The number of biogas stations in the Czech Republic has recently increased considerably; as of 1 January 2014 some 500 biogas plants were registered of which 378 are agricultural biogas plants with installed electric energy of 392 MW. The estimated annual digestate production ranges around 7.9 mil t and the biogas stations are deployed irregularly in the regions. On that account practical problems frequently arise how to utilise the digestate effectively and/or where to apply it (in compliance with legislation -for example: The Nitrates Directive and Restrictions in the Vulnerable Areas). Considering the fact that digestate is produced during the whole year, possibilities are being sought of using it not only to fertilise arable crops but also vegetables. Whereas information on using digestate to fertilise arable crops is relatively abundant, in the case of vegetables it is vice versa (Dostál et al. 2015).Manures from stables, crop residues, wastes from the food industry, municipal wastes, and dedicated energy crops are the main feedstocks for anaerobic digestion (AD) in biogas plants. The residual product of AD, called digestate (= biogas effluents = biogas residues, or biogas slurry, when animal manures are digested), is usually used as (1) untreated control; (2) digestate; (3) digestate + phosphorus (P). The nitrogen (N) rate was the same in treatments 2-3. There were significant differences between years in all parameters. The weight of single kohlrabi bulbs in the unfertilised control was significantly lower in both years (33.1-46.9%) than in the digestate treatment (100%). Digestate supplemented with P (treatment 3) increased the bulb yield significantly by 11.0-14.3% compared with pure digestate (treatment 2). In both years the content of bulb nitrates (mg NO 3 -/kg FM (fresh matter)) was significantly the lowest in the unfertilised control (135 and 163, respectively). After digestate applications the nitrates content (mg NO 3 -/kg FM) increased significantly in both years, i.e. to 315-327 (2014) and to 486-509 (2015) compared to unfertilised control. In two years the content of ascorbic acid (mg/kg FM) did not differ among the three treatments (274-288 in 2014 and 311-329 in 2015). Digestates can be recommended for kohlrabi fertilisation prior to planting.

The vegetation pot experiment with kohlrabi (Moravia variety) was established in 2014 and 2015. The experiment included four treatments of fertilization: (1) untreated control, (2) digestate, (3) digestate + Mg + S, and (4) digestate + S. Treatments 2–4 were fertilized with the same N dose. Differences between the years were recorded in all parameters. Compared with the variant with digestate (100%), the kohlrabi yield of the unfertilized variant was demonstrably lower in both years (33.1% and 46.9%). Digestate enriched with the fertilizer containing Mg + S (treatment 3) demonstrably increased the yield of kohlrabi by 10.2% and 15.7% compared with pure digestate (treatment 2). Digestate enriched with the fertilizer containing elementary S (treatment 4) demonstrably increased the yield of kohlrabi (by 7.4%) only in 2015 compared with pure digestate (treatment 2). Except for the year 2015, there were no yield differences between variants 3 and 4. In both years, the lowest content of nitrates in kohlrabi was observed in the unfertilized control (135 and 163 mg NO3−/kgFM, respectively). Following the application of digestate (treatment 2), the content of nitrates (mg NO3−/kgFM) increased to 327 in 2014 and to 509 in 2015. The addition of fertilizers with Mg + S as well as fertilizer with elementary S to the digestate (treatment 3 and 4) significantly reduced the content of nitrates to 295–301 mg NO3−/kgFM (2014) and to 449–468 mg NO3−/kgFM (2015). The content of ascorbic acid did not statistically differ among the four treatments in the two years (268–281 and 311–329 mg/kgFM in 2014 and 2015, respectively). Digestate supplemented with Mg + S (magnesium sulfate) or only with elementary S can be recommended for kohlrabi fertilization prior to the planting in order to reduce dangerous accumulations of nitrates in kohlrabi.