Optimizing p and k for improving biological nitrogen fixation and productivity of bambara groundnut (v igna Subterranea ) in South-Eastern Tanzania

A Dissertation

Bambara groundnut (Vigna subterranea (L) Verde) is a pulse crop cultivated by smallholder farmers in Tanzania, ranked the third most important legume in the SEZ after pigeon pea (Cajanus cajan) and cowpea (Vigna unguiculata). Farmers in south-eastern Tanzania grow Bambara groundnut primarily for food and sell part of the produce to supplement income of their household. However, the productivity of Bambara groundnut is below the potential yield (< 2 t ha -1 ), and is considered a threat to their livelihood. Among the factors causing low yields of Bambara groundnut include insect pests and disease incidences, poor agronomic practices and low soil fertility. Low soil fertility is amplified by continual mining of nutrient elements from the soil and reduction of soil organic matter content, through the removal and burning of crop residues and without applying balanced nutrients. Balanced supply of plant nutrients is important in order to improve and sustain yields of Bambara groundnut in South-Eastern Zone (SEZ). However, the establishment of the technologies of soil fertility management is hindered by inadequate information on soil characterization and classification of soils. These agronomic experiments would lead to establish technology that would be transferred to other areas of similar soil conditions for Bambara groundnut production, particularly in Tanzania. Furthermore, all developed interventions concentrated on improving Bambara groundnut varieties; plant spacing, morphology and diversity of the crop are known. The information on soil nutrient status including phosphorus (P) and potassium (K) for SEZ is not well known, and the required optimum rates are not yet established. Therefore, the intention of this study was to determine the status of soil fertility and optimum rates of N, P and K for Bambara groundnut production in the SEZ. A standard survey was carried out in the Bambara groundnut growing areas of SEZ, to establish representative experimental sites on the basis of agro-ecological settings and soils. The morphology, genesis, physio-chemical properties and classification of soils based on two international soil classification systems, namely USDA Keys to Soil Taxonomy and the World Reference Base (WRB), were assessed. Three pedons were characterized, namely NNL-P1 at Nannala village in Tandahimba district, MKG-P1 at Mikangaula village and NWJ-P1 at Nawaje village in Nanyumbu district. Disturbed and undisturbed soil samples from genetic soil horizons were analyzed for physico-chemical properties. Soil nutrient status for macro-nutrients (N, P, K, Ca, Mg and S) and micro-nutrients (Fe and Zn) were assessed from 22 soil samples from Bambara groundnut growing areas of south-eastern Tanzania. Field experiments were established on two pedons, namely NNL-P1 and NWJ-P1. The experiments were used to establish optimum rates of P and K for N 2 fixation and yields of Bambara groundnut in south-eastern Tanzania. The treatments for the field experiments were varied rates and combinations of P (0, 20, 40, or 60 kg P ha -1 ), K (0, 40 or 80 kg K ha -1 ) and N (0, 20 or 80 kg N ha -1 ), as sub-plots, in a split plot design replicated three times. The main plots were two varieties of Bambara groundnut (local and improved). The characterization results indicated that two pedons NNL-P1 and MKG-P1 were classified to great group level using the USDA Keys to Soil Taxonomy as Dystrustepts and Haplustepts, respectively, and these translated into Cambisols in the WRB for Soil Resources. Pedon NWJ-P1 was classified as Argiustolls and translated into Phaeozems in the WRB. Over 60% of 22 soil samples analyzed for nutrient status, the limiting nutrients for productivity were N, P K, S, Mg and Zn, which were rated as being very low to low, while Ca was rated as medium in >80% of the soils of the studied areas. It was concluded that low nutrient status especially N, P, K, S, Mg and Zn, needs proper management to improve soil fertility for Bambara groundnut production. Application of P increased N 2 fixation and Bambara groundnut yield significantly. Thus, use of P at the rates of 20 – 40 kg P ha -1 is deemed necessary for high yields in the study area.