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 b ulk 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 powde r properties developed. This review identifies previous research where the agronomic effectiveness of ground rock fertilisers has been evaluated. Ther e are many contradictory findings that need to be evaluated by reference to basic geochemical knowledge. Geochemical studies of mineral dissolution in dicate the general reaction pathways by which nutrients are released, assum ing that equilibrium between the soil solution and primary mineral is achie ved. In soils, mineral dissolution is enhanced by disequilibrium between so il solution and mineral surfaces through the removal of ions by processes s uch as leaching and nutrient uptake. Rhizosphere processes and other biolog ical activity may further enhance mineral dissolution through the release o f 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 fertilise rs. A range of weathering rates for minerals have been identified in the la boratory and the field and are dependent on physical, mineralogical and bio geochemical factors. The rate limiting steps may be those that involve reac tions between the soil solution and mineral surface. Dissolution primarily occurs at defects at the mineral surfaces and an understanding of these sur face reactions may lead to preparative procedures to enhance nutrient relea se from the mineral surface. Normalising the release rates of nutrients to a unit surface area basis can aid in predicting nutrient release during dis solution from various ground rock materials. Identifying the relationships between release rates of minerals and plant uptake is vital to developing a n understanding the effectiveness of rock dust applied to vegetated soils.