Plant availability of phosphorus from fluid fertiliser is maintained under soil moisture deficit in non-calcareous soils of south-eastern Australia

Abstract

Low soil moisture and phosphorus (P) deficiencies restrict grain production in south-eastern Australia. The effect of the soil moisture regime on the plant availability of P from fluid mono-ammonium phosphate (MAP) fertiliser was examined during vegetative growth of wheat and canola in P-responsive soils from the Wimmera, Mallee, and high rainfall zone (Glenelg) regions of Victoria. Three plant growth experiments were performed. In Experiment 1, wheat growth and P uptake increased synergistically as soil moisture increased above permanent wilting point (PWP) and as P rate increased. In Experiment 2, the uptake of P from MAP fertiliser of wheat growing under a soil moisture deficit was examined using radioactive (32P) labelling. A factorial design compared uptake from intact soil cores of 3 soil types (Vertosol, Sodosol, Chromosol), 2 depths of fertiliser placement (40 and 80 mm), and either 0 or 25 kg P/ha. Results showed distinct plant responses to both the presence and depth of MAP fertiliser. Banding MAP close to the seed was most efficient in terms of recovery of the fertiliser P, regardless of the soil moisture conditions. Soil moisture regime affected the plant uptake of soil P, rather than fertiliser P, with more soil P taken up by the plants when soil moisture increased. The plant availability of the residual MAP fertiliser in the soil was subsequently examined in Experiment 3. The availability of the residual MAP to canola seedlings was equivalent to 8 kg P/ha of freshly applied MAP. Following both crops (9 months), Colwell P values indicated no further residual MAP availability in soil that had been under the wet regime, and only a small residual value in soil from the dry regime. Banded application of fluid MAP close to the seed and into soil that is above PWP is recommended, even when a relatively dry season is expected, although the residual value to following crops may be limited.