Fertilizer-Nitrogen Management in Onion-Tropical Pumpkin Rotation in Puerto Rico.

Onion (Allium cepa var. cepa L.) and tropical pumpkin (Cucurbita moschata) contribute about 13% of the total gross agricultural income for horticultural crops in Puerto Rico which is estimated at about $54.5M.  Both crops are usually rotated on an annual basis.  An experiment was conducted to test the effect of three fertilizer-N levels (140, 196, and 253 kg N/ha) on onion yield, followed by fertilizer-N rates of 100, 175, and 250 kg N/ha on pumpkin yield, on the same plots. Prior to onion planting, initial immediately (0-15 plus 15-30 cm) inorganic soil N (NO₃-N + NH₄⁺-N) ranged from 138 to 361 kg N/ha for the three N levels.  After onion production, total profile inorganic N (0-90 cm) did not change significantly by fertilizing with 140 and 196 kg N/ha, but increased by 150% with the highest fertilizer-N treatment with a mean of 474 kg N/ha.   Most of the onion agronomic indicators (plant height, number of leaves per plant, leaf color index,and leaf %N) tended to increase with fertilizer-N though these were non-significant (P>0.05).  SAP nitrate was significantly higher with the 253 kg N/ha fertilizer treatment.  There was a non-significant increase in total and marketable yields and number of onions with increasing fertilizer-N levels.  The mean marketable yields were 34,416 kg/ha with 185,548 onions/ha.  About 83% of the onion size was in the medium and large size-classification.   Using 140 kg N/ha as a baseline fertilizer-N application, the value/cost ratio was 8.4 with 253 kg N/ha.  The improved income with the highest fertilizer N is offset by greater residual soil N and losses to the environment.