Legume Breaks in Stockless Organic Farming Rotations: Nitrogen Accumulation and Influence on the Following Crops

Cultivation of one year's green manure could form the base for supplying nitrogen in an all-arable or ‘stockless’ crop rotation in organic farming systems. Three four-year crop rotations from two long-term field trials on stockless organic agriculture in the U.K. and Germany were analysed using data collected over 5 and 7 years, respectively. All three rotations were based on one year's clover green manure. The main focus of the investigation was to assess the N-supply by the green manure and its influence on the succeeding crops. The estimated N-input by the symbiotically fixed N from the clover ranged from 20 to 350 kg N ha−1. The average for two of the three rotations demonstrated that N-input was greater than N-export. In the third case, N-supply was much lower than N-export, due to weather and pest related damage to the clover. In the individual rotations, the influence of varying green manure N-accumulation on N-uptake of the cash crops was not significant. However, combining the rotations by using partial correlation analysis revealed significant correlations in some groupings. In particular, the N-uptake of potatoes as the first crop following the green manure reacted clearly to N-differences in the green manure biomass; this was not true for wheat following clover. Quantitatively, the reaction of the potato-N-uptake amounted to approximately 10 to 20% of the green manure-N differences. Even with the combined calculation there were no significant responses from the second and third crops following green manure.