The chemical properties of acid soils with emphasis on soils of the humid tropics

Abstract

- it is advisable to measure suspension pH in a solution that has the characteristics of the soil solution in the field - strong relationship between electrical conductivity (EC) of a 1:5 soil suspension and ionic strength (IS) of the soil solution at 0.1 bar (acc. to Gillman and Bell, 1978): IS = 0.0446 x EC - soil pH could be standardized by using 0.002M CaCl2 electrolyte; the IS of this solution (0.006) is close to IS found in highly weathered soils (0.005); alternatively EC in a 1:5 soil suspension could be measured, IS calculated acc. to formula above and sufficient CaCl2 added to bring IS of suspension to IS of calculated soil solution - caution is required when interpreting low pH, because, though it is normally linked to possible problems with Al toxicity, this can be alleviated by sufficient quantities of SOM The properties of acid soils are discussed with respect to soil pH, exchange properties and sulfate sorption with emphasis on soils from the humid tropics. Laboratory methods for determining soil pH and cation exchange capacity are considered and the application of such data to soil management is discussed. Sulfate sorption mechanisms for variable and constant charge minerals are distinguished. The role of surface charge in aluminium retention and its influence on the amount of total aluminium in the soil solution is discussed.