Effects of organic and inorganic phosphorus sources on nitrogen fixation by field grown common bean on an Alfisol and an Ultisol in Kakamega, Kenya

Abstract

A field study was carried out in 2005 in Kakamega to quantify the effects of organic and inorganic sources of phosphorus on nitrogen fixation of common bean by the 15N Natural Abundance method. Field experiments were conducted on two different soil groups (Alfisol and Ultisol) in 20 farmers’ fields. Six treatments: control with no input, inorganic phosphorus, inorganic nitrogen, inorganic phosphorus plus nitrogen, seed priming with phosphorus and farmyard manure. These treatments were replicated three times in a randomized complete block design. Maize was used as a non-fixing reference plant for 15N analysis. The results showed that the total nitrogen accumulation by common bean ranged from 5 to 20 kg ha-1 in the Alfisol and from 8 to 26 kg ha-1 in the Ultisol and treatments in the Ultisol accumulated 20% more biomass comparedto treatments in the Alfisol. Further, the rate of nitrogen fixation was 50% to 54% Ndfa for the inorganic phosphorus sources and 40% to 51% Ndfa for the organic phosphorus source. Seed priming with phosphorus,inorganic nitrogen and the control obtained less than 40% Ndfa for nitrogen fixation. These results illustrate that phosphorus sources could differentially enhance nitrogen fixation by common bean in different soil groups in smallholder farming systems in Kakamega.