Influence of Soil Organic Matter on Cowpea's Response to N Fertilizer

There are divergent views on N application to legumes, especially cowpea. Results of various investigations in the tropics, especially Nigeria, have indicated either no response or a significant response to about 20 kg N/ ha. In Nigeria, a starter N is usually recommended irrespective of the cropping history of the field. As a result of these conflicting reports three field and one greenhouse experiments were carried out over a period of 4 years to investigate the effect of different levels of N, soil organic matter (O.M.), and split application of N on grain yield and performance of cowpea. Ife-Brown, an improved variety of cowpea, was used as the test crop. The soils of the fields used were moderately to strongly leached luvisols, derived from metamorphic and igneous basement complex rocks under forest vegetation. The experimental design for each field experiment was a randomized, complete-block design, while the greenhouse investigations was laid out in a split plot design. Cowpea was grown on fields having different cropping histories and levels of O.M. with various levels of N including no nitrogen as control. Cowpeas responded to N fertilizer; however, the level of O.M. affected the response. On soils having 0.5% O.M., grain yield was increased from 800 kg on the check to 1,850 kg dry grain per hectare, where 20 kg N/ha was applied, whereas on soils with 2% or more O.M. there was no constant response to N fertilizer. In the greenhouse, N fertilizer did not effect the time of flowering and pod initiation of cowpea but the total number of flowers and pods were significantly affected. When 33.6 kg N/ha was applied at planting or split into two equal applications applied 2 and 4 weeks after planting, there was a significant increase in dry matter accumulation but low grain yield. In the latter treatment the cowpea fixed 33.07 mg N per pot compared with 40.10 mg N when 33.6 kg N/ha was applied at planting. The treatment that produced the highest grain yield fixed the lowest N. Application of N at planting and 2 weeks after planting depressed nodule number up till the 4th week, but 10 weeks the highest number of nodules was observed on cowpeas from the plots where N was applied at planting. It is concluded, therefore, that starter N should be used in soils having less than 2% O.M. Such starter N should be applied not later than 2 weeks after planting. Time of N application affects nodulation and N fixation.