Molybdenum-Phosphorus Nutrition for Amelioration of Biological Nitrogen Fixation for Common Bean on a Ferralsol

Abstract

Common bean (Phaseolus vulgaris L.) is a principal food security crop in sub-Saharan Africa (SSA); a crop whose production is almost exclusively dependent on natural biological nitrogen fixation (BNF) as the source of nitrogen. The objective of this study was to determine the response of BNF in common beans, to different regimes of Mo-P application and rhizobia inoculants regimes in a Ferralsol. A screen house study was conducted at Makerere University Agricultural Research Institute, Kabanyolo (MUARIK), in Uganda. A Ferralsol obtained from previously cropped fields with different types of legumes and cereals, and low in plant available P and Mo was used. Treatments included Mo applied at rates of 0, 0.6, 1.3 and 2.5 mg kg^-1 of soil, equivalent to 0, 0.5, 1 and 2 kg Mo ha^-1; P was applied at rates of 0, 38 and 76 mg kg^-1 of soil, equivalent to 0, 15 and 30 kg P ha^-1. Rhizobia inoculants, namely BioStacked (characterized as “stress tolerant inoculant” by the manufacturer, Becker Underwood in USA); and Mak Bio Fixer (strain CIAT 899). The study was laid out in a completely randomized design (CRD), in a split block arrangement, with 3 replicates and repeated three times. Common bean, variety NABE 4, a low land, fast cooking and tasty variety, commonly grown in Uganda, was the experimental crop. Results revealed that co-application of Mo and either of the two types of rhizobia inoculants had a significant (P<0.005) effect on the number of effective nodules and their dry weights; total shoot N and shoot dry weight. Pots treated with Mo at the rates of 0.5 to 1 kg ha^-1, yielded the best overall. However, estimated values of biologically fixed nitrogen (total shoot N from rhizobia inoculated plots discounted for rhizobia un-inoculated plots), were not significantly (P>0.005) influenced by Mo application, rhizobia inoculation and/or both treatments. Additionally, intervening with P application in the Mo-rhizobia inoculant setup had no significant (P>0.005) effect on the status of estimated BNF generated by the common bean; implying that energy requirement for BNF was not a key limiting factor for the process. Overall, Biostacked rhizobia inoculant (exotic) tended to perform better at lower rates of Mo and P in terms of effective nodules and their dry weights, compared to Mak Bio Fixer (indigenous) that performed better at higher rates of Mo and P.