The challenge of HIV/AIDS: Where does agroforestry fit in? - World ...

54World Agroforestry into the Futurecontinuous cultivation, or which have awidespread history of fertilizer use, do notexhibit this problem (Scoones and Toulmin1999). Localized differences in farmerwealth ranking, field-use history and theuse of organic additions (typically in fieldsclose to the homestead) generally produce‘islands’ of high soil fertility (Shepherd andSoule 1998.Given the acute poverty and limited accessto mineral fertilizers, a promising approachis one that integrates organic and inorganicfertilizers. Organic fertilizers include theuse of improved fallows of leguminoustrees, shrubs, herbaceous legumes andbiomass transfer. The improved fallows systemis the product of more than 10 years’of agroforestry research and developmentefforts by the World Agroforestry Centre(ICRAF) and its many partners in SSA. Bothresearch and development dimensions arediscussed in this chapter. We do this bydrawing particular reference to the Centre’scollaborative work in three regions ofAfrica – East and Central Africa, southernAfrica and the Sahel. Declining soil fertilityis a major concern faced by smallholderfarmers in all these regions (Franzel 1999;Sanchez and Jama 2002).Improved fallowsThe concept and practiceAlthough neither the idea nor the researchon improved fallows is new (Nye andGreenland 1960), critical examination ofthe practice, and the wide-scale evaluationof suitable species, is relatively recent(Sanchez 1995). Planted fallows of leguminoustrees or shrubs can biologically fixconsiderable amounts of N – for example,between 60–80 kg ha –1 – in above-groundbiomass (Gathumbi 2000). The rest of therecycled N in such leguminous trees orshrubs is accessed from sub-soil N – Oxisolsand Oxic Alfisols – which is unavailableto crops (Mekonnen et al. 1997). Underconditions such as those in western Kenya,where the soils possess substantial anionexchange capacity, net N mineralizationexceeds N uptake by crops and high rainfallcarries nutrients to the sub-soil, resultingin a build-up of sub-soil N that rangesfrom 70 to 315 kg ha –1 (Hartemink et al.1996). Nitrogen that accumulates in theabove-ground biomass of planted tree fallowsis returned to the soil upon clearing;the fallow biomass is incorporated into thesoil for subsequent cropping. Additionally,fallows increase the amount of labile fractionsof organic soil matter, which supplynutrients to crops following fallows (Barrioset al. 1997). They can also contribute toimproving soil structure, build up of soilorganic matter and its carbon (C) stocks,thus contributing to C sequestration.The choice of which species to plant inthe fallow period is influenced by bothbiophysical and socioeconomic conditions.The ideal tree species is typicallyfast-growing, N-fixing and efficient atnutrient capture and cycling. Examples ofpromising species include Crotalaria grahamiana,Tephrosia vogelii, Cajanus cajan(pigeonpea) and Sesbania sesban (sesbania).Coppicing species can also be used,and Gliricidia sepium (gliricidia) and Calliandracalothyrsus (calliandra) are becomingincreasingly popular with farmers inKenya, Malawi and Zambia because theyare perennial and, unlike the non-coppicingspecies, there are no costs involved inreplanting them once they are cut back.Agronomic and economic benefitsSeveral agronomic studies demonstratethat improved fallows of 1–3 seasons (8–21months) can increase soil fertility and improveyields considerably. For instance,Kwesiga and Coe (1994) in premier fieldstudies demonstrated that 2- and 3-yearsesbania fallows can increase maize yield,compared to unfertilized maize monoculture,for at least three cropping seasons afterharvest of the fallows on an N-deficientsoil in Zambia. This was confirmed laterin multilocational trials in eastern Zambia(Kwesiga et al. 2003). In western Kenya,similar observations have also been madewith use of several species and fallowdurations (Jama et al. 1998a; Niang et al.1996a; Rao et al. 1998). Recent trials in theSahel that were conducted within the subhumidregion of Mali also demonstrate theability of several species to improve soilfertility and crop yields considerably (Figure1). These studies have led to the generalconclusion that total farm productioncan be greater with improved fallow–croprotations than with continuous cropping,even though crop production is skipped forone or more seasons with improved fallows(Sanchez et al. 1997).In areas such as southern Malawi with lowrainfall and sandy soils, gliricidia fallowsthat coppice when cut back perform betterthan those of sesbania, which does notcoppice well. This has been demonstratedthrough long-term trials that also show thatthe highest yields are obtained when improvedfallows are used in conjunction withrepeated application of the recommendedrates of inorganic fertilizers (Figure 2).In soils that are severely depleted of nutrients,the addition of inorganic fertilizersincreases the productivity of improved fallows.In western Kenya, for instance, thereis increasing evidence that 1–2 season-longfallows do not overcome N deficiency inhighly degraded soils, especially when deficienciesof other nutrients are overcomeand when high-yielding crop varieties areused. Fertilizer use is, however, limited by