Mineralogical Perspectives On Conserving Phosphate In Agriculture.

Phosphorus is essential for life and hence for agricultural production. Readily accessible deposits of phosphate are of limited extent and declining at a rate that arouses concern about sustainability. Also, formidable environmental problems arise from the mining and excess application of phosphate. The present flow of phosphate is mainly unidirectional, from ore to various wastes. Assuring sufficient P for sustained global food production requires minimizing its unidirectional loss to ecological systems where it can have deleterious effects. Solutions must arise from human will, science, and technology. Mineralogy is pertinent to scientific understanding of P dynamics in soils and to technologies that recycle P from wastes in manageable forms. Native soil minerals dictate tenacity of phosphate binding which in turn relates to the balance between plant availability and loss via runoff or leaching. Solubility and dissolution kinetics of amendment minerals control P release to the soil and potential for P accumulation at the surface. For example, P from highly soluble commercial fertilizers does not accumulate appreciably in acidic sandy soils whereas “legacy” manure P does accumulate in sparingly- soluble Ca-Mg bound forms that nonetheless leach P at environmentally problematic rates. Hence, leaching risk relates to P retention characteristics of subjacent native soil minerals for both manure and fertilizers.  Phosphate recovery from flushed animal manure as struvite or apatite is a means of reducing the waste and environmental damage resulting from sprayfield P application in excess of crop needs. Carbonate and Mg have an interactive effect on apatite precipitation in leaching and P-recovery scenarios. Each is an inhibitor when acting singly, but the MgCO₃ (aq) species formation enables recovery of P as apatite by preventing preemptive CaCO₃ precipitation at elevated pH. The balance of Ca and Mg in animal diets can affect P solubility of manures by favoring apatite- or whitlockite-like stoichiometry.