Abstract To analyze the influences of prolonged grassland conditions after deforestation on tropical soils, soils under a natural Dipterocarp forest and grassland in Central Luzon, the Philippines were compared with each other in terms of morphology, clay mineralogy, physicochemical properties, nitrogen fertility, and humus composition. The influence of forest land degradation on the soil was limited to the shallow superficial horizon in the case of the Plinthic Acrisols, but extended to the E or the upper argillic B horizon for the Orthic Luvisols. The soils of natural forest and grassland differed from each other in the following characteristics: 1) The forest soils exhibited strong structural development and various signs of high activity of the soil fauna unlike the grassland soils. 2) The physical properties of the grassland soils had deteriorated compared with those of the forest soils. 3) Comparison of the clay distribution pattern in solum suggested the existence of a weaker clay eluviation and/or...