Nitrogen Fertilization of Corn and Sorghum Grown in Oxisols and Ultisols in Puerto Rico

The requirement of fertilizer N for high yields, its generally high price, and the ever-present possibility of large leaching losses of inorganic N makes it essential that fertilizer N be managed as efficiently as possible in the humid tropics. Three rates and 2 times of application of fertilizer N were applied to 15 corn (Zea mays L.) and 3 sorghum (Sorghum bicolor Moench) crops on 2 Ultisols and 3 0xisols during 5 successive growing seasons in Puerto Rico to determine the most effective means of applying fertilizer N to these crops. Additional objectives were to determine the apparent recovery of fertilizer N by the crops; the interrelationships between crop yield, N fertilization and soil inorganic N content; and the effectiveness of slow-release sulfur-coated urea as a source of fertilizer N for corn and sorghum. All soils had been under cultivation for at least 50 years and received relatively high rates of fertilizer for the last 20 years. The results indicated that: 1) postplant sidedress applications of fertilizer N resulted on the average in higher yields and plant recovery of fertilizer N than preplant applications; 9) when there were no limiting factors of climate or disease, and there was a response to N, the recovery of postplant applied N was comparable to that observed in temperate areas; 3) maximum corn grain yields were approximately 6.3 metric tons/ha; near-maximum corn grain yields were obtained with 67 kg/ha of postplant-applied N; 5) preplant-applied sulfur-coated urea was no more effective than preplant urea in increasing yields or N recovery; 6) soil inorganic N content was generally not a good index of soil N supplying power in these soils; 7) drought and disease frequently reduced yields; 8) there was very little residual effect of the fertilizer N applied in this experiment; and 9) the clayey Oxisols and Ultisols in Puerto Rico had a relatively high N-supplying power.