Determination of Soil Nutrient Levels for Maximum Yield of Okra (Abelmoschus esculentum) Using Sole and Amended Plant Residues

The determination of critical or optimum level of nutrients for a particular crop (i.e. soil calibration) is very important. An investigation was carried out in Akure rainforest zone of Southwest Nigeria to determine the critical soil nutrient levels for pH, organic matter (OM), N, P, K, Ca and Mg for okra crop using sole plant residues, wood ash, cocoa husk, spent grain, rice bran and saw dust amended with pig, goat and poultry manure. Twenty organic fertilizer treatments were compared to control (no treatment) and a reference treatment 400kg/ ha NPK 15-15-15 fertilizer in four experiments. Application of sole and amended plant residues at 2,4,6,8 and 10 t ha-^1 to soil increased soil N, P, K, Ca, Mg, Na, pH and OM, growth and pod weight of okra relative to the control. The wood ash, spent grain and cocoa husk were the most effective in improving soil nutrient contents for okra crop. The NPK fertilizer decreased soil OM, pH, Ca and Mg. The pod weight increased up till 6 t/ha under the different plant residues with R^2 valu e of 0.907 and decreased at 8 and 10 t ha^-1. Spent grain + poultry manure treatment gave the best performance in increasing all soil properties as well as yield and leaf area. For instance spent grain + poultry manure treatment had the highest okra yield (4555.5 kg ha^-1) with critical or optimum nutrient combination 5.2% N, 51.03 mg kg^-1 P, 0.78 mmol kg^-1 soil K, 0.43 mmol kg^-1 soil Ca, 0.0 mmol kg^-1 soil Mg, 2.96 mmol kg^-1 soil Na, 3.2% soil OM and 6.9 for soil pH respectively.