Effects of lime and phosphorus on biological nitrogen fixation by soybeans (Glycine max (L.) Merrill) in an Ultisol, Morogoro, Tanzania

Abstract

Low soil pH and phosphorus are among the major soil constraints which limit BNF and soybean production in sub-Saharan Africa. Most Probable Number (MPN) plant infection technique and glasshouse pot experiment were conducted at SUA to estimate the population of native Bradyhizobium japonicum and determine the effect of lime and phosphorus on soil pH and BNF by soybean in an Ultisol. The pot experiment was a 33 factorial in a split plot experimental design replicated two times. The main plots were inoculation, non-inoculation and inorganic N fertilizer at the rate of 200 kg N ha-1. The levels of lime were 0, 5 and 10 ton/ha and the levels of phosphorus were 0, 50 and 100 kg/ha, whose combinations constituted the sub-plots. The soil was acidic (pH 4.6) and contained 2-79 cells of indigenous Bradyhizobium japonicum per gram of soil, which is low and appeared to limit BNF by soybean. The soil pH and N content in soybean plants increased significantly (P < 0.05) with increase in liming rate. The interaction of lime and phosphorus on nodulation, N2 fixed, DMYs, P content in inoculated plot and on Ca contents of soybean plants in inorganic N applied plot was significant. It was concluded that for soybean to be grown in the studied area should be inoculated before planting due to the low Bradyhizobium japonicum population. The application of 5 tons lime/ha and 100 kg P/ha would serve as the optimum rates for the Ultisol used in this study. It is recommended that the study be validated under field conditions before conclusive decisions can be made.