An assessment of various measures of soil phosphorus and the net accumulation of phosphorus in fertilized soils under pasture

Phosphorus fertilizer use efficiency of pastures is often low because P accumulates in soils as sparingly-available forms of inorganic and organic P. The aim of this study was to use sequential chemical fractionation to identify which forms of P have accumulated in soil under permanent pasture from a medium-term (13 y) field experiment. Treatments included an unfertilized pasture (P0), and treatments designed to maintain soil P fertility at near 'optimum' (P1) and 'supra-optimum' (P2) levels for pasture growth; all levels of soil P fertility were continuously grazed with either a moderate or high stocking rate (SR09 and SR18). Summed concentrations of extractable inorganic and organic P, and residual P (i.e., non-extractable P) accounted for 20, 31, and 49% of total soil P (as determined by laboratory X-ray fluorescence), respectively, across all surface (0-10 cm) and subsurface (10-20 cm) soil layers. A large proportion of extractable inorganic and organic P was contained in the NaHCO3 and NaOH soluble fractions across all surface soil layers, and these fractions were also the most important sinks for fertilizer P. The accumulation of organic P appeared to plateau with increasing fertilizer addition, whereas inorganic P continued to increase. The majority of the P that had been applied as fertilizer (70%) was recovered in the surface soil layer (0-10 cm). Approx. 31 and 30% of the added P was recovered in the summed concentrations of extractable inorganic and organic P, respectively, of the surface soil layer at the optimum (P1) agronomic level of soil P fertility.