Nitrogen and potassium fertilizer vs. nematode and weevil effects on yield and foliar nutrient status of banana in Uganda

Banana (Musa AAA) production is declining in central Uganda. A decline in soil fertility is often cited as one of the causes of the declining production. From 1996 to 1998, we studied banana yield responses to N and K fertilization at three sites in Uganda, where plantations ranged in age from 8 to 50 years. Phosphorus was applied to all treatments at 25 kg ha−1, while N and K were applied (kg ha−1) at 0N-0K, 0N-100K, 100N-0K, 100N-100K, and 100N-200K. Fresh fruit yields ranged from 4 to 67 Mg ha−1 yr−1. At one site the yield response to 100N-100K fertilization was significant (p < 0.01). The other sites showed positive but non-significant yield responses to N or NK fertilization. Economic analysis indicated that fertilizer use would be profitable only at the single site with significant yield increases. This site had negligible banana weevil (Cosmopolites sordidus (Germar)) and nematode (Radopholus similis; Helicotylenchus multicinctus) populations whereas the other sites were characterized by either severe nematode or weevil pressure. Fertilizer did not affect the damage caused by either nematodes or weevils. The best predictor of yield (p< 0.001, r 2 = 0.75) was the number of functional leaves during anthesis. Measured soil nutrient parameters were highly variable but fell within sufficiency ranges. Foliar N contents were near diagnostic norms, but K levels fell below them, even when yields were satisfactory, suggesting that norms developed from other regions may not be suitable for Uganda. We conclude that in established plantations soil nutrient status is but one, and often a secondary, factor in the decline of banana yields in the central region of Uganda. Pest and disease infestations that result in reduced plantation productivity will need to be addressed before fertilizer use is likely to make a significant improvement in banana fruit yields.