Role of weathering of fine-grained micas in potassium management of Indian soils

Abstract Although the major soils of India (alluvial, ferruginous and black soils), endowed with fine-grained micas, are known as natural K suppliers to plants, crop response to K fertilizer treatment has been anomalous. The silt and clay fractions of 14 benchmark soils were subjected to repeated batch type Ba–K exchange to study the rate of K release. The significant positive correlation between cumulative K release in 25 extractions and X-ray intensity ratio of peak heights of the 001 and 002 basal reflections of mica in the silt ( r =0.629 at 1% level) and clay ( r =0.729 at 1% level) suggested that although the fine-grained muscovite and biotite co-exist in soils, the rate of K release and crop response to K are related primarily to the presence of biotite only while muscovite remains as an inert source of K in soils justifying the crop response to K fertilizers. Soil micas under a scanning electron microscope (SEM) indicated that biotite particles were generally thick, showing layer separation with bending at edges due to the formation of vermiculite around their rims. The muscovites, on the other hand, were characterized by very minor layer separation at their edges. To resolve the issue of layer separation of micas through the release of K during the formation of soils, experimental studies of repeated Ba–K exchange were conducted with specimen muscovite and biotite and also on the mixture of the two micas. Experimental studies indicated that at ambient temperature and pressure, muscovite released negligible amount of K from its exposed surfaces and the weathering of muscovite was inhibited in the presence of biotite. This confirmed the observation that when muscovite and biotite co-exist, muscovite hardly weathers in soil environments. Hence, its enrichment in soils does not enhance the supply of K. Therefore, weathering products like di- and trioctahedral vermiculite and smectite are generally the products of the weathering of biotite mica. The X-ray intensity ratio of 001 and 002 basal reflections of mica has been found to be an effective diagnostic parameter to find out the quality of mica and therefore to judge the K releasing potential of soils. However, actual quantification of fine-grained biotites of soils appears to be mandatory for the precise determination of K reserves in soils in order to predict release of K for sustainable K management.