Effect of organic matter on mineralogical changes of clay sized phlogopite and muscovite in alfalfa rhizosphere

Weathering of K-bearing minerals as the major source of potassium in soils is of special importance under K deficient conditions. Many studies have been carried out on the effect of plant types and microorganisms on the transformation of micaeous minerals and their potassium release. However, there is no information on the effect of organic matter on mineralogical changes in micaceous minerals. The objective of this study was to investigate the effect of organic matter on transformation of muscovite (a dioctahedral mica) and phlogopite (a trioctahedral mica) in the rhizosphere of alfalfa. A pot experiment was carried out in a completely randomized design with factorial combination and three replicates. The growth medium was a mixture of quartz sand, micaceous mineral (muscovite or phlogopite) and organic matter (0, 0.5 and 1 %). During 120 days of cultivation, plants were irrigated with either complete or K-free nutrient solution and distilled water as needed. At the end of cultivation, plants were harvested and their K uptake was measured by flame photometer following the dry ash extraction. Also, the mica particles and their weathering products in each pot were separated from the quartz sand and their clay fraction analyzed using x-ray diffraction (XRD). The results showed a significant increase of total K uptake occurred in pots containing trioctahedral mica (phlogopite) and organic matter as compared to those with no organic matter amendment, under both nutrient solution treatments. XRD data clearly showed the transformation of phlogopite under both nutrient solution treatments, but no XRD detectable transformation of muscovite was recognized. Organic matter amendment seems to have created considerable mineralogical changes in clay sized phlogopite. Root activities and organic matter decomposition appear to have increased the acidity of rhizosphere which, in turn, facilitated the K release from trioctahedral mica (phlogopite) and induced the transformation of phlogopite to vermiculite and a minor quantity of smectite and chlorite. In conclusion, the effect of organic matter on mineralogical changes greatly depends on the type of micaceous mineral.