How understanding soil chemistry can lead to better phosphate fertilizer practice: a 68 year journey (so far)

Abstract

This paper describes a journey which started with the question: why do farmers reapply phosphate? This led to understanding how phosphate and other specifically sorbed ions, both cations and anions react with soil. Under most soil conditions, they do not precipitate to give specific compounds. Rather they are specifically adsorbed onto the surface of soil particles and then penetrate the particles via diffusion mechanisms. The pathways by which they move differ between ion species; for example nickel penetrates the lattice of iron oxides but phosphate penetrates defects between particles. Such reactions change important properties of the soil. Reaction with phosphate conveys negative charge to the reacting surface and so decreases the buffering capacity for phosphate; this makes subsequent phosphate applications more effective. Nor does the diffusive penetration continue unabated; as phosphate builds up, diffusive penetration slows and then virtually stops. These effects mean that on long-term fertilised soils, phosphate fertiliser is much more effective and annual doses (if they are needed at all) should be much smaller. I suggest that failure to appreciate this is the cause of phosphate losses from farmland and consequent pollution of water bodies.