| Summary: | Eighteen months old coffee seedlings grown in greenhouse were subjected to reducing soil water suctions. Six Costa Rica soil series representing two each of andosol (Birrisito and Cervantes), latosol (Paraíso and Colorado), and aluvial (Margot and Instituto) soils were used. Oxyen diffusion rates (ODR), soil water suction and soil temperature near the root zone were measured daily. Independent evaluation of air-filled porosity (Ea) was made from moisture content and bulk density data during the growing soil moisture range of the crop. Aeration porosity (Ea) and oxygen diffusion rate were influenced by soil water suction in all soils studied. The degree of the magnitude of this influence may have varied with soil textural class, water retention capacity, compaction and root physiology of the coffee. The critical range of ODR of 20-40x10-8 gm/cm2/min. correponded to soil suction range of 6-38 cb for Margot, 14-? cb for Institute, 22-? cb for Paraiso, 24-66 cb for Colorado, 8-46 cb for Birrisito and 14-56 cb for Cervantes soil series. Whereas the soil suction (Ys) values at which aeration porosity reached its minimum critical level of 10 per cent were 12 cb for Margot, 8 cb for Institute, 12 cb for Paraiso, 25 cb for Colorado, 12 cb for Birrisito and 18 cb for Cervantes. There was somewhat linear relationship over wider range of porosities and oxygen diffusion rates for most of the soils studied. ODR seems to be a sensitive measure of soil aeration over a wet range of soil moisture conditions and is recommended as a good field and greenhouse method for characterizing aeration status for the purpose of drainage in all soils. Aeration porosity seems to be a good method in coarse textured soils
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