Mineralogy and ion exchange properties of the particle size fractions of some Brazilian soils in tropical humid areas

The inorganic fractions of 25 samples from 12 representative Brazilian soils were separated into six subfractions (clay, silt, very fine sand, fine sand, medium sand, coarse+very coarse sand) and their effective cation and effective anion exchange capacity (ECEC and EAEC, respectively) evaluated by the compulsive exchange method using unbuffered BaCl2 solution. The soils had developed from a range of parent material, such as sandstones, Tertiary unconsolidated sediments, siltstones, silty shales, basalts, metapelites and mica schists. The silt fraction of sandstone-derived soils had an ECEC of at most 2 mmolc kg−1 while the silt fraction of basaltic soils had ECEC which reached 106 mmolc kg−1. The silt fraction of the soil derived from mica schists had an EAEC of 9 mmolc kg−1, which was the highest of all the soils analyzed. The highest ECEC recorded was close to 300 mmolc kg−1 for the fine sand fraction of soil derived from silty shales. Sand fractions from Tertiary unconsolidated sediments were practically inert, while those from sandstones made only a small contribution to the effective exchange capacity. The origin of these differences may lie in the characteristics of the parent materials of the soils. Primary micas, illite and feldspars together with inactive quartz were the major components of the coarse fractions of the soils. We attributed the high ECEC of the coarse fractions to several factors, including feldspars and mica being subjected to intense weathering, accelerated cation release from the mineral structure, and silt and sand-sized clay mineral pseudo-aggregates. In highly weathered tropical soils, the coarse fractions can play an important role in active soil fertility because they are not chemically inert and in potential fertility because of their importance as reservoirs of nutrients.