Nitrogen fixation by cowpea as influenced by the stage of growth and duration of crop

Abstract

A field experiment was conducted in the College of Agriculture, Vellayani to study the quantity of nitrogen fixed by, cowpea at various stages of growth and also to find its residual effect on a succeeding crop of fodder maize The field trial was laid out as RBD and split plot design with six replications. The study revealed that the growth characters and dry matter production of cowpea increased with increasing age of the crop. The content of plant nitrogen decreased with advancing age eventhough the total uptake of nitrogen » showed an increasing trend from 579 g/plot at the maximum flowering to 657 g/plot at the maturity stage of the crop. Uptake of other nutrients like phosphorus, potassium, calcium and magnesium also showed an increasing trend towards the maturity stage of the crop. The ratios of these nutrients with respect to nitrogen narrowed with increase in the duration of the crop. The total residual nitrogen status of the soil recorded a gradual increase towards the maturity stage of the crop. The value increased from 0.0514 per cent at the maximum flowering stage to 0.0553 per cent at the maturity stage. The available nitrogen status of the soil at various stages of growth of cowpea remained unaltered. The quantity of nitrogen fixed by cowpea was found to be 23, 67 and 112 kg/ha at the maximum flowering, mid pod filling and maturity stages of the crop respectively. All these indicate that nitrogen fixing processes in cowpea are active during its entire growth period gaining pace after the maximum flowering stage of the plant. The highest residual effect due to nitrogen fixed by oowpea was obtained from plots where It was grown upto maturity as seen from the general growth characters and fodder production of maize. Uptake of the nutrients such as nitrogen, phosphorus and potassium in fodder maize was also highest in plots where oowpea was grown upto maturity.The nitrogen made available from the decomposition of plant residues at any of the three stages of cowpea was always lower than that obtained from the addition of 30 kg M/ha.The highest fodder yield (20.7 t/ha) was obtained from plots which. received an additional amount of 60 kg U/ha over and above the residual nitrogen in.the soil where cowpea was grown upto maturity. The residual effect of nitrogen in the soil where cowpea was grown .upto maturity was reckoned on equivalent to the effect of 60 kg N/ha. The study has brought out the fact that maximum nitrogen gain through fixation, by growing legume crops can be achieved only if the plant is allowed to complete its full growth period.