Boaz S. Waswa

Advances in integrated soil fertility management in sub Saharan Africa: challenges and opportunities

Since the 1970s, research throughout West Africa showed that low soil organic matter and limited availability of plant nutrients, in particular phosphorus and nitrogen, are major bottlenecks to agricultural productivity, which is further hampered by substantial topsoil losses through wind and water erosion. A few widely recognized publications pointing to massive nutrient mining of the existing crop–livestock production systems triggered numerous studies on a wide array of management strategies and policies suited to improve soil fertility. Throughout Sudano-Sahelian West Africa, the application of crop residue mulch, animal manure, rockphosphates and soluble mineral fertilizers have been shown to enhance crop yields, whereby yield increases varied with the agro-ecological setting and the rates of amendments applied. In more humid areas of Western Africa, the intercropping of cereals with herbaceous or ligneous leguminous species, the installation of fodder banks for increased livestock and manure production, and composting of organic material also proved beneficial to crop production. However, there is evidence that the low adoption of improved management strategies and the lack of long-term investments in soil fertility can be ascribed to low product prices for agricultural commodities, immediate cash needs, risk aversion and labour shortage of small-scale farmers across the region. The wealth of knowledge gathered during several decades of on-station and on-farm experimentation calls for an integration of these data into a database to serve as input variables for models geared towards ex-ante assessment of the suitability of technologies and policies at the scale of farms, communities and regions. Several modelling approaches exist that can be exploited in this sense. Yet, they have to be improved in their ability to account for agro-ecological and socio-economic differences at various geographical scales and for residual effects of management options, thereby allowing scenario analysis and guiding further fundamental and participatory research, extension and political counselling. Soil fertility – the perpetual issue Owing greatly to the two major Sahelian droughts in the early 1970s and 1980s, the poor productivity of agropastoral systems in Sudano-Sahelian West Africa (SSWA) has raised worldwide concern and subsequently stimulated numerous research and development projects dealing with issues of soil This article has been previously published in the journal “Nutrient Cycling in Agroecosystems” Volume 76 Issues 2–3. A. Bationo (eds.), Advances in Integrated Soil Fertility Management in Sub-Saharan Africa: Challenges and Opportunities, 1–28.

Appropriate technologies to replenish soil fertility in southern Africa

In southern Africa, soil nutrient reserves are being depleted because of continued nutrient mining without adequate replenishment. The consequent downward spiral of soil fertility has led to a corresponding decline in crop yields, food insecurity, food aid and environmental degradation. The central issue for improving agricultural productivity in southern Africa is how to build up and maintain soil fertility despite the low incomes of smallholder farmers and the increasing land and labour constraints they face. Under this review five main options namely: inorganic fertilizers, grain legumes, animal manures, integrated nutrient management and agroforestry options appropriate to smallholder farmers are presented. Issues addressed in the use of inorganic fertilizers are reduction in fertilizer costs, timely availability and use efficiency. Legumes can be used to diversify farm system productivity but this requires P and lime application to support better legume growth and biological nitrogen fixation (BNF) as well as development of markets for various legume products. Manure availability and quality are central issues in increasing smallholder farm productivity and increasing its efficiency through proper handling and application methods. Integrated nutrient management of soil fertility by combined application of both inputs will increase use efficiency of inputs and reduce costs and increase profitability; but the challenge is often how to raise adequate amounts of either inorganic or organic inputs. Issues such as quality of inputs, nutrient balancing, labour to collect and transport organic inputs and their management need to be optimized. These are the challenges of adoption as are the scaling up of these options to millions of small-scale farmers.

Fighting poverty in sub-Saharan Africa : the multiple roles of legumes in integrated soil fertility management

Preface.- Chapter 1: Agro-ecological distribution of legumes in farming systems and identification of biophysical niches for legumes growth, Nandwa SM (The late), Obanyi SN and Mafongoya PL.- Chapter 2: Socio- economic contribution of Legumes to Livelihoods in Sub-Saharan Africa Odendo M, Bationo A and Kimani S.- Chapter 3: Inter And Intra-Specific Variation Of Legumes And Mechanisms To Access And Adapt To Less Available Soil Phosphorus And Rock Phosphate, Nandwa SM (The late), Bationo A, Obanyi SN, Rao IM, Sanginga N and Vanlauwe B.- Chapter 4: Legume based cropping systems and soil biodiversity dynamics Karanja NK, Kimenju JM, Esilaba AO, Jefwa J and Ayuke F.- Chapter 5: Strategies to adapt, disseminate and scale out legume based technologies Mugendi DN, Waswa BS, Mucheru MW and Kimetu JM.- Chapter 6: Comparative Analysis of the Current and Potential Role of Legumes in Integrated Soil Fertility Management in West and Central Africa Bationo A, Kimetu J, Vanlauwe B, Bagayoko M, Koala S and Mokwunye AU.- Chapter 7: Comparative Analysis of the Current and Potential Role of Legumes in Integrated Soil Fertility Management in East Africa Mugendi DN, Waswa BS, Mucheru M, Kimetu JM and Palm C.- Chapter 8: Comparative Analysis of the Current and Potential Role of Legumes in Integrated Soil Fertility Management in Southern Africa Mapfumo P.- Chapter 9: Options for improving plant nutrition to increase common bean productivity in Africa Jansa J, Bationo A, Frossard E and Rao IM.- Conclusion and the way forward

Strategic phosphorus application in legume-cereal rotations increases land productivity and profitability in Western Kenya

SUMMARY Many food production systems in sub-Saharan Africa are constrained by phosphorus (P). We hypothesized that within legume-cereal rotation systems: targeting P to the legume phase leads to higher system productivity, and that use of grain legumes leads to better economic returns than use of herbaceous legumes. Four P application regimes: (i) no P, (ii) P applied every season, (iii) P applied in season 1 only and (iv) P applied in season 2 only were tested for four seasons in three cropping systems (continuous maize, mucuna-maize rotation and soybean-maize rotation) in a split plot experiment set up in Nyabeda, western Kenya. Treatments where P was applied were better than no P treatments. While continuous cereal systems showed the need for application of P every second season, rotation systems involving mucuna and soyabean indicated that application in one out of three seasons could be sufficient. Nitrogen fertilizer equivalence was 52 to >90 kg N ha−1 for soyabean and 37 to >90 kg N ha−1 for mucuna, depending on P fertilization and season. Analysis of marginal rates of return (MRR) showed that soybean-maize rotation with one application of P was the most economically viable option, with an MRR of at least 147% compared to other non-dominated options.

EFFECT OF REDUCED TILLAGE AND MINERAL FERTILIZER APPLICATION ON MAIZE AND SOYBEAN PRODUCTIVITY

SUMMARY Reduced tillage is said to be one of the potential ways to reverse land degradation and ultimately increase the productivity of degrading soils of Africa. We hypothesised that crop yield following a modest application of 2 t ha −1 of crop residue in a reduced tillage system is similar to the yield obtained from a conventional tillage system, and that incorporation of legumes in a cropping system leads to greater economic benefits as opposed to a cropping system involving continuous maize. Three cropping systems (continuous maize monocropping, legume/maize intercropping and rotation) under different tillage and residue management systems were tested in sub-humid western Kenya over 10 seasons. While soybean performed equally well in both tillage systems throughout, maize yield was lower in reduced than conventional tillage during the first five seasons but no significant differences were observed after season 6. Likewise, with crop residue application, yields in conventional and reduced tillage systems are comparable after season 6. Nitrogen and phosphorus increased yield by up to 100% compared with control. Gross margins were not significantly different among the cropping systems being only 6 to 39% more in the legume–cereal systems relative to similar treatments in continuous cereal monocropping system. After 10 seasons of reduced tillage production, the economic benefits for our cropping systems are still not attractive for a switch from the conventional to reduced tillage.

Optimising crop productivity in legume-cereal rotations through nitrogen and phosphorus management in western Kenya

Combined application of organic resources and mineral inputs is integral to sustainable soil fertility management but in-situ production of adequate organic matter is often limited by P availability. An experiment was set up at Nyabeda in Western Kenya aimed at (1) quantifying the contribution of herbaceous and grain legumes to nitrogen supply in a cereal-legume rotation system and (2) quantifying the impact of targeting phosphorus (P) to certain phases of the rotation on overall maize grain yield. In this split-split plot experiment, Mucuna pruriens was used as the herbaceous legume while soybean was used as the grain legume. Results obtained in the two seasons of the study indicated that the use of either mucuna or soybean as previous crop significantly increased maize grain yield with or without the addition of nitrogen fertilizer. More than 5 tons ha-1 of maize grain yield was realised in season two following the addition of phosphorus fertilizer at both season one and season two compared to about 3 tons ha-1 of maize grain yields obtained when no P was added. It could be concluded that in this region, the addition of P fertilizer is an integral management option to ensure optimal utilization of the nitrogen fixed by the legume crop. Using P during the legume season may be sufficient to supply P requirements to the succeeding cereal crop. Also, applying P to the mucuna or soybean legume crop was not any different from applying it both to the legume and cereal crops indicating that farmers can save labour and cash by applying P only to the legume. The good performance of maize planted after mucuna was an indication that mucuna could be used by farmers in the region as an N source (Nitrogen Fertilizer Equivalency (NFE) >100 kg N ha-1) thus reducing cost of buying N fertilizers. Although soybean showed a lower NFE of 40 kg N ha-1, it had higher economic benefits and could thus be more acceptable to the farmers. These findings could be confirmed by using more than two cereals and legume rotation cycles

Strategies to Adapt, Disseminate and Scale Out Legume Based Technologies

The full potential of legume based technologies can only be achieved if farmers widely adopt these technologies. Widespread adoption of legume-based technologies calls for the identification of spatial and temporal niches of legumes in the farming systems. The entry point may be immediate provision of food and fodder but also contribution to soil fertility improvement and erosion control among others. Whichever the objective, there is need to demonstrate the immediate benefits to be accrued by the adopters if large scale adoption is to be achieved. There is need to address challenges hindering widescale adoption of legume technologies such as availability of improved germplasm, poor markets and policy. Whereas the national systems play a key role in promoting legumes, the private sector continues to effectively complement government efforts to increase availability and accessibility to quality seed. Flow of knowledge cannot be achieved through a narrow prescriptive approach, but requires development and testing of a battery of possible interventions for soil fertility improvements suited to the specific agroecological environment together with the farmers. Better and innovative mechanisms are required for sharing of knowledge between all those involved in trying to improve the productivity of smallholder agriculture. This should cover the use or both print and audio media as complement to the traditional approaches such as the use of field days and onfarm demonstrations. The focus of this chapter is therefore to look at how the above issues have been addressed in adapting and disseminating legume-based technologies in Africa.

Soil carbon and agricultural productivity: perspectives from Sub-Saharan Africa

Soil carbon plays a key role in maintaining crop productivity in the soils in sub-Saharan Africa (SSA). This is more so considering that most smallholder farmers cannot afford the use of adequate amounts of inorganic fertilizers to restore the proportion of nutrients lost through crop harvests, soil erosion and leaching. Complicating the situation is the huge proportion of land under threat of degradation in the form of soil erosion and nutrient decline. There are numerous opportunities for improving soil carbon as a basis of ensuing sustainable agriculture. This paper discusses the role of soil carbon in agricultural production, with special focus on sub-Saharan Africa. First, the paper presents a discussion on the functions of soil carbon (biological, chemical and physical). This is followed by a look at the causes of carbon variation across agroecosystems. Management of soil carbon and productivity is evaluated in the context of resource availability, quality and soil organic matter pools. Drawing from the integrated soil fertility management practices in Africa, the paper discusses various strategies for organic carbon management and the implication of the same on crop productivity and soil properties. A special focus is given to the lessons learned from long-term experiments across Africa.

Crop and Soil Response to Tillage and Crop Residue Application in a Tropical Ferralsol in Sub-humid Western Kenya

Conservation agriculture (CA) offers an opportunity to reverse prevailing land degradation and consequent loss of productivity often occasioned by intensive soil tillage in cropping systems in most parts of sub-Saharan Africa (SSA). A long term experiment was established in Nyabeda Western Kenya in 2003 to evaluate the effect of tillage and crop residue application on maize and soybean yields, and on soil properties. The experiment was set up as a split-split-split plot design with four replicates and involved a factorial combination of tillage system (reduced and conventional tillage), cropping system (continuous cereal, soybean-maize rotation and intercropping), crop residue – maize stover – management (plus and minus crop residue) and nitrogen (N) application. Results showed that tillage influenced performance of maize although significant tillage effects were observed in only 5 out of the 15 seasons analyzed. Overall average maize grain yields were 2.9 ton ha−1 in reduced tillage and 3.6 ton ha−1 in conventional tillage systems. Application of crop residue increased seasonal maize grain yield in reduced tillage (340 kg ha−1) and in conventional tillage (240 kg ha−1), but the only significant crop residue (CR) effect was observed in season 10. Differences in maize yields between the two systems were attributed to phosphorus availability as it was demonstrated that application of crop residue in the reduced tillage resulted in better availability of P than without crop residue application.

The Potential of Increased Maize and Soybean Production in Uasin Gishu District, Kenya, Resulting from Soil Acidity Amendment Using Minjingu Phosphate Rock and Agricultural Lime

In Kenya, soil acidity is a major contributor to declining soil fertility and 20% of the soils are acidic and are considered to be of low fertility. Most farmers are unaware of the benefits of liming acid soils. A study was carried out during the 2005 and 2006 long rain seasons at Kuinet in Uasin Gishu District of the Rift Valley Province in Kenya to delineate the effects of Minjingu phosphate rock (MPR) and agricultural lime as liming materials on yields of soybeans intercropped with maize. The maize responded to application of soil amendment materials for the first season with the diammonium phosphate and lime (DAPL) treatment giving the highest maize yields of 6.19 t ha–1 compared to the control which gave 1.36 t ha–1. Soybean yields were low in the first season with the DAPL treatment and control treatment giving yields of 0.32 and 0.14 t ha–1, respectively. This, however, changed significantly after the variety was changed in the second season, with yields going up to 0.68 t ha–1 for the triple superphosphate and lime (TSPL) treatment. From the study, it was concluded that there is potential for growing soybean in Uasin Gishu District of Kenya. However, a study and/or research is recommended to screen and identify a suitable variety for increased soybean yields in this district.