Exploiting P in heavily P-dressed fields in Sweden

Phosphorus status of soils is regarded a component of soil fertility. This component can be influenced by cropping measures. In 1983 a number of Swedish fields with a high P status were identified. In all there were 17 fields chosen distributed over Sweden. On these fields P fertilizer experiments were started with the treatments 0, 15 and 30 kg P ha^-1 yr^-1.

In average of all experimental sites the starting P-AL value was 19.6 mg 100^-1g soil. Typically P-AL in Swedish arable soils are less than 10 mg, so the 19.6 average is a high value, pH varied from 5.1 to 7.3 with 46% of the sites having initial pH values less than 6.2. With no P applied the P-AL values decreased significantly with time. The slope of the regression line indicates that P-AL in the top soil decreased with 0.5 units year^-1. The decline was statistically significant at the 5% level. When 30 kg P year^-1 was applied the decline was insignificant. In the high pH range (≥ 6.2) the decline in P-AL was 0.67 units year^-1 compared with 0.18 in the low pH group. Measurement of the uptake and removal of P with harvest products was chosen as the best method to analyse the crop response with time. When the experiments were pH grouped and the effects in cereals were analysed a positive effect on the P uptake with time was found.