Morphological characteristics and distribution patterns of lowland soils in the Philippines

Abstract

Thirty lowland soils from six lowland rice areas were selected for the study. Among these soils, 13 were considered to be irrigation water aquic soils, while seven ground water aquic soils, based on the morphological characteristics. The irrigation water aquic soils showed cloud-like ferruginous mottles inside the matrix of grayed subsurface horizons, while the ground water aquic soils displayed ferruginous coatings and/or hypo-coatings of voids, corresponding to tubular or filmy mottles. Hence, the difference in ferruginous pedofeatures in the subsurface horizon between the two types of aquic soils was confirmed. By setting a value of < 100 mm rainfall for the dry month, the ground water aquic soils were observed only in relatively humid areas with ≦4 dry months per year. The change in topography from natural levee to back swamp in Central Luzon with >4 dry months per year was associated with increasing influence of irrigation water, while in Bicol with <2 dry months per year, with increasing influence of ground water. Thus, the importance of rainfall and topographic conditions for the lowland soil distribution was verified. Five orders were identified according to the system of U.S. Soil Taxonomy. Most of the irrigation water aquic soils were classified into Tropaquepts, while most of the ground water aquic soils into Fluvaquents. Since many soils examined were characterized by a clayey nature rich in swelling clays, two soils were classified into Vertisols and 17 soils into vertic subgroups. Three soils with an argillic horizon were separated into Aqualfs. Two Aquolls with a mollic epipedon were also found.