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 inorganicNchanges 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 increasingNfertilization 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.