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

56World Agroforestry into the Futurefor 6–8 months. The wood can also be usedto support climbing beans and other climbingcrops.LimitationsTwo important factors that need to be consideredwith short-term improved fallowsystems are: how much leaf biomass thefallow species produces and the quantityof nutrients recycled with it. Several factorsinfluence biomass production. In degradedsites (nutrient-depleted and eroded), mostfallow species grow poorly and producelittle biomass. This is also the case in dryareas and those with Vertisols (heavy clays)that drain poorly during the wet season.Such conditions prevail around the LakeVictoria basin where leaf biomass yieldsare typically less than 1 t ha –1 . This willgive less than the 80–100 kg N ha –1 requiredto produce a 2 t ha –1 maize grainyield (Palm 1995). There are options thatcan be explored to increase biomass yieldwithout necessarily increasing the fallowperiod. These include the use of coppicingspecies, and under-sowing the treefallow with herbaceous green manurelegumes such as mucuna (Mucuna puriens)and macroptilium (Macroptilium atropurpureum).In P-depleted soils, trees respondto P application and can benefit from havingP applied to crops planted within them(Jama et al. 1998b).The incidence of pests and diseases is anotherimportant limitation and there aretwo aspects to this problem. Firstly, thereare pests and diseases that affect the treesthemselves and limit their productivity. Forexample, sesbania is damaged, sometimesseverely, by the defoliating beetle Mesoplatysochroptera. Crotalaria grahamiana,until now a promising species for improvedfallows in western Kenya, is attackedand defoliated severely by lepidopterousAmphicallia pactolicus caterpillars. Controllingthese pests is vital to ensure thatthe productivity of species used and promotedfor improved fallows is maintained.Secondly, there is need to understand andcontrol the effects of these pests on thecrops that succeed the fallows. A case inpoint are the root-knot nematodes associatedwith sesbania that also affect beansand tomatoes (Desaeger and Rao 2000).There is also the potential for some of thespecies used in fallows to become invasiveweeds – although no such occurrence hasbeen reported so far. Prolific seeders likecrotalaria and leucaena species are examplesof the types of fallow plants most likely tobecome problematic. Other species may startto seed prolifically when taken out of theirecological range. Thus control mechanisms,including prevention, early detection andrapid response, need to be developed. Thisrequires cross-regional collaborative efforts.Biomass (green manure)transferApart from improved fallows, existinghedges on farm borders are another sourceof organic nutrients for biomass transfer.More than 10 species with potential for thispurpose have been screened in westernKenya (Niang et al. 1996b), and the mostpromising of all is Tithonia diversifolia ofthe family Asteraceae (tithonia). Althoughit is not a legume, the fresh leaf biomassof tithonia has levels of N as high as thosefound in many N-fixing legumes. This commonshrub is also rich in P and K: the freshleaves contain 3.5% N, 0.3% P and 3.8%K. The leaf biomass decomposes rapidlywith a half-life of about one week especiallyduring the rainy season (Gachengo1996).Many field studies report that the applicationof tithonia biomass results in highercrop yields than application of inorganicfertilizers, and it has longer residual effects(Gachengo 1996; Jama et al. 2000). Part ofthe yield benefits associated with tithoniacould be due to increased availability ofnutrients. Phosphorus release from tithoniafresh-leaf biomass is rapid, and the supplyof plant-available P from tithonia can be atleast as effective as an equivalent amountof soluble fertilizer. Nziguheba et al. (1998)reported that incorporation of green tithoniabiomass equivalent to 5 t dry matter ha –1to an acid soil in western Kenya increasedP in soil microbial biomass and reducedP sorption by soil (Table 1). In this study,the plots were kept free of weeds and notcropped in order to eliminate plant uptakeof P as a factor affecting soil P fractions andprocesses. Increased P in soil microbial biomass2 weeks after tithonia incorporationpresumably indicates enhanced biologicalcycling and turnover of P in labile poolsof soil P. Enhanced microbial biomass Pfollowing integration of tithonia with triplesuperphosphate, and not with sole applicationof triple superphosphate, supports thehypothesis that tithonia increases soil labileP. Soil microbial P before maize plantinghas been shown by Buresh and Tian (1997)to be directly correlated to maize yield ona P-deficient soil in western Kenya.Availability of sufficient quantities of tithoniabiomass and the labour required toharvest and transport it to cropped fieldsare likely to be two major constraints tothe wide-scale adoption of this technologyby farmers. Recognizing these limitations,most farmers in western Kenya are usingtithonia on small parcels of land and onhigh-value crops such as tomato and kale(Brassica oleraceae var acephala; ICRAF1997). They are also experimenting withtithonia in maize–bean (Phaseolus vulgaris)intercrops, where it could be more financiallyattractive than in sole maize because