The Problem of Maintaining Soil Fertility in Eastern Kenya: a Review of Relevant Research
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THE PROBLEM OF MAINTAINING SOIL FERTILITY IN EASTERN KENYA: A REVIEW OF RELEVANT RESEARCH J. R. SimpJol/, J. R. Okafehl e.3 G. Luhllfwa) J -10 HOIIIlCJ CreJcent, Campbell, ACT, AllJtralia 2612 2 Department of Soil Sciencc, fi10i UrziIJerJity, PO Bo.r: 1125, EIJoret, Kenya , ACIAR, GPO Bo.\: 157 L Canberra, AlItJrall.'a AllJtralian Centre for International A.lJriCllI tllml ReJearcb 1996 The Australian Centre for International Agricultlll'al Research (ACIAR) was established in June 1982 an Act of the Australian Parliament. Its primary mandate is to help identi(y agricultural problems in developing countries and to commission collaborative research between AustI'alian and developing country researchers in fields where Australia has special competence. \Vhere trade names are used this does not constitute endorsement of nor discrimination against any product by the Centre, SERTES series contains the results of original research supported by ACIAR. or' material deemed relevant to ACIAR's research and development objectives, The series is distributed internationall:v, with an emphasis on developing countries. © Australian Centre for International Agricultural Reseal'ch GPO Box 1571. Canberra. Australia 2601. Simpson. J. R .. Okalebo. J. R. and Lubulwa. G. 1996. The Problem of Maintaining Soil Fertllity in Eastern Kenya: a review of relevant reseal'ch. ACL\R Monograph No, 41. 60p. ISBN I 86320 1920 Design and production: (J,'.'l.qll ONE. Canberra. Australia Printed Goanna Print PI'y Ltd. Canberra. Australia THE AUTHORS The ;\ustralian Centre for International Agricultural Research (ACIAR) supported collaborative research vvith the Kenya Agricultural Research Institute (KARI) in Machakos and Kitui Districts between 1984 and ]993. Drs Simpson and Okalebo \vere collaborators in that research, affiliated \vith ACL-\R and KARI respectivel,)·. Or Lubulwa is a Senior Economist with ACIAR in Canberra. 2 PURPOSE OF THE REVIEW TllIS REVIEW TRACES n lE IIISTORY with two crops taken every yeal~ learned how to manage the and progress of research in puts heavy pressure on the soil resource base better'. eastern Africa into the problems resource. TiFfen and colleagues This review aims to show that of maintaining soil fertility and (1994) described the excellent progress is indeed being made sustainable productivity on the progress that has been made in towards better land croplands, with particular reducing soil erosion by contour managemen t 111 reference to the 'medium banks and especially districts like :Vlachakos. Several potential' areas of the 'moist in the rainfall hill areas of examples are of farmers savanna'. These are the areas of j\lacha koso \Vhi le they working towards affordable medium but unreliable rainFall recognised that soil Fertility has techniques for better soil (average 600-800 mm/year). declined, these authors fertility management. 11000,vever, They span the E"-O (Food and suggested no feasible and the overall progress in soil Agriculture Organization) com prehensi ve solu tion to this fertility management is slow; classifications of 'semi-arid' and increasingly important problem. very uneven, and is hampered into the less-favoured end of the They did not mention that the by major socioeconomic hurdles. adjacent 'sub-humid '. The conservation of soil and water The low rates of adoption of review deals mainly with has, by increasing the supply of better soil management studies in eastern Kenya, but soil moisture, actually' practices, despite the also calls on reports from other the potential for the 'mining' of conclusions of recent on-farm areas of eastern and sou thern the remaining soil nutrients by research and modelling, Africa ;vith related agricultural crops. \Vith current 10Y.,' that further on-farm research systems, and ;,;hel'e similar nutrient inputs, this can and extension are urgently problems of maintaining soil produce short-term gains in needed. Apparently many small fertility are experienced. crop yields but will scale farmers need education on As an exam pIe of the the problem of maintaining soil their possible options, and problem, the present study fertility in the long term. assistance in selecting centl'es on the situation in English and co-workers (1994) appropriate sol u tions. \Vi thou t Machakos District, Kenya, also did not recognise this such effort. and enabling where population and reality~ stating (p. 35) that 'any Facilities such as financial credit agricultural production have deleterious eFFects of changes in and better fertilizer increased some sixFold during the nature of the resource base distribution, the improvement the years 1930-1990. I n this (due to cu/til'atioll and croppill/l) rate in soil fertility management 'medium potential' area, have been more than offset by in eastern Kenya is likely to intensification of agriculture, improvements due to terracing, remain generally lo\'.: etc., and the farmers have 3 Some of the options in better are explored in the light of relative economic attractions soil fertility management for recent research, and their and risks surve.ved. farmers wi th modest resources BACKGROUND TI lE j\i\PlD['{ INCREASING forecasts increasing Food deficits Fertilizer use has been declining, populations in sub-Saharan if Llrming practices are not e.g. in Uganda, where usage has Africa linked ,vith low inputs of improved, and estimates that decreased by 40-60% between soil nutrients and increasingly food sufficiency can be achieved 1980 and 1990 and farmers now intensive cropping over much only if ferti lizer usage rises to rely heavily on local organic of the cultivated lands have 90 kglha by the year 2010 and sources (Swift et a!. 1994). It is resulted in the stagnation of, 160 kglha by 2030, not surprising that soil then gradual decline in, per Total inorganic fertilizer productivity is decreasing capita production of Food grains usage in su b-Saharan Africa generally apart from the over the period 1960--90 during 1992 was about relatively few cases where (Makhn 1993a). Gerner and 1.28 million tonnes (Gerner farmers wi th better financial Harris (1993) surveyed the and Harris 1993). Kenya and resources are able to afford the recent histor,Y of inorganic Tanzania used roughly equal nutrient inputs necessary tor fertilizer consumption in sub amounts, and together sustainable crop production. Saharan Africa. Average rates of accounted for some 100000 Generally, rhroughou t su b fertilizer application across tonnes of this total. Saharan AFrica, the existing Africa are outstandingly low by Consumption during 1975-90 low-input agriculture results in world standards, ranging from had been growing at 3.4% depletion of the major soil about 1 kg/ha cropland in annually, overall; in East Africa nutrients nitrogen (N), Central Africa to 15 kglha in the rate was nearly 5%; in phosphorus (P) and potassi um southern Africa. These national southern Africa, 1. 7%. However, (K). Overall, this depletion has averages disguise large structural adjustment policies of been calculated at 22 kg N, variations between agro many governments have led to a 2.5 kg P and K 15 kg/ha/year ecological zones wi thin stagnation in fertilizer (Stoorvogel et al. 1993), wi th countries and even between consum ption since the late much higher values For Kenya. individual farmers. FAO (1993) 1980s. In some countries The extent of the problem varies 4 both within national boundaries highland areas of Ethiopia, In the densely populated, and on national scales, Kenya, Rwanda, Burundi, heavily cropped district of Kisii, depending on the inherent soil Malawi and Lesotho (Smaling SW Kenya, aggregated nutrient fertility and the intensity of 1993). For these countries, even losses were calculated at 112 kg cropping imposed. Annual national average cropland losses N, 3 kg P and K 70 kg!ha/year, nutrient losses are particularly are calculated to exceed 40 kg wi th some serious P acute in the densely populated N,6 kg P and K 15 kg/ha/year. deficiencies. EARLY SOlL FERTlLlTY RESEARCH CAB! (1994) recently reviewed started in 1936, was designed to Later work (Simpson 1961; soil fertility research in East establish what ratios of time Stephens 1967) at Kawanda, near Africa (1930-1990) and under crop to time under fallow, Kam pala, showed that the published an annotated supplemented by varied inputs beneficial effects of resting bibiliography with over 1000 of manure, could produce fallows of deep-rooted grasses, abstracts. sustainable crop yields. The e.g. Cf:,/O/"l~1 .qayana and Pen/ll~ld{11ll Early soil fertility work, experiment continued for at PIII"PIII"CIIIIl, were due largely to starting soon after 1930, was least 25 years (i .e. fi ve of its the capture of leached nitrate and largely concerned wi th the rotational cycles). The results bases, particularly K, from the effecti veness of vegetati ve were reviewed by Jameson and subsoil and the return of these fallows, combined with animal Kerkham (1960), 1vlills (1960) nutrients to the surface as manure applications, designed and McWalter and Wimble increased organic Nand to restore ferti li ty after a period (1976). Essentially, the associated exchangeable cations. of cropping. This work was in experiment showed that the The physical effects on soil the context of a gradual demise beneficial effects of resting from structure and rainfall acceptance, of shifting cultivation, as cropping under a grass/bush which had previously been communities became more fallow could be largely (not thought to be important, were settled and populations entirely) replaced by the found to be only short-term increased (Padwick 1983). application of manure once in transient improvements. A major long-term fertility each 5-year rotation, but would experiment in Eastern Uganda require application rates greater at Serere Research Station, than 5 t!ha. 5 At Katumani, Machakos, in a var'iations in parent material, or large quantities of cations \vere drier climate, Bennison and reactions ,vi Ih ferric and leached into the subsoil, Evans (1968) found that aluminium ions in the more oxic accompanyi.ng the nitrate accumulations of' nitrate in the profiles.