Fertilizer-nitrogen Management in an Onion and Tropical Pumpkin Rotation in Puerto Rico

Abstract

Onion (Allium cepa) and tropical pumpkin (Cucurbita moschata) combined contribute 13% of the total gross agricultural income (GAI) for vegetable crops in Puerto Rico, which is estimated at $54.5 million. Both crops are usually rotated on an annual basis. In this study, an onion-tropical pumpkin rotation was used to test the effect of fertilizer-nitrogen (N) on agronomic indicators of onion (plant height, number of leaves per plant, leaf color index, and leaf nutrient concentration), yield of both onion and tropical pumpkin, and inorganic N changes in the soil profile. Three fertilizer-N levels (140, 196, 252 kg·ha−1) were applied to onion, followed by 112 and 280 kg·ha−1 of N applied to tropical pumpkin. For tropical pumpkin, N was applied in plots with the lowest and highest fertilizer-N levels from the previous onion crop. Changes in onion agronomic indicators with increasing N fertilization were either not significant or showed no clear trend. There was no increase in total and marketable yields and number of onions with increasing fertilizer-N levels. Tropical pumpkin yields significantly increased with 280 kg·ha−1 compared with 112 kg·ha−1 of N. Using 112 kg·ha−1 as a baseline fertilizer-N application, the value/cost ratio for tropical pumpkin was $12.70 per dollar of fertilizer-N. In low fertilizer-N plots, immediately available inorganic soil N (0 to 30 cm) did not change between the onion and tropical pumpkin crop, but then decreased at the end of the rotation. In high fertilizer-N plots, immediately available soil N greatly increased after onion, but then decreased at the end of the rotation. Potentially leachable soil N (30 to 100 cm) also increased after the onion crop and then decreased after pumpkin. However, in high fertilizer-N plots, potentially leachable soil N remained 44% higher at the end, compared with the beginning, of the rotation. The increased income attainable with the highest fertilizer-N in tropical pumpkin may be offset by greater residual soil N in the lower part of the soil profile, and the potential for this N to have a negative environmental impact.