How to maintain soil fertility in stockless organic farming: Research concepts and insights from the first crop rotation of a long-term field experiment

With the increase of organic agriculture throughout Europe, there is also an increasing share of stockless organic farms. On mixed farms, growth of deep rooting perennial forage legumes or legume-grass mixtures as well as farmyard manure are important contributors to soil fertility and play a key role for nutrient management. On stockless farms, growth of these crops has no direct economic use and is therefore questionable. Disentangling physical, chemical, and biological long-term impacts on soil fertility and consequently on crop yield and quality requires long-term research. In 2017, a long-term field experiment was established in Hesse, Germany, in which three stockless organic farm types differing in crop rotation, each combined with three fertilization treatments, are compared to a traditional mixed farm type with three livestock density levels. The results of the first crop rotation show that the mixed farm achieved more synchronized nutrient input and output with increasing livestock density. Stockless farm types showed deficits, especially in P and K balances, unless compensated by organic fertilizers from farm-external sources. The application of compost from external sources but also of grass-clover silage prepared from own fertility-building leys resulted in significant increases in soil organic carbon. Significant correlations between soil organic carbon and crop yields in stockless farm types using compost emphasize the importance of soil organic carbon content to ensure productivity in organic farming systems. On the other hand, at least in this first rotation, other farm types relying more on the high natural site productivity did not experience yield declines.