Changes in soil phosphorus pools of grasslands following 17 yrs of balanced application of manure and fertilizer

Limiting the use of phosphorous (P) in intensive agriculture is necessary to decrease losses to surface waters. Balanced fertilizer application (P supply equals P offtake by the crop) is a first step to limit the use of P. However, it is questioned whether this balance approach is sufficient to maintain soil fertility. A long-term field experiment (17 yr), on grazed grassland, has been conducted on sandy soil, marine clay soil and peat soil to obtain insight into the effects of balanced P fertilizer application on soil test P values and to explain the results by changes in P pools in the soil. The balance approach led to a gradual decline in plant available P, measured as P-AL, in the topsoil (<0.10 m deep). This decline was accompanied by a decline in oxalate extractable P, dithionite extractable P and inorganic P (0.5 m H₂SO₄). The decline in these mineral P pools in the topsoil was (partly) compensated by an increase in the amount of organic P. There was evidence for the accumulation of P in an occluded form, especially at one of sites which received P as Gafsa rock phosphate [Ca₃(PO₄)₂].