Long-Term Effect on Soil of Restricted Use of Phosphate Fertilisers

Keywords: Phosphorus, phosphate, maintenance fertilisation, fertiliser, residual effect, sorption, precipitation, P pool, adsorbed P, absorbed P, STP.

The effect of any decrease in phosphorus (P) application rates has aroused anxiety amongst farmers. They fear a loss of yield and quality of the crop due to a rapid loss of plant available soil phosphorus. The aim of this paper is to investigate the effects of reduction of P application on changes in soil test P values (STP) in the soil plough layer and grass sod, on changes in soil P pools in the soil profile and to reduce and prevent unnecessary anxiety that farmers may have about lower rates of P application.

Data of long term field experiments were analysed on the change in STP and P pools (adsorbed and absorbed P, total-P). Reducing P application rates to quantities similar to crop offtake (maintenance fertilisation) can cause a decline in STP. However, this decline tends to be slow. The soil has a large buffering capacity to prevent a rapid decline. An increase of the surplus of P leads to an increase in the quantity of reversibly bound P on arable land. However only approximately 20% is directly available, a larger quantity is more strongly bound to soil components but this quantity is not unavailable to crops. The soil of these field experiments still contained a quantity of reversibly bound P after 30 years of mining the soil (soil layer 0-40 cm) of P. This amount surpasses the average offtake of arable crops by a factor of 6.

P surpluses of 0, 10 or 20 kg P/ha/yr on grass land did not cause dramatic changes in soil test P (data given) and P pools (data not given). Year-to-year variations of soil fertility are large, hampering the interpretation of the results. Despite this we see that after 8 years a significant decline in soil fertility is found at surpluses below 20 kg P/ha/yr which depends of the initial P status and the P-surplus in the upper 5 cm while the P status of deeper soil layers remained virtually constant.

Phillip Ehlert¹ and Caroline van der Salm, Wageningen University and Research Center (WUR), Alterra, Soil Science Center, PO Box 47, 6700 AA Wageningen, the Netherlands, and

Oscar Schoumans, WUR, Alterra, Center for Water and Climate, PO Box 47, 6700 AA Wageningen, the Netherlands.

¹ Corresponding author.

23 pages, 7 figures, 5 tables, 46 references.