Effect of phosphate fertiliser addition and land slope on soil phosphate fractions

A large‐scale phosphate (P) fertiliser grazing trial at Whatawhata Research Centre, North Island, New Zealand provided quantitative information on the effect of slope on changes in the forms of P accumulating, or being depleted, in the soil. Changes in the size of P fractions (Olsenextractable, water‐extractable, total, inorganic, organic, easily soluble, non‐occluded, occluded, and calcium‐bound) were monitored on three slope groups (0–10°, 11–20°, and 31–40°) topdressed annually at five different rates of P fertiliser applied as single superphosphate (10, 20, 30, 50, or 100kg P/ha) for 4 years. Olsen P and total and inorganic soil P increased with increasing rate of fertiliser addition and decreased with increasing depth and slope. Non‐occluded P was the largest inorganic P fraction and appeared to be involved in active P cycling and P transfer. In contrast, calcium‐bound P was less actively involved in P cycling and appeared to accumulate as residue from fertiliser.