Liquid digestate application to autumn cover crops
Due to limited capacities, slurry storage facilities usually have to be emptied in autumn, as slurry application in winter is prohibited for water law reasons. Since field crops require little fertiliser in autumn, there is a risk of inefficient N utilisation and groundwater pollution. Therefore, a sensible and environmentally conscious utilisation of biogas slurry in autumn was sought: a legume-free cover crop of oil radish, phacelia, sand oats, cress and camelina following a winter wheat crop is tested for its ability to absorb and bind the accruing N quantities in its biomass in autumn. In spring, the absorbed nitrogen is made available to the following crop, spring barley, through the decomposition of the cover crop biomass in the soil. In a plot trial, the effect of biogas slurry application (15 m³/ha) on soil nitrate contents with and without ploughing at the end of November was investigated. In November, the variants with manure had stored about 150 kg N/ha in the above-ground biomass and thus about 50 kg N/ha more than the unfertilised variants. The legume-free cover crop was thus able to successfully absorb and utilise the N fertiliser. The Nmin measurements showed that there was no risk of groundwater pollution due to increased Nmin values in deeper soil layers both after their ploughing in autumn or spring and during or after the following spring barley crop. For the yield of spring barley, the influence of ploughing on this site in the trial year was greater than that of biogas slurry fertilisation.