Impact of targeted nitrogen and phosphorus fertilizer micro-dosing on maize and cowpea yields under two cropping systems

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Maintenance of soil fertility poses a pressing challenge in smallholder farming. Although the routes to increasing soil productivity include optimizing fertilizer use, prevailing fertilizer recommendations are high and beyond the reach of most smallholder farmers. Hence, the need for alternative lower but more efficient and cost-effective fertilizer recommendation. This study focused on determining the influence of fertilizer micro-dosing on nutrient use efficiency (NUE) and yields of maize and cowpea crops on the Gleyic Plinthic Acrisol (at Assin-Kushea) and Plinthic Acrisol (at Twedie) in the semi-deciduous rainforest zone of Ghana. The study consisted of four activities: i. Identifying between N and P, the most limiting soil nutrient to maize crop yield in the study areas. ii. Determining maize yield response to varying rates of N and P fertilizer application. iii. Examining the effect of mineral NPK inputs on yields of maize-cowpea crops, both as sole and in rotation. iv. Assessment of farmers‟ fertilizer use and management practices in maize and cowpea production. Although the Gleyic Plinthic Acrisol was less fertile than the Plinthic Acrisol, maize response to N120P0, N0P90, N120P90, and N0P0 showed better growth and yield parameters at the former than at the latter. On the Gleyic Plinthic Acrisol where maize attained maturity, grain yield ranged from 1.2 to 2.4 t ha-1with increases of 10, 77 and 95 % over the control in the N120P0, N0P90 and N120P90 treated plots, respectively. The application of N0P90 treatment led to low apparent recovery of P compared to that of N due to N120P90 treatment. This study therefore established that P is the major nutrient limiting maize growth and yield on the Gleyic Plinthic Acrisols. Hence, P should be externally supplied. Maize showed differential yield response to the individual application of N and P2O5 fertilizers at 0, 30, 60, 90, and 120 kg ha-1. On both xvii soil types, whereas P response was quadratic in function, N response showed no trend. Despite that the highest NUE was obtained under N0P30 treatment plot on both soil types, this study has demonstrated that the critical level of P for optimum maize yield was at N0P60 and N0P90 on the Plinthic Acrisol and Gleyic Plinthic Acrisol, respectively. Therefore, fertilizer P application should not exceed the critical level for the soils. Fertilizer micro-dosing with N, P and K treatment combinations, under continuous maize cropping (CMC) and cowpea/maize rotation (CMR) systems has proven to substantially increase maize yields (33 to 99 %) on the Gleyic Plinthic Acrisols and Plinthic Acrisols. Grain yield increase with micro-dose fertilizer treatments was generally higher on the former than the latter soil type. Remarkably, N20P40K20 treatments under CMC which gave the highest maize grain yield and net returns, was thus proposed for increased maize yield in both soils studied. Socio-economic survey was conducted through oral interview with structured questionnaire involving one hundred farmers each at Assin-Kushea and Twedie. About 65 and 80 % of maize and cowpea farmers respectively, identified high cost of fertilizer as a major constraints to fertilizer utilization. Consequently, only 32 % maize farmers and 19 % cowpea farmers used fertilizer with average application rate of 18.45 kg ha-1 and 9.05 kg ha-1 NPK 15:15:15 (mostly used fertilizer type), respectively. The prevalent fertilizer application method and the fertilizer quantity used by the smallholder farmers were comparable to fertilizer micro-dosing. Awareness of fertilizer micro-dosing among the farmers was very low. The survey results therefore suggested that awareness creation and dissemination of fertilizer micro-dose technology were needed to minimize fertilizer input costs. This would promote fertilizer use, increase maize yield and income and subsequently improve the smallholder farmers‟ livelihood.
A Thesis presented to the Department of Crop and Soil Sciences, Faculty of Agriculture, Kwame Nkrumah University of Science and Technology, Kumasi, in partial fulfillment of the requirements for the award of the degree of Doctor of Philosophy in Soil Science,