Organic matter induced mineralogical changes in clay-sized phlogopite and muscovite in alfalfa rhizosphere

Weathering of micaceous minerals is the major source of potassium in soils. Many studies have shown the beneficial effects of organic matter (OM) on soil physical, chemical, biological, and nutritional properties. However, there is no information on the effect of OM on the breakdown of K-rich micaceous minerals. The objective of this study was to investigate the transformation of muscovite and phlogopite as influenced by added OM in the rhizosphere zone of alfalfa. Growth medium was a mixture of quartz sand, micaceous mineral (muscovite or phlogopite) and OM (0, 0.5 and 1% cocopeat on a weight basis). During 120 days of alfalfa cultivation, plants were irrigated with either a complete or a K-free nutrient solution and distilled water as needed. At the end of the experiment, plants were harvested and their K uptake was measured by flame photometer following dry ash extraction. Micas and their weathering products were separated from the quartz sand to analyze their clay fractions using X-ray diffraction (XRD). Results showed a significant increase in total K uptake in pots containing trioctahedral mica (phlogopite) and OM as compared with non-OM treatments. Addition of OM transformed phlogopite to 2:1 expanding clays but left dioctahedral mica (muscovite) intact with undetectable formation of any secondary minerals. Root activities and OM decomposition appear to have increased rhizosphere acidity which, in turn, facilitated K release from trioctahedral mica (phlogopite) and induced the formation of vermiculite and a minor quantity of smectite and hydroxy-interlayered vermiculite (HIV). In conclusion, the influence of OM on the formation of expanding clays greatly depends on the type of micaceous mineral.

Research Highlights
► Plants can take up more K from phlogopite when organic matter (OM) is added,
► K release from muscovite is not affected by OM addition.
► The type of nutrient solution has a high impact on the K release from phlogopite.
► The rate of phlogopite transformation increases as a result of OM treatment.
► Muscovite is not easily weathered even if OM is added.