Effects of continous cropping and NPK fertilizer on growth and yield of cowpea (Vigna unguiculata (L) Walp) grown on soils from different management systems

Studies were conducted to investigate the effect of continuous cropping and NPK fertilizer on the growth and yield of cowpea ( Vigna unguiculata (L) Walp) grown on soils from different management systems. The investigations were carried out in two parts. The first dealt with the effect of continuous cropping while the other focused on the effect of NPK fertilizer on cowpea. Representative soil samples were obtained at 0-15 cm depth from land which had been continuously cropped, under forest tree fallow, and secondary bush fallow. These were bulked, processed, and analyzed. Six (6) kg of each type of soil were weighed into 25 x 22 cm polybag of 500 gauge. The continuous cropping experiment involved three cropping cycles of 35days each of cowpea on the same soils from 3 cropping systems. In the second experiment, a 2 x 3 factorial involving 2 levels of fertilizers (0, 40 kg NPK/ha) and 3 sources of soil were carried out to test their effects on cowpea. The growth parameters measured were plant height, number of leaves/plant and number of branches/plant. Number of nodules/plant, number of pods/plant, dry matter and grain yield were also estimated. Successive cropping of soils significantly reduced cowpea growth. Cowpea plants grown on soils from continuously cropped land had the least growth performance. Number of nodules/plant increased with successive cropping. Higher dry matter yield was obtained from the continuously cropped land. A significant (r = +0.55) positive correlation was found between soil P and dry matter yield of cowpea. NPK fertilization had only significant effect on plant height at 10 WAP but not on other growth parameters measured. The highest grain yield was obtained from unfertilized secondary bush fallow soil. Sources of soil (soil management system) had significant effect on some of the agronomic characters of cowpea.