Tomatoes

Project responsible: Birgitta Båth, Department of Crop Production Ecology, SLU

For the past ten years, a participatory research group has been working on issues relating to organic growing of greenhouse tomatoes. An important issue for the growers in the group is how to design their fertilisation strategy. Analyses of plant sap from commercial crops show that the levels of phosphorus (P) are low. The nutrient deficiency in the plant is probably not primarily the result of deficiency in the soil, but rather of low nutrient release rate in relation to crop requirements. In turn, phosphorus deficiency can give rise to N deficiency. The experiments carried out by the participatory research group have investigated issues such as the effect on pH of adding four different fertilisers/soil improvers; and whether addition of citric acid together with irrigation water increases P availability (Figure 1).

As the results are promising we want to continue with the task of drawing up a balanced fertilisation strategy for organic tomato production. Our research issues are:

• Can the effect of adding citric acid with the irrigation water be achieved at pH values higher than 3.6?
• Can the fertiliser dose be decreased when citric acid is added with the irrigation water?
• How is the pH in the soil affected by addition of fertiliser and soil improver to cultivation systems with and without plants?