David R. Sotomayor, PO Box 9030, University of Puerto Rico at Mayaguez, Mayaguez, PR, Miguel Oliveras, Crops and Agroenvironmental Sciences, University of Puerto Rico, Mayaguez, Mayaguez, PR and Remy Rodriguez, Crops and Agroenvironmental Sciences Dept., University of Puerto Rico, Mayaguez, PR
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.