Growth and Responses of Castor Oil Plant to Sources and Rates of Organic Manures in Ferralitic Soils

Four organic manures and NPK fertilizer, each at four rates, were assessed under field conditions for their comparative effects on the growth and yield of castor oil plant. Castor seed yields (5.11 and 5.01 t ha−1) were greatest with poultry or swine manure applied at the rate of 10 t ha−1. Yields of 4.01 or 4.00 t ha−1 were obtained with fresh rice husk dust or burnt rice husk dust applied at 20 t ha−1, while with NPK the highest yield (3.84 t ha−1) was obtained with the formulation of N50 P40 K40 kg ha−1. Very high manure application of 30 t ha−1 depressed growth and yield, irrespective of the manure sources. The potential fertilizer values of the organic manures were not fully reflected by early growth parameters as they were with NPK treatment, apparently due to slow release of the elements bound in organic forms in the manures. The ultimate yield advantages associated with the organic manures compared with NPK were, in part, ascribed to their probable effects on the soil physical characteristics, and their supply of macro- and micro-nutrient elements not contained in NPK fertilizers.