Factors influencing the release of plant nutrient elements from silicate rock powders: a geochemical overview
Rock-forming minerals of igneous and metamorphic rocks contain most of the nutrients required by higher plants for growth and development. Ground rock fertilisers may provide a source of nutrients to depleted topsoils where bulk soil solutions are not in equilibrium with fresh primary minerals. Slow dissolution rates of silicate minerals may inhibit the use of rock powders in agriculture unless suitable soils are identified and optimum rock powder properties developed. This review identifies previous research where the agronomic effectiveness of ground rock fertilisers has been evaluated. There are many contradictory findings that need to be evaluated by reference to basic geochemical knowledge. Geochemical studies of mineral dissolution indicate the general reaction pathways by which nutrients are released, assuming that equilibrium between the soil solution and primary mineral is achieved. In soils, mineral dissolution is enhanced by disequilibrium between soil solution and mineral surfaces through the removal of ions by processes such as leaching and nutrient uptake. Rhizosphere processes and other biological activity may further enhance mineral dissolution through the release of H-ions and complexing organic compounds which react with mineral surfaces. Geochemical principles can be used to predict some of the reactions that occur when ground silicate minerals are added to soils as mineral fertilisers. A range of weathering rates for minerals have been identified in the laboratory and the field and are dependent on physical, mineralogical and biogeochemical factors. The rate limiting steps may be those that involve reactions between the soil solution and mineral surface. Dissolution primarily occurs at defects at the mineral surfaces and an understanding of these surface reactions may lead to preparative procedures to enhance nutrient release from the mineral surface. Normalising the release rates of nutrients to a unit surface area basis can aid in predicting nutrient release during dissolution from various ground rock materials. Identifying the relationships between release rates of minerals and plant uptake is vital to developing an understanding the effectiveness of rock dust applied to vegetated soils.