharan Africa, the factors contributing to their current status, and the existing and emerging opportunities for addressing the soil fertility challenges. The major points are (1) the soils in SSA have continued to be characterized by huge and widespread negative nutrient imbalances and low productivity, and (2) the factors contributing to declining soil fertility in Africa are not abating. The way forward includes: balanced fertilization; input–output market development; improved crop management, and the use of nutrient budgets to inform policy and ensure that farmers and other land managers enjoy the profitable use of farm inputs including mineral fertilizers and agro-minerals.
The principal objections to the proposition that organic agriculture can contribute significantly to the global food supply are low yields and insufficient quantities of organically acceptable fertilizers. We evaluated the universality of both claims. For the first claim, we compared yields of organic versus conventional or low-intensive food production for a global dataset of 293 examples and estimated the average yield ratio (organic:non-organic) of different food categories for the developed and the developing world. For most food categories, the average yield ratio was slightly <1.0 for studies in the developed world and >1.0 for studies in the developing world. With the average yield ratios, we modeled the global food supply that could be grown organically on the current agricultural land base. Model estimates indicate that organic methods could produce enough food on a global per capita basis to sustain the current human population, and potentially an even larger population, without increasing the agricultural land base. We also evaluated the amount of nitrogen potentially available from fixation by leguminous cover crops used as fertilizer. Data from temperate and tropical agroecosystems suggest that leguminous cover crops could fix enough nitrogen to replace the amount of synthetic fertilizer currently in use. These results indicate that organic agriculture has the potential to contribute quite substantially to the global food supply, while reducing the detrimental environmental impacts of conventional agriculture. Evaluation and review of this paper have raised important issues about crop rotations under organic versus conventional agriculture and the reliability of grey-literature sources. An ongoing dialogue on these subjects can be found in the Forum editorial of this issue.
Different fields within a farm have been observed to have different soil fertility status and this may affect the response of a maize crop to applied N, P, and K fertiliser. A limiting nutrient trial was carried out at six farms each, in three districts of Western Kenya. In each of the farms, the following treatments were laid out in three fields with different soil fertility status at different distances from the homestead (close, mid-distance, remote fields): no inputs, application of NPK, NP, NK, or PK fertiliser (urea, triple super phosphate, KCl) to maize. Total soil N decreased at all sites with distance to the homestead (from 1.30 to 1.06 g kg-1), as did Olsen-P (from 10.5 to 2.3 mg kg-1). Grain yields in the no-input control plots reflected this decrease in soil fertility status with distance to the homestead (from 2.59 to 1.59 t ha-1). In the NPK treatments, however, this difference between field types disappeared (from 3.43 to 3.98 t ha-1), indicating that N and P are the major limiting nutrients in the target areas. Response to applied N was related to the soil total N content in Aludeka and Shinyalu, but not in Emuhaia, probably related to the high use of partially decomposed organic inputs with limited N availability. Consequently, response to applied N decreased with distance to the homestead in Aludeka (from 0.95 kg kg-1 relative yield to 0.55 kg kg-1) and Shinyalu (from 0.76 kg kg-1 to 0.47 kg kg-1), but not in Emuhaia (from 0.75 kg kg-1 to 0.68 kg kg-1). Response to applied P was related to the soil Olsen-P content at all sites. While for farms with a relatively high Olsen-P gradient, response to applied P decreased with distance to the homestead (from 0.99 kg kg-1 to 0.68 kg kg-1), large variability in Olsen-P gradients across field types among farms within a specific site often masked clear differences in response to P between field types for a specific site. Clear scope for field-specific fertiliser recommendations exists, provided these are based on local soil knowledge and diagnosis. Scenario analysis, using farm-scale modelling tools, could assist in determining optimum allocation strategies of scarcely available fertiliser for maximum fertiliser use efficiency
Soil fertility depletion has been described as the single most important constraint to food security in West Africa. Over half of the African population is rural and directly dependent on locally grown crops. Further, 28% of the population is chronically hungry and over half of people are living on less than US$ 1 per day as a result of soil fertility depletion.Soil organic carbon (SOC) is simultaneously a source and sink for nutrients and plays a vital role in soil fertility maintenance. In most parts of West Africa agro-ecosystems (except the forest zone), the soils are inherently low in SOC. The low SOC content is due to the low shoot and root growth of crops and natural vegetation, the rapid turnover rates of organic material as a result of high soil temperatures and fauna activity particularly termites and the low soil clay content. With kaolinite as the main clay type, the cation exchange capacity of the soils in this region, often less that 1 cmol kg−1, depends heavily on the SOC. There is a rapid decline of SOC levels with continuous cultivation. For the sandy soils, average annual losses may be as high as 4.7% whereas with sandy loam soils, losses are lower, with an average of 2%. To maintain food production for a rapidly growing population, application of mineral fertilizers and the effective recycling of organic amendments such as crop residues and manures are essential especially in the smallholder farming systems that rely predominantly on organic residues to maintain soil fertility. There is need to increase crop biomass at farm level and future research should focus on improvement of nutrient use efficiency in order to increase crop biomass. Research should also focus on ways of alleviating socio-economic constraints in order to increase the legume component in the cropping systems. This will produce higher quality fodder for the livestock and also increase biomass at farm-level. This paper reviews various strategies and lessons learnt in improving soil organic carbon status in West Africa soils.
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