Changes in Understanding the Behaviour of Soil and Fertiliser Phosphorus: Implications for their Efficient Use in Agriculture

Keywords: Phosphorus, bonding energy, agronomic efficiency, recovery, plant-soil system.

Current concepts about soil phosphorus (P) envisage that, in relation to the availability of P to plants, it exists in soil in four pools. These are the soil solution, a readily plant available pool, a less readily available pool and a very slowly available pool. Phosphorus in the soil solution and the readily available pool is measured by routine soil analysis. Much evidence from field experiments shows that there is reversible transfer of P between the first three pools. When P is added to soil it is rapidly distributed between these pools so that not all the freshly added P will be measured by routine soil analysis. This does not mean that this P is no longer available for plant uptake. As P is removed from the soil solution by plant uptake, P in the readily available pool replenishes it. This behaviour is explained if P is adsorbed/adsorbed on to soil mineral and organic components and held with a range of bonding energies. Readily available P is that P which is weakly bonded. These concepts developed in the last 30 years or so replace earlier ideas that added water-soluble P is irreversibly fixed in soil and becomes unavailable to plants, an idea that must now be discarded. These recent concepts about the behaviour of P in soil relate well to the known responses by crops to soil and added fertiliser P. They also make it possible to develop a concept of critical levels of plant available soil P appropriate to the soil type and farming system. Only applying P to maintain such a critical level will ensure that P is used most efficiently, that yields are not jeopardised by lack of P and that soils are not excessively enriched with P. A better method of estimating percent P recovery is presented here and evidence to support these statements is given in this review.

A E Johnston, Rothamsted Research, Harpenden, UK; and

J K Syers, Mae Fah Luang University, Chiang Rai 57100, Thailand.

31 pages, 5 figures, 10 tables, 46 references.