Effects of organic versus conventional fertilizers on insect pests, natural enemies and yield of Phaseolus vulgaris

Field studies were conducted during 2003 and 2004 for three consecutive growing seasons. The treatments were (1) market crop wastes (MCW) compost incorporated in the soil, (2) un-composted MCW incorporated in the soil, (3) un-composted MCW applied as a surface mulch, (4) a conventional chemical fertilizer (NPK) incorporated in the soil, and (5) the untreated control. Response variables recorded included plant height and width, leaf area, tissue nitrogen content, nodulation; occurrence of Aphis fabae, Maruca vitrata and the associated natural enemies; and grain yield. Results indicated that significant differences in plant attributes and yield were only detected in the second and subsequent season of the crop or when weather conditions were stressful. For the insect pests, it was only A. fabae infestation that varied among treatments with MCW amended plants sustaining higher infestations than NPK and the untreated control, a trend that held for all the seasons. Natural enemy occurrence followed the trend of A. fabae infestation. After the first and subsequent harvests, more P, K, Ca, and Mg were extracted from the soil from MCW plots than NPK plots. The study indicated that yields accruing from MCW amended plots were comparable and sometimes even higher than those from NPK despite the higher A. fabae infestation of the former.