Analyses of plant sap from organic greenhouse tomato crops show that the levels of phosphorus (P) are frequently low, despite the fact that soil analyses indicate P status in the soil to be good. In the present study, two soils (A and B) with a pH of just over 6 and a high total content of N and P were investigated with respect to release of P and uptake in a tomato crop. The fertilisers primarily used on soil A was silage and blood meal, and on soil B Vinasse and blood meal. An incubation experiment showed that the release of P per unit time was greater from soil A than soil B. This difference between the two soils may be due to lower soil organic matter concentration, with less P sorbed by Al and Fe on soil A, and a fertiliser regime which favours a high concentration of dissolved organic carbon in this soil. Tomato plants grown for 10 weeks in the soils had greater DW production, total uptake of P and concentration of P in the leaves on soil A than on soil B. Addition of citric acid to the soils in order to mobilise P increased plant yield and uptake of N and P on soil A. Despite the increased growth, the concentration of P in plant leaves increased and the N concentration was unchanged. On soil B addition of citric acid decreased Zn uptake in the tomato plants despite good availability, resulting in a lower concentration of this micronutrient in the leaves. The plants on soil A maintained their concentrations of all micronutrients analysed, but the ratio of P to micronutrients increased. Thus for commercial organic tomato growers, adding citric acid with the irrigation water offers a possibility to increase P availability.
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