Changing Concepts for the Efficient Use of Phosphorus in Agriculture

Keywords: Phosphorus, P-use efficiency, critical levels of plant-available P, recovery of P residues, removal to input ratios, routine soil analysis for P.

Changes in our understanding of the behaviour of soil, fertiliser and manure phosphorus (P) during the last 150 years are presented. When linked with agronomic data, there is clear evidence that P is not irreversibly fixed in most soils, and that a reserve of plant-available P accumulates from past applications of P fertilisers and organic manures. Data are presented supporting the proposal that soil inorganic P can be considered as existing in four pools, which are related to the strength of bonding of the P to soil components and thus the plant-availability of the P and its extractability by chemical reagents. The stronger the bonding of phosphate ions to soil components the lower the availability of P to plants.

P-use efficiency in agriculture is related to soils reaching and being maintained at a critical level of readily plant-available P, and factors affecting the critical level are discussed. When soils are maintained at about the critical level for a particular soil type and cropping system soil P residues are used more effectively. Efficiency can be assessed by the direct, difference and balance methods. The latter, when calculated as the removal to use ratio, shows that P use efficiency can exceed 80-90%.

Combined data from controlled experiments in England and derived ‘statewide’ aggregate information in the United States relating removal to use ratios to changes in plant-available P could best be described by a single, simple function, making a powerful and convincing argument that there is an underlying ‘simple rule’ for the behaviour of plant-available inorganic soil P that is related to the four-pool concept discussed in this paper.

A E Johnston¹ and P R Poulton²

¹ Lawes Trust Senior Fellow, Rothamsted Research, Harpenden, AL5 2JQ, UK.

² Visiting Scientist, Rothamsted Research, Harpenden, AL5 2JQ, UK.

38 pages, 11 figures, 8 tables, 65 references.