Methods for Silicon Analysis in Soil, Plant and Fertilizers

To develop recommendations for field applications of silicate materials, knowledge of the soil Si status and the availability of Si in the amendment are essential. To determine a crops response to application of Si requires calibration of soil Si status and plant uptake. While a method for determining plant Si levels using the auto-clave induced digestion procedure (Elliott and Snyder, 1991) is well established, the challenge for routine testing of soils and amendment materials is the development of simple, dependable and robust methods that correlate well with changes in soil Si status and corresponding plant tissue levels. The total Si content of soils can have little relationship to the concentration of soluble Si in soils, which is the component important for plant growth. The concentration of soluble Si is dynamic, and although leaching of Si from the soil and plant uptake are important processes determining Si concentrations, the equilibrium concentration is largely controlled by adsorption/desorption reactions. A number of chemical extraction procedures have been developed to determine the ‘plant available’ soil Si status, and a range of these are compared when used on different soil types. However, a complication occurs when Si is added to soil, as it reacts rapidly with amorphous surfaces, and amount and rate of Si adsorption is dependant on the sesquioxide content of the soil, soil pH, and the presence of other anions. Although many soil tests are effective in predicting ‘sub-optimality’, there may be a need to refine soil-testing techniques that enables an assessment of ‘responsiveness’ to silicate additions. These soil reactions also influence the solubility of, and hence availability of Si in various amendments, causing poor correlation with laboratory chemical extraction procedures for determining available Si in a material and plant uptake once that material is added to the soil. Various methods to determine the potential for a material to supply ‘plant available’ Si are compared.