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 compared
to 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.