SIGNIFICANCE OF MACROPOROSITY AND HYDROLOGY FOR SOIL MANAGEMENT AND SUSTAINABILITY OF AGRICULTURAL PRODUCTION IN A HUMID-TROPICAL ENVIRONMENT

Abstract

This paper analyzes soil-related agronomic constraints in the Sitiung region of Indonesia that are directly related to low nutrient-holding capacity, macroporosity, and rainfall regime. This region receives 2500 to 3000 mm of rainfall per year, but nearly 50% of the rainfall is disposed of rapidly via internal drainage. Although rapid internal drainage reduces the risks of erosion, it leads to infertility, acidity, and Al toxicity. The physical structure of the soils is characterized by stable aggregates, with numerous macropores in the surface and a predominantly microporous subsoil matrix interspersed with a few larger macropores. Macropores account for about 29% of porosity in the surface and between 3 and 6% in the subsoil. The saturated hydraulic conductivity of the matrix containing macropores averages about 300 to 400 cm/day, whereas that of the microporous matrix is generally <1 cm/day. The structure facilitates rapid infiltration and leaching of rainfall. However, little opportunity exists for nutrients moving downward with drainage water to accumulate in the subsoil. The main reason for this seems to be the low hydraulic conductivity and the preponderance of excessive wetness in the subsoil. Drying seems to be essential for movement of nutrients into the subsoil matrix. However, most of the agronomic crops are sensitive to Al toxicity and fail to grow roots deeper than 10 to 15 cm. Thus, they suffer from water stress, despite heavy and frequent rainfall, and fail to cause drying of the subsoil. Problems of acidity, Al toxicity, and infertility worsen progressively where agricultural production consists mainly of Al-sensitive crops. Although liming with calcium carbonate improves the soil chemical environment, downward movement of lime is very slow. Deep liming is effective in improving rooting depth, crop water availability, and drying of the subsoil, but the technology is cost- and labor-intensive. Native vegetation, on the other hand, is able to grow roots to considerable depths and causes significant drying of the subsoil, even without soil amendments. Thus, production systems in which locally adapted vegetation of economic value is the main focus seem to be more sustainable and conducive to improving soil conditions.