Soil interpretation for non-agricultural and agricultural uses in the soils of the Benchmark Soils Project

Abstract

Variable charge clay soils are widely distributed in the intertropical region. They primarily consist of Andisols, Oxisols, and low activity clay Alfisols and Ultisols. The last three groups of soils are collectively called variable charge, low activity clay soils. Due to insufficient knowledge of these soils, and inadequate analytical methods to study, classify and interpret them, variable charge, low activity clay soils have not been used as efficiently as the permanent charge, high activity clay soils of the temperate region. Several International Committees have been established to revise the Soil Taxonomy and make it more accurate for making and interpreting soil survey for the tropics. Although progress has been made in improving Soil Taxonomy for making soil surveys, the improvement for interpreting soils for specific uses has lagged behind. The National Soils Handbook used by the Soil Conservation Service contains the procedures for translating soil information for the soil user. These procedures are mainly based on experience with temperate climate soils of the continental United States. For this reason problems arise when the same procedures are used on tropical soils. The current procedures for estimating soil properties from other properties and their use in soil interpretation for engineering and agricultural uses are tested on the soils of the Benchmark Soils Project. It was recognized that the current procedures for estimating soil properties and rating soils are not suited for variable charge, low activity clay soils, and Andisols with variable charge, short-range order clays. The latter represents a unique group of soils which must be treated and interpreted differently. In order to retain simplicity and accuracy in the estimation procedure, it was necessary to stratify soils according to the activity of their clays as expressed by the relationship: % clay = CEC/Ac = PI/Ap, where CEC is cation exchange capacity, PI is plasticity index, Ac and Ap are chemical and physical clay activity, respectively. Soil data from the International Soil Classification Workshop Tour Guide from Sudan, Chili and Ecuador, and data from soil survey reports from Indonesia were used to develop the equations for estimating soil bulk density, available water capacity, liquid limit, plasticity index, coefficient of linear extensibility and hydraulic conductivity. Due to limited data, only three categories of soils were taken into account. They were (1) Andisols with variable charge, short-range order clay, (2) soils with variable charge, low activity clays and (3) soils with permanent charge, high activity clays. The measured and estimated soil properties were used to rate soils for use as septic tank absorption fields, dwellings without basements, local roads and streets, roadfill, topsoil, playgrounds and irrigation. Improved procedures for estimating soil properties and pitfalls 1n rating Andisols and variable charge, low activity clay Oxisols and Ultisols are discussed.