Effect of phosphorus fertilization on the concentration of total and algal-available phosphorus in different particle-size fractions in Norwegian agricultural soils

The distribution of total P and potentially algal-available P (measured as total reactive P, TRP) in different particle-size fractions (< 20 mu m, <2 mu m and <0.2 mu m) was studied in soil samples from two PK fertilization field trials at Tune and As in southeastern Norway. The concentration of total P and TRP increased with decreasing particle size. The effect of increasing amounts of added P on the P content in the different particle-size fractions was examined, and it was found that the excess of added P was concentrated in the clay fraction. This resulted in higher enrichment ratios of both total P and TRP with increasing P additions. The part of total P that was algal-available increased significantly with the increasing level of P fertilization.