Weathering of soil minerals and distribution of elements: Pedochemical aspects1
Weathering of soil minerals is a key determinant of soil formation, rhizosphere processes, ground and surface water quality, and release of essential as well as toxic elements. It is coupled with rise and fall of atmospheric CO2. In association with pedogenic processes, it determines the transport and buildup of elements across the genetic horizons, and accumulation of elements in surface and ground water bodies. In the last few decades, public concern with these issues has ensured enormous research activity in this youthful arena.
In the early twentieth century, attempts were made to develop general schemes of the weathering sequence of soil minerals either from their formation history, or from dissolution in weak acids, or from thermodynamic data. These schemes proved inadequate because they did not take into consideration weathering environment, and secondary minerals.
Weathering environment can best be explained on the basis of Al2O3-SiO2-H2O system; H4SiO4 for the alkaline and neutral soils, and Al2O3 for the acid soils. pH is always an essential variable, and rate limiting ion(s) can be deduced from mineralogical assemblages. The most popular method of calculating weathering state remains loss or gain of elements. For the profile distribution of elements, some workers attribute it to pedogenesis, and others insist for a random association.
Weathering is a function of pedochemical processes that involve factors as well as processes of soil formation. Some of the agents that cause rapid weathering are high-yielding crops and fast-growing tree-species (fast uptake through their rhizosphere-soil interface), acid and aerosol precipitation, accretion of industrial waste, irrigation and organic farming, and green house gases. While zeoponics provides a perfect equilibrium condition in a closed system, natural eco-systems are open non-equilibrium ones. Some of the weathering phenomena are useful for carbon sequestration, supplying micro and rare-earth elements to plants, testing theories of soil genesis, verifying life in extra-terrestrial planets, and deducing past climates.