Increasing phosphate rock availability using a lupin green manure crop

Legumes have been shown to increase the dissolution and utilisation of phosphate rock (PR) phosphorus (P) compared with non-legumes because of their acidifying effect on the rhizosphere. A promising agronomic approach for optimising nutrient availability in organic cropping systems appears to be the integration of legume green manures (GM) into the cropping rotation. A field experiment was conducted to study the combined effect of legume growth (in terms of improved P utilisation from PR) and subsequent organic matter additions (green manure incorporation) on crop growth compared with a corresponding winter fallow system. Three rates of PR (0, 60, 120 kg P/ha) were applied in autumn before sowing of lupin green manure and maize as the crop. A fourth PR treatment involving the application of 60 kg P/ha as PR in spring, before maize sowing was also employed. The objective of the study was to quantify the impact of preceding lupin (green manure), treated or not treated with PR, on a subsequent maize crop, and to evaluate winter fallow and green manure effects in terms of P availability. The observed improvements in maize yields following green manuring and PR application of approximately 1.5t/ha and the 3.1-5.3 kg/ha increase in P uptake by maize in the first season were due to a combination of P release into soil solution following the mineralisation of organic P additions via the lupin GM and increased solubilisation of residual P due to rhizosphere and possible mycorrhiza colonisation processes