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Experimental data on the adsorption/desorption of trimagnesium bis(orthophosphate) (CAS 7757-87-1) are not available. Testing the adsorption/desorption behaviour according to OECD Guideline 121 is not feasible as the test method is not validated for inorganic substances. A batch equilibrium study according to OECD Guideline 106 was not conducted since analysis of the test material may not be possible due to interference from the soil extracts that may leach into the aqueous media during the test. This would prevent quantification of the test material. In addition, the mobility of the test item would be dependent on the anion exchange capacity of the soils as the main component of the test material is an anion. This absorption relationship would not be anticipated to correlate with the organic carbon content of the soils and is considered to be beyond the scope of the OECD 106 method.

Trimagnesium bis(orthophosphate) is well soluble in water and the solubility of the substance is regulated by the pH under the environmental conditions. Under the pH, redox and conductivity regimes typically found in water, Mg is present almost exclusively as Mg2+ in water and sediment. In terrestrial compartment, soluble ionic magnesium is highly mobile on one hand, and on the other hand it can adsorb to surface of clay and organic matter becoming immobilised in natural soil (Mikkelsen 2010, Schulte 2004). The mobility in soil strongly depends on the cation exchange capacity (CEC) of the soil. Soils with a high CEC, soils with more clay or organic matter, will hold more magnesium caused by a higher total amount of exchangeable cations that the soil can adsorb. Magnesium, and also other cations, is held by the negatively charged clay and organic matter particles in the soil through electrostatic forces. As a result, magnesium and the other cations are plant available. The actual CEC of the soil is also depended on the pH of the soil, and will increase with an increase in pH (Cornell University Cooperative Extension, 2007). Therefore, Magnesium will become more available with increase of soil pH.
Solution in soils contains very small amounts of dissolved phosphates, which occur in three states of protonation in dependence upon the pH values. In soil water H2PO4 and HPO4 are the dominant species for pH values of 4.5 – 6.2. This is the form in which phosphorus is used by plants. Precipitation-dissolution and sorption-desorption processes control the concentration pf phosphate ions in solution. Phosphorus ions are mainly immobilised in soils by adsorption to solid matter or by reaction with aluminium or iron to aluminium- and ironphosphates (Cornforth 2008).


Cornell University Cooperative Extension (2007) Cornell University Agronomy Fact Sheet # 22: Cation Exchange Capacity (CEC)

Cornforth I.S. (2008) The fate of phosphate fertilizers in soil. New Zealand Institute of Chemistry. II-Chemicals and Soils-D-Phosphate-2 (with reference to: Dahal 1977; McLaren and Cameron 1990; Syers and Cornforth 1983)

Dahal, R.C. 1977. Soil organic phosphorus. Advances in Agronomy. Volume 28, 83-117.

McLaren, R.G.; Cameron, K.C. 1990. Soil Science, an introduction to the properties and management of New Zealand soils.

Mikkelsen R (2010) Soil and fertilizer magnesium. Better Crops 94:26–28

Schulte E. E. (2004) Soil and Applied Magnesium, University of Wisconsin-Extension, Understanding Plant Nutrients, A2524

Syers, J.K.; Cornforth, I.S. 1983. Chemistry of Soil Fertility. Read at the New Zealand Institute of Chemistry Annual Conference, Hamilton.