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Use of Ion Exchange Membranes as Root Simulators to Assess Nutrient Ion Supply Rates in Soils

Schoenau, J.J. and P. Qian. 2007. Proceedings of 2007 Dahlia Greidinger Symposium: Advanced Technologies for Monitoring Nutrient and Water Availability to Plants, March 12-13, Technion University, Haifa, Israel

Abstract

Ion exchange resins in membrane form may be regarded as rudimentary plant root simulators when placed in direct contact with soils and sediments. Anion and cation exchange membranes act as ion sinks or dynamic exchangers when placed directly into contact with the medium. Nutrient ions in soil solution and on soil colloids will exchange with counter ions on the membrane surface and become adsorbed. The measurement of ions adsorbed on the surface of the membrane after a specific period of contact time is used to provide an indication of potential ion supply rate, with units of weight of nutrient ion adsorbed per unit surfacearea of membrane over the contact time. Encapsulation of the membranes into probes (Plant Root Simulator Probes™) allows the direct insertion of membranes into intact soils in laboratory, greenhouse or field. The ion supply rate measured in this manner is highly sensitive to ion concentration in solution, diffusive conditions and sorption-desorption, mineralization-immobilization processes as they are affected by soil and environment. Several examples are covered of the successful use of ion exchange membranes to study the impacts of management on nutrient availability. Applications dealt with include evaluating the effect of different crop residues, amendments, fertilization and tillage practices on available nitrogen and phosphorus supplies in the soil. The sensitivity of the technique to biological and physico-chemical processes that are operative the soil environment such as mineralization-immobilization and diffusion is an advantage in mimicking what a plant root would experience.