Accumulation and translocation of sparingly soluble manure phosphorus in different types of soils after long-term excessive inputs

Summary

When phosphorus (P) is applied to soils in excess of plant P demand, P accumulation takes place. By means of P fractionation, we studied the fate of P in 35 soils that had received long-term surplus P rates as fox and mink manure P (F&MM-P), considered as a sparingly soluble P source. We compared these data with those from the same soils under a more typical P management, i.e., fields amended with soluble P sources superphosphate and dairy manure (SP+DM). Fractionation of manures according to the Hedley procedure suggested limited solubility of F&MM-P, two-thirds of the F&MM-P being soluble in acid (HCl) only. In mineral soils, surplus F&MM-P accumulated for the most part as HCl-P (poorly available), whereas in organic soils accumulation occurred largely as NaOH-soluble (moderately available) and labile P. Translocation of F&MM-P was evidenced by P fractionation and by agronomic P test: subsurface soil of F&MM-amended fields contained more P than the same soils in fields amended with SP+DM. In the 35 soils that had received surplus F&MM-P, agronomic P status was classified as “excessive” in 90% of the samples taken from the plough layer, in 75% of those from 20–40 cm depth, and in 50% of those from 40–60 cm depth. In the reference soils amended with SP+DM, “excessive” P status was recorded for 23% of the plough layer samples and 4% of the subsurface samples. These results show that manure P which appears as sparingly soluble in P fractionation may in a longer term increase the content of labile soil P and translocate in the soil profile when applied in high rates.