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.