Nutrient Replenishment in Smallholder Farms of Western Kenya Using the 'PREP-PAC' Product

摘要

Soil fertility depletion is recognized as a leading cause of food insecurity in sub-Saharan Africa, and several approaches are being tested to replenish the fertility of degraded soils. Aparticular challenge is to develop products formulated for the needs of smallholder farmers who are directly affected by continuous food shortages. These products could be distributed through local farm input supply networks. Thus, the Phosphate Rock Evaluation Project (PREP) at Moi University, Kenya, has formulatedand field-tested the PREP-PAC, which is an inexpensive product that combines fertilizer, legume seed, and rhizobial inoculant techniques. It is specifically targeted to the low soil fertility patches that are widespread in smallholder farming systems inthe highly weathered and leached soils of western Kenya, consisting of about 500,000 ha of mainly Acrisols (Ultisols) and Ferralsols (Oxisols). PREP-PAC contains 2 kg of the reactive/biogenic locally available Minjingu (Tanzania) phosphate rock (PR), 0.2kg imported urea, 0.13 kg seed of a N-fixing food legume, rhizobial inoculant (Biofix), seed adhesive (gum arabic), lime pellets and an instruction sheet for use in English, Kiswahili and other local languages. The pack is assembled at US 0.58 dollar per unit and is intended to ameliorate patches of 25 m~2. The product is particularly effective in acid (pH <5.5) and low P (<10 mg P/kg) soils. In PREP-PAC field-testing, several experiments were conducted from 1998 to date. The general principle is to apply the slowly available PR in sufficient quantities for several cropping seasons, with readily available nitrogen fertilizer (urea) and to intercrop with a legume that provides fixed-nitrogen and organic inputs to the soil. This approach was tested in 52 on-farm experiments where PREP-PACs were tested in symptomatic patches containing maize-bean intercrops with or without an improved variety of climbing Phaseolus vulgaris cv. Flora. Untreated patches (25 m~2) produced 1.6 kg maize and 0,08 kg bean. With addition of PREP-PAC containing Flora, yields increased to 4.1 kg maize and 1.1 kg bean (p<0.001) and the improvement in bean yield during the first cropping season nearly offset PREP-PAC's investment costs. A second experiment examined interactions between PREP-PAC components in a maize-soybean intercrop in nutrient-depleted soils, with sandy and loamy surface horizons, at 3 on-farm sites. The treatments were +-PR, +- urea and +- inoculants arranged as a 2~3 factorial with 4 replicates at each location. The total value of the intercrops ranged between 0.83 dollar in the untreated plots and 2.44 dollar in those treated with PREP-PAC. Significant positive effects were observed with the addition of PR (p<0.001), urea (p=0.04) and inoculant (p=0.01) and in interactions between PR and urea (p=0.02) or inoculant (p=0.07). The return ratio to PREP-PAC investment was 2.6 in the sandy soil and 3.7 in the loamy soil. Athird field experiment compared the responses of dry bean and soybean to rhizobia inoculants, which tolerated acidity, in three acid and low fertility soils. Significant site and PREP-PAC treatment effects were found across sites for both legumes. The effectiveness of inoculants also varied with sites. Presently, PREP-PAC is being field-tested by five developmental organizations and test-marketed by several retailers of agricultural supplies. Efforts are underway to lower the cost of PREP-PAC production by 30% (to 0.41 dollar per unit), primarily through bulk purchase and reduced packaging and transportation costs.