Productivity of groundnut has been generally low in Nigeria owing to soil fertility problems arising from low soil pH and low organic matter content of the soils leading to deficiency of major nutrients, especially, N and P. Current fertilizer recommendations are based on single nutrient trials and do not take into consideration the use of lime, manure or micronutrients. To this end, this study was designed to identify the major factors that influence biological nitrogen fixation and yield of groundnut in a savanna Alfisol through fertilizer application and soil amendment on an acidic and non-acidic soil so as to optimize productivity. A nutrient omission trial using factorial combinations was used to achieve this aim. The study was carried out in field trials at two locations at the Institute for Agricultural Research (IAR) farm, Samaru, on an acidic (S13) and non-acidic soil (S7) using groundnut genotype, SAMNUT 24 from July to October, 2015. The treatments used on the acidic soil were two levels each of lime, phosphorus, potassium and micronutrients. On the non-acidic soil, there were also two levels each of organic manure, phosphorus, potassium and micronutrients. These levels were zero (0) and the recommended rate for each nutrient. Phosphorus was applied at 54 kg P2O5/ha as SSP, K at 25 kg K2O/ha as MOP, micronutrients with the trade name Agrolyzer at 2 g/L, lime at 250 kg/ha and cow dung at 1.7 tons/ha (equivalent to 10kg N ha-1). The treatments were arranged in a factorial combination and laid out in a randomized complete block design replicated three times on plots of 9m2 for each location. The SAMNUT 24 seeds were inoculated with rhizobial inoculant, NC 92 to enhance biological nitrogen fixation (BNF) and were planted on all plots. A non-nodulating groundnut genotype, ICGL 5 was included for estimation of BNF. The effects of the main treatments and their interactions were observed on nodule number, nodule dry weight, root and shoot dry weights, nitrogen fixation, 100 seed weight, shelling percentage, pod, grain and haulm yields, as well as harvest index. On the acidic soil, the amount of N2 fixed and nitrogen derived from the atmosphere (Ndfa) were highest with the application of P while soil N balance was most favoured by the application of P and micronutrients. The combination of lime, P and K was observed to favour pod and grain yields the most. Haulm yield however was most favoured by the application of P only while harvest index was improved mostly by liming. On the non-acidic field, the highest Ndfa, N2 fixed, grain and haulm yields were obtained by the combination of cow dung, P, K and micronutrients, while the soil N balance was most influenced by addition of P. The combined application of cow dung, K and micronutrients was best for pod yield. Harvest index was highest under the combination of cow dung and K. Stepwise regression and correlation studies showed that the nodule dry weight was important in predicting the grain yield on the acidic soil. On the non-acidic soil, nodulation influenced BNF and yield of groundnut. Similarly, N2 fixation significantly influenced yield parameters on both soils. These indicate that rhizobium inoculation with NC 92 was effective in enhancing BNF and yield in the soils that were characterized with low indigenous rhizobial population. This study also showed that groundnut yield was significantly increased by liming and fertilization on the acidic soil. Liming of the acidic soil gave 21% higher pod yield than the non-acidic soil. The positive N balance in both locations indicates improved soil quality and can be beneficial to non-fixing crops grown in rotation. Micronutrients addition showed no significant difference on pod yield on both locations, indicating sufficiency of inherent micronutrients levels in the soils. This has further shown that poor nutrient management strategies are among the key factors that have affected groundnut productivity in Nigeria. This trend can be reversed through adequate application of fertilizer nutrients and soil amendment.
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