IS BIOGAS DIGESTATE A SAFE AND AN EFFICIENT FERTILIZER FOR AN ORGANIC ICEBERG LETTUCE CROP?

We compared the fertilization effects of two different manure-based biogas digestates with commercial organic fertilizers, such as meat bone meal (MBM) and composted and granulated chicken manure (CM), on an organic iceberg lettuce (Lactuca sativa var. capitata) crop. Organic fertilizers were also compared with a commercial mineral fertilizer (NPK). The second aim was to evaluate the risks of the use of biogas digestate on the hygienic quality of the lettuce crop. We tested whether one option to avoid hygiene risks could be a use of the biogas digestate as a fertilizer in the previous year of the rotation. A two-year field experiment was conducted as a split-plot design with three rotation options and five fertilization treatments. Rotation option 1 included fertilized lettuce in 2010 and in 2011. In rotation option 2, a fertilized pea-oat mixture or oat in 2010 was followed by fertilized lettuce in 2011. In rotation option 3, an unfertilized pea-oat mixture or oat was followed by fertilized lettuce in 2011. The fertilization effect of the tested biogas digestates proved to be similar to other tested commercial organic fertilizers. This indicates that manure-based biogas residue is a suitable fertilizer for organic iceberg lettuce when it is applied to the soil not earlier than two weeks before planting. In all fertilization treatments, the hygienic quality of the lettuce was good and did not differ from the NPK treatment. Therefore, the risk of the use of biogas digestate on the hygienic quality of the lettuce crop was minor. No residual fertilization effect from the fertilization of the rotation plants was seen in the following lettuce crop. We conclude that the fertilization of the previous year’s rotation plant with biogas digestate is not efficient in terms of the utilization of recycled nutrients when a pea-oat mixture is used as a rotation plant and spring wheat as a catch crop.