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An application of up to 120 kg/ha of nitrogen fertilizer in the dry season improved the milling quality of the chalky varieties ‘IR8,’ ‘IR5,’ and ‘Sigadis’ but not that of the nonchalky variety ‘C4‐63.’ An application of up to 60 kg/ha of nitrogen increased the protein content of brown rice of the four varieties but increased the head rice yield of only Sigadis, the most chalky of the four varieties; the head rice yield of the other three varieties remained unchanged. Irrespective of variety and nitrogen level, the optimum time of harvest of transplanted rice for obtaining maximum grain and head rice yields and highest germination percentage was between 28 and 34 days after heading in the dry season, and between 32 and 38 days in the wet season. These heading periods correspond to moisture contents of between 22 and 19% and between 21 and 18%, respectively. Lodging increased grain yield losses in both seasons, with the losses being greater when harvested late; therefore, lodging‐susceptible varieties should be harvested earlier than lodging‐resistant varieties. Nitrogen fertilization in the dry season delayed by 2 to 4 days the optimum time of harvest of the improved varieties IR8, IR5 and C4‐63.

The equilibrium phosphate sorption and release behavior and the kinetics of adsorption were studied in some acidic soils of South and Southeast Asia. Phosphate sorption data at 25 °C were fitted to the Tempkin, Freundlich, and Langmuir adsorption isotherms. The soils varied widely in their capacity to sorh and retain P. During desorption, the amount of sorbed P at a given equilibrium P concentration was always higher than that during sorption, indicating low desorbability of the sorbed P. The kinetic P sorption data at 21 and 35 °C were fitted to a modified Freundlich type of kinetic equation to obtain a parameter related to the sorption rate coefficient. The effect of temperature on the kinetics of sorption was small, suggesting the process is diffusion controlled. Equilihrium and kinetic P sorption parameters were found to he closely correlated with clay and organic matter contents and surface area of soils. A negative correlation between specific P sorption by clay or organic matter and clay or organic matter content of the soils suggests an association between the two colloidal components of the soils, causing loss of effective P‐sorhing surface area. An attempt was further made to evaluate the differential change in free energy of P sorption at infinite dilution from the corresponding thermodynamic equilibrium constant for P sorption by the given soils. The results suggest that the stability of the P sorption reaction products in the given soils contribute to the P‐fixing characteristics of these soils.