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 increase of stockless organic farms. On mixed farms, growth of perennial 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, such crops have no direct economic use and their cultivation is therefore questionable, which poses challenges for maintaining nutrient balance and soil fertility. 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 different 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 type 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 external sources. The application of compost and grass-clover silage prepared from on site 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 for productivity in organic farming systems. In contrast, at least in this first rotation, treatments with lower organic matter inputs benefited from high site fertility and showed no yield declines.