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Experimental data on the toxicity of magnesium hydrogenorthophosphate (CAS 7757-86-0) to soil organisms are not available. However, the chemical safety assessment does not indicate the need to investigate further the effects on terrestrial organisms. Based on the aquatic hazard assessment toxic effects of the substance on terrestrial organisms are not expected. The substance is not expected to bioaccumulate or to be toxic. In release to terrestrial environment the substance is present mostly in dissociated ionic forms magnesium and phosphate in soil water. Both ions are ubiquitous in the terrestrial environment.

The ionic magnesium is highly mobile on one hand, and on the other hand magnesium can adsorb to 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 thus will increase with an increase in pH (Cornell University Cooperative Extension, 2007). Therefore, Magnesium will become more available with increase of soil pH.

Phosphorus is required by all living plants and animals. Phosphorus containing compounds are essential for photosynthesis in plants, for energy transformation and for the activity of some hormones in both plants and animals (Cornforth I.S., 2008). Solution in soils contains very small amounts and concentrations of phosphates. The Phosphate ion can occur in three states of protonation, which is pH dependent. In soil H2PO4 and HPO4 are the dominant species for pH values of 4.5 – 6.2, which are occur normally in soil. This is the form in which phosphorus is used by plants. Inorganic phosphates will dissociate to soluble orthophosphate (PO43-) in sewerage systems, sewage treatment plants and in the environment. Orthophosphates are also formed by natural hydrolysis of human urine and faeces, animal wastes, food and organic wastes, mineral fertilisers, bacterial recycling of organic materials in ecosystems, etc. Precipitation-dissolution and sorption-desorption processes control the concentration of phosphate ions in solution. Phosphate ions are mainly immobilised in soils by adsorption to solid matter or by reaction with aluminium or iron to aluminium- and ironphosphates (Cornforth 2008).

Thus, detrimental effects on terrestrial organisms are not expected due to the absence of acute aquatic toxicity, the low bioaccumulation potential and the fact that both ions are essential micronutrients.

References:

Cornforth I.S. (2008) The fate of phosphate fertilizers in soil. New Zealand Institute of Chemistry. II-Chemicals and Soils-D-Phosphate-2