Increasing phosphorus supply in subsurface soil in northern Australia: Rationale for deep placement and the effects with various crops

Three field experiments involving wheat, lucerne or cotton were established at different sites in the semiarid cropping regions of northern Australia, to test whether the deep placement of P fertiliser improved P availability, compared to the conventional practice of placing the fertiliser beside or adjacent to the seed. At Mulga View, near St George in southern Queensland on a red Kandosol soil with a Colwell soil test value of 19 mg P kg soil(-1) in the top 10 cm, there was no response to 10 kg P ha(-1) applied in the 5-7 cm layer. However, increasing the depth of placement of 10 kg P ha(-1) from 5-7 to 10-15 cm resulted in increased shoot growth and grain yield of spring wheat (Triticum aestivum) by 43 and 30%, respectively. A further grain yield increase of 43% to 3.2 t ha(-1) resulted when the deep P rate was increased from 10 to 40 kg P ha(-1). At Roma, in southern Queensland, on a grey/brown Vertosol with a Colwell soil test value of 15 mg P kg soil(-1), there was no difference in the winter growth of lucerne (Medicago sativa) when P fertiliser had been applied at 5-7 cm depth at rates of 10 and 40 kg P ha(-1). Shoot dry matter yields were around 2 t ha(-1). However dry matter yields increased significantly to 2.6 and 3.7 t ha(-1) when 10 and 40 kg P ha(-1), respectively were applied at the 10-15 cm depth. The third experiment was carried out on a grey Vertosol at Kununurra in Western Australia. Significant increases in the yield of seed cotton (Gossypium hirsutum) occurred when 50 kg P ha(-I) was applied at depth (10-15 and 25-30 cm), compared with the conventional placement at 7-10 cm, with maximum yield response to deep placement occurring with DAP, and the minimal response with MAP. The cotton was grown on raised beds and the crop was irrigated according to district practice. The response to deep P at all sites was attributed to the rapid drying of the soil surface layers, reducing the availability of soil or fertiliser P in these layers. The deep fertiliser P remained available during the growing season and alleviated the P deficiency that appears to be a feature of these soils when the surface layers become dry.