Effects of phosphorus, potassium and sulphur nutrition on nitrogen fixation of promiscuous soyabean varieties in Meru south district
Biological nitrogen fixation (BNF) by promiscuous soybean cultivars offers a potential for minimizing the investment made by resource-limited farmers in Meru South districtKenya. Promiscuous soybean cultivars have the ability to fix nitrogen from the atmosphere if properly nodulated with rhizobia. Nitrogen fixation is influenced by factors such as availability of mineral elements and prevalent weather conditions. Nitrogen (N), Phosphorus (P) Potassium (K) and Sulphur (S) are intimately involved in plant metabolism and growth. There is, however, hardly any quantitative information on the effects of PKS fertilizer application on nodulation and nitrogen fixation of promiscuous soybeans. The objective of this study was to assess the effects of PKS fertilizer application on nodulation, nitrogen fixation and yields on promiscuous soybeans. Laboratory and field experiments were carried out to evaluate the response of promiscuous soybeans to PKS fertilizer application on both high carbon and low carbon areas. Soybean cultivars, early maturing SB19 and late maturing SB20 were the main factors while fertilizer inputs were the sub-factors. In limiting nutrient trials fertilizer inputs were 60kgP/ha+60kgK/ha+24kgS/ha, 60kgP/ha+60kgK/ha, 60kgP/ha+24kgS/ha and 60kgK/ha+24kgS/ha. For the site-specific response trials the inputs were 5kgP/ha, 10kgP/ha, 25kgP/ha and 50kgP/ha. Nodulation status, plant biomass production and yield components were used for data generation. The transformed data was subjected to analysis of variance- ANOVA using PROC GLM package to determine the main effects of the treatments and their interactions. Specific pair-wise comparisons of treatment levels were done using the least significant differences (LSD) test at P=0.05 and correlations using `PROC CORR'. In both site specific response trials and limiting nutrient trials the main effects of carbon level in the field, soybean varieties and treatments on nodulation, plant biomass production and yield components were significantly different at (P<_ 0.05). The effects due to the interaction of these factors were not significant. Field carbon level significantly (P<_ 0.05) influenced nodulation, plant biomass and the yield components across the two soybean varieties. Application of varying amounts of P fertilizer and application of PKS fertilizers significantly (P<_ 0.05) influenced nodulation, plant biomass production and the yield components. Varietal differences significantly influenced plant biomass production and yield components but did not significantly influence nodulation. Laboratory and greenhouse results indicated that the isolates obtained were slow growing Bradyrhizobium japonicum, Bradyrhizobium elkanii and the fast growing Sinorhizobium fredii.