Effect of phosphorus fertilization and content of plant-available phosphorus (P-AL) on algal-available phosphorus in soils

The fate of excess fertilizer P was studied using a sequential P extraction procedure of soil samples from two PK fertilization field trials in southeastern Norway. At both sites there was an increase in the resin-P and NaHCO3-P-i (P-i = inorganic P) fractions with increasing P additions; both of these fractions were closely correlated to algal-available P (measured as total reactive P, TRP). Further, in a study of 17 soil samples it was found that TRP was positively correlated to inorganic P and plant-available P (P-AL). P-AL explained 83% of the variation in TRP. The quantity of particulate P that was soluble in a soil suspension increased considerably when the solution:soil ratio was increased from 2:1 to 2000:1. The ''equilibrium'' level of P in the solutions increased with increasing level of P-AL in the soil.