Green manures and NPK fertilizer effects on soil properties, growth, yield, mineral and vitamin C composition of okra (Abelmoschus esculentus (L.) Moench)

The nutritional effects of green manure on soil and crop plants depends on residue quality. Hence field experiments were carried out during the cropping seasons of 2015 and 2016 to compare the impact of different green manures and NPK 15-15-15 fertilizer on soil properties, growth, yield, mineral and vitamin C composition of okra (Abelmoschus esculentus (L.) Moench). The experiment each year consisted of four green manure (GM) types {Pawpaw (Carica papaya L.) leaves, Neem (Azadirachta indica A. Juss.) leaves, Moringa (Moringa oleifera Lam.) leaves, and Mesquite (Prosopis africana Guill., Perr. & A. Rich) Taubert leaves}, NPK 15-15-15 fertilizer and a control. The six treatments were arranged in a randomized complete block design with four replications. Application of GMs reduced soil bulk density and increased soil organic matter (OM), N, P, K, Ca, Mg, growth and yield of okra compared with the control. NPK fertilizer did not reduce soil bulk density and increase soil OM, but did increase soil N, P, K, Ca, Mg, growth and yield of okra compared with the control. Mesquite increased growth and yield of okra compared with NPK fertilizer and other GMs. This was due to increased availability of N and K in the soil at the level of this treatment. Using the means of the two years and compared with the control and NPK fertilizer, Mesquite increased pod yield of okra by 214 and 53%, respectively. Also GMs and NPK fertilizer increased okra mineral and vitamin C contents compared with the control. Moringa had the best fruit quality in terms of higher K, Ca, Fe, Zn, Cu, and vitamin C contents compared with other GMs and NPK fertilizer. Therefore, for those that desire to grow okra for the quality of its fruits, Moringa (Moringa oleifera) is recommended, however for those that desire quantity, Mesquite (Prosopis africana) is recommended. Keywords: Bulk density, Moringa oleifera, Carica papaya, Azadirachta indica, Prosopis african