Saturday, 15 July 2006

Responses of Maize-Bean Intercrops to Minjingu Phosphate Rock and Lime in Terms of Nutrient Use Efficiency and Economic Benefits on Acid Soils of Western Kenya.

Abigael O. Nekesa1, John Robert Okalebo1, Caleb O. Othieno1, Ruth Njoroge1, Mary Kipsat1, Moses Thuita1, and André Bationo2. (1) Moi Univ, Eldoret, Kenya, (2) Tropical Soil Biology and Fertility Institute of CIAT, Tsbf-ciat, UN Avenue, Gigiri, Nairobi, POB 30677, Kenya

In sub-Saharan Africa, Kenya included, the long term development constraints have been food insecurity, poverty and recently the HIV/Aids threat. Soil fertility depletion is well known to contribute to low and unsustainable food productivity in the region. Thus in western Kenya, with about 5 million people, cultivating about 0.5 million hectares of land, the staple maize and bean crops hardly exceed 0.5t/ha/season on smallholder farms. The acid and low available phosphorous (P) soils (pH < 5.5 and available P < 5 mg/kg) are cultivated continuously with nil to negligible nutrient returns. The soils are highly weathered, mainly the ferralsols (oxisols) and acrisols (ultisols). However, only a few farmers apply the soluble but acid forming diammonium phosphate (DAP) to improve crop yields. The benefit of liming acid soils has been neglected. From this background, we tested simultaneously the liming and phosphate effects of the biogenic, reactive, readily available and cheap Minjingu Phosphate rock (MPR; 38.3% CaO) mined in the neighbouring Tanzania, against another cheap and available agricultural lime (20.8 % CaO) mined at Koru, Kisumu, Kenya and the widely used DAP as a source of P (at 0, 60, 120 and 180 kg P/ha). Thus the MPR at 0, 0.5, 1.0 and 1.5 t/ha, supplied the P rates above plus 0, 0.2, 0.4 and 0.6 t/ha CaO. Similar liming rates were adopted for agricultural lime (L) used in MPR liming material comparison, while DAP supplied the same rates of P as for MPR above (0-180 kg P/ha). To eliminate possible deficiency of the commonly limiting nitrogen (N) nutrient, the highest level of 160 kg N/ha from the highest DAP (or P rate) of 180 kg P/ha was applied to all non- DAP treatments as urea (including lower DAP rates). Maize and beans were planted during both long and short rains 2004 using the staggered two maize and bean rows (the MBILI system) which allow adequate light and heat radiation to reach the legumes. Treatments were applied in a RCBD with 4 replications at on-farm site at Nyabeda, Siaya (pH 5.2, < 3 mg P/kg). Treatments significantly (p°Ü0.05) increased both maize and bean yields in both cropping seasons; whereby the second rains trial tested the residual effects of treatments added from first season. MPR and DAP+L treatments gave the highest yields; 5.67 and 6.24 t/ha respectively compared to 0.48t/ha in the control for maize and 0.93 and 0.86 t/ha respectively compared to 0.28 t/ha in the control for beans in the first season. Increased External P use efficiency; 10.9, 9.6 kg grain/kg P for beans and 82.4, 91.7 kg grain/kg P for maize in the first season was obtained for MPR and DAP+L respectively at the lowest P rate (60 kg P/ha). Highest N use efficiency, 34 and 4.05 kg grain/kg N for maize and beans respectively for the first season was associated with the highest DAP+L rates (180 kg P/ha and 0.6 CaO t/ha). Treatments further raised the crude protein contents of both maize and bean grains. The single factor ANOVA on profitability indicated a significant probability difference at 95% confidence interval between the considered acid soil ameliorants. All in all, both MPR and Lime are effective liming materials in western Kenya. Key words: Maize, beans, soil acidity, responses, nutrient use efficiency, lime.

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