Performance of Soybeans (Glycine max (L.) Merrill) in the Wet Tropics Affected by N, P, and K1

The cultivation of soybeans [Glycine max (L.) Merr.] in wet, humid, equatorial countries that have short days is relatively new. Little is known of the agronomic requirements for maximum production of the few adapted varieties. Trials were set up on Arakaka clay loam (a yellow podzol, Typic Normudult) to determine soybean performance from 1) application of N, P, and K and 2) source of N. The variables were: nitrogen, 0, 22, and 44 kg/ha; phosphorus, 0, 29, and 58 kg/ha; and potassium, 0, 37 and 74 kg/ha. Urea, ammonium sulphate, and uramite were the sources of N. Nitrogen significantly influenced yields only during one particularly wet season, when it caused a decrease. Potassium significantly increased yields only during this same period of heavy rainfall, with the effect being greater at the higher application of N. Yields were increased by P at all harvests up to application rates of 58 kg/ha for the first two crops and 29 kg/ha for subsequent crops. The N × K interaction was the only two‐factor interaction that consistently had any significant effect on yield. Source of N had no effect on yield. Oil and protein contents of the seeds were not influenced by N, P, or K. Truog‐P in the soil was unaffected by the application of N or K. Fertilizer P and K increased their levels in the soil. Results indicated clearly that adapted soybean varieties can be successfully cultivated in the wet tropics.