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 Oxisols 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; 2) 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.

An attempt was made to evaluate the N supplied by crop residues through crop rotation experiments on Oxisols and Ultisols. Field experiments were conducted on three typical soils following a split-plot design. Main plots were three rotations: soybeans, corn , corn; fallow, corn, corn; and continuous corn. Subplots included two treatments: 0 and 110 kg/ha of fertilizer N. Fair yields of soybeans were obtained while corn yields were good, especially on the Humatas soil (Ultisol), 6240 kg/ha. Substantially higher yields were obtained in the Bayamón (Oxisol) and Humatas soils from the first corn crop following soybeans or corn than when following fallow. Although the second corn crop yield following soybeans was slightly higher than the first, the second corn crop after initial corn and fallow were substantially higher. The effect of applied N at all sites was striking, regardless of the previous crop. There was no apparent relationship between the amount of N returned to the soil and yields of subsequent corn crops.

Relationships between rooting depth, growth, and yields of corn and soil water availability in an Ultisol and an Oxisol are presented. Soil and crop management techniques considered to be adequate for maximum yield were used at the two experimental sites. At each site one treatment was irrigated as often as necessary to maintain a water tension of less than 1 bar. Soil properties, plant growth, effective rooting depth, soil water availability, plant water deficits, crop yields, and weather conditions were recorded. During a prolonged drought and at grain filling, corn growing in a typical Oxisol extracted soil water to a depth of 120 cm, and plants became water stressed after a soil water tension of 15 bars developed at 90 cm. In a typical Ultisol plants could not extract water effectively below 30 cm. Plants grew better in the Oxisol than in the Ultisol. Grain yields approached 9.4 tons/ha in the former but only 3.1 tons/ha in the latter.

Experiments were conducted on two Ultisols in the interior of Puerto Rico to determine the fertilizer N requirements of a high yielding, high root protein variety of cassava (Manihot esculenta Crantz cv. Llanera) from Colombia. This variety appeared to be daylength sensitive and top growth ceased from mid November to mid-February, when days were shorter than 11 1/2 hours. At Corozal (Humatas clay), the crop was grown from mid- March to mid-December and top growth responded strongly and root growth moderately to applied N. High N rates produced low root/top ratios (.67-87) but, due to the experience at the Cidra site, it is assumed that root/top ratios would have increased to higher levels had the crop continued to grow. Apparent recovery of fertilizer N was high (68-69 percent) for the 40 and 80 kg/ha rates. At Cidra (Torres clay), where the crop grew from mid-June until the following May and was dormant for the three winter months, there was no response to N, the top yields were lower but the root/top ratios were higher (average, 1.37) and were not affected by N rates. The lack of response to N was due to the high N supplying power of this soil. The maximum root yields were 23 metric tons/ha at Corozal and 33 metric tons/ha at Cidra. Plants in the highest yielding treatments contained 125-175 kg/ha N. Assuming all N was protein N, unpeeled root protein contents averaged 3.4 percent and peeled 2.3 percent on a dry weight basis and were increased only slightly by higher N rates. The N content of the 4th and 5th fully expanded leaves 4-5 months after planting was very well correlated with final root yield at Corozal (R2 = .99 for 0-160 kg/ha N rates). It appears that a minimum of 5 percent N in these leaves 2 1/2 months after planting is necessary for maximum yields.

PRELIMINARY EVALUATION OF TWO NEW HIGH-QUALITY PROTEIN OPAQUE-2 CORN VARIETIES ON AN ACID AND RELATIVELY INFERTILE SOIL (ULTISOL) OF PUERTO RICO