DOC trial: nutrient supply in winter wheat – where is the deficit?

The nutrient supply of winter wheat was one of the topics investigated by the DOC long-term system comparison from 1978 to 2003. The aim of this trial is to provide evidence of nutrient-related yield limitations in organic farming systems. Substantial differences in yield between «organic» and «conventional» farming systems and different fertilisation intensities were primarily attributed to the delivery of nutrients – in particular, nitrogen – to the plants. Because the soil phosphorus supply was adequate in all DOC systems over the entire trial period, phosphorus was ruled out as a co-limiting factor. The plant analyses of straw and grain exhibited high figures and a low differentiation for phosphorus, thus confirming the soil findings. By contrast, potassium was identified along with nitrogen as a co-limiting factor in the organic systems at the low fertilisation intensity and in the unfertilised control. This was indicated by the differentiation of potassium content in the above-ground biomass and the available soil potassium content. Despite this, both the biodynamic and bio-organic system exhibited a balanced potassium supply at the high fertilisation intensity. Both bio-systems may therefore be considered sustainable at this fertilisation intensity.