Effect of different rates of P fertilization on the yield and P status of the soil in two long-term field experiments

Two field experiments on P fertilization were conducted on clay soils in Southern Finland. The rates of P applied yearly in granular NPK fertilizers were 0, 13/16, 26/32, 47/56 and 60/72 kg P/ha in 1974—82/1983 —85. Oats, barley, spring wheat and winter wheat were grown, in two years also oil seed crops. In one experiment, the maximum yield of cereal grains in the first nine years (4 460 kg/ha) was reached at the P rate of 13 kg/ha, but thereafter at 32 kg P/ha. The average difference between the maximum yields and the ones obtained without P fertilization was 470 kg/ha (12 %) in 1974—80, but during the last four years the difference increased to 1 360 kg/ha (40 %), owing to the depletion of P in the plots not fertilized with P. Also in the other experiment, in which the maximum yield of cereal grains (4 790 kg/ha) was obtained at the P rate of 26/32 kg/ha, the response to P fertilization increased towards the end of the trial, the mean response during the last three years being 570 kg/ha (12 %). Phosphorus fertilization, up to the P level at which the maximum yield was reached, decreased the moisture content of cereal grains at harvest. The quantity of P extracted with 0.5 M NH4-acetate-0.5 M acetic acid (pH 4.65) decreased in the plots not fertilized with P, from 5.8 mg/l to 2.2 mg/l and from 6.2 mg/l to 1.8 mg/l in the course of the two trials. The original level of acetate-extractable P was somewhat maintained but not elevated by P rates of 26/32, 47/56 and 60/72 kg/ha. Residual P was recovered mainly in the fractions extractable with NH4F (“Al-P”) and NaOH (“Fe-P”)