Phosphorus in agriculture: a review of results from 175 years research at Rothamsted, UK

Insight into the role of phosphorus (P) in soil fertility and crop nutrition has come from long-term field experiments initially started by J.B. Lawes at Rothamsted in 1843 and continued by others together with experiments on different soils. From the 1940s, results have refuted a widely held view that P residues were irreversibly fixed in soil. Rather, such residues build-up a reserve of plant-available P (Olsen P), and there is a strong relationship between crop yield and Olsen P. The critical level of Olsen P, determined from this relationship, is the level at which soils should be maintained to optimise P-use efficiency, which, for applied P, is high when the amount applied nearly equals offtake by the crop. This reserve of inorganic P is associated with a variety of soil components and held with a range of bonding energies, such that when no P is applied the decline in Olsen P follows a smooth curve. Based on the accumulated information, inorganic P in soil can be conceptualised as being in four pools of vastly varying size, availability for uptake and extractability by reagents used in routine soil analysis, and with reversible transfer of P between the pools.