Agricultural production removes plant nutrients; therefore, these must be replaced to sustain production. Phosphorus (P) and nitrogen (N) are perhaps the two nutrients of greater importance because Australian soils are inherently low in these elements. In the case of P, reactive phosphate rock (RPR) is the most practical fertiliser to replace P in broad-acre organic farming (Penfold 2000). For N, the optimal replacement strategy is legume N2 fixation; however, this process is highly dependent on P nutrition (Donald and Williams 1956). Hence, replacing P is crucial to sustaining productive organic cropping systems.
Phosphate rock (PR) is an allowed input for managing phosphate fertility on organic farms and is potentially suitable for mildly acidic to strongly acidic soils, because acidity is required to breakdown the phosphate rock to release P in a form that is available to plants. However, PR is generally only effective under substantially higher rainfall conditions than occur in the southern cropping regions of Australia, where previous work has sometime found it to be ineffective. In many soils, elemental sulphur (S) is naturally oxidised by soil bacteria to produce acid. Mixing PR with S may thus have a positive role in improving the effectiveness of PR in the southern cropping areas.
