Isotopic tracing of phosphorus uptake in corn from ³³P labelled legume residues and ³²P labelled fertilisers applied to a sandy loam soil

In low input (e.g. organic) farming systems where soil phosphorus (P) fertilisers such as superphosphate are not used, maintaining sufficient available soil P for plant growth can be a major challenge. The use of P accumulating cover crops may increase P availability for subsequent crops. We hypothesised that P release from organic residues of legumes (faba bean ('Vicia faba') and field peas ('Pisum sativum')) could supply adequate P to meet the needs of a subsequent crop in a low P soil. A pot experiment was conducted to determine the contribution of P by legume green manure to subsequent corn using ³³P labelled legume residues and ³²P labelled inorganic fertiliser (KH₂PO₄). The treatments included two rates of P application, (a) 10 kg P ha⁻¹ as legume root and shoot residues or as inorganic fertiliser with and without a C source, and (b) 38 kg P ha⁻¹as a combination of legume shoot and root residues or a combination of root and inorganic fertiliser and inorganic fertiliser alone. An absolute control (zero P) was also used. Shoot dry matter, P uptake and P source (residues or fertilisers) of total P in corn were measured at harvest. Faba bean and field pea residues alone or in combination with fertilisers contributed up to 10% and 5% of the total P uptake by corn respectively, compared with up to 54% by inorganic fertilisers. Incorporation of field pea and faba bean residues with P concentrations higher than those observed under field conditions, may not always lead to adequate net P release to supply the early growth phase of subsequent crops.