Comparison of the effectiveness of digestate and mineral fertilisers on yields and quality of kohlrabi (Brassica oleracea, L.)

Abstract

In a one-year vegetation pot experiment we compared the effect of the digestate from a biogas station and mineral fertilisers on yield and quality parameters of kohlrabi, variety Seguza. Four treatments were used in the trial: 1) untreated control, 2) urea, 3) digestate, 4) urea, triple super phosphate, KCl, MgSO4. The rate of N was the same in treatments 2–4, 1.5 g N/pot. In treatment 4 the rate of P, K and Mg corresponded with the rate of these nutrients in the digestate treatment (3). The weight of single bulbs of the control unfertilised treatment were significantly the lowest (22.9%), as well as the nitrate (6.0%) and ascorbic acid content (66.2%) compared to the urea treatment (100%) and the other fertilised treatments. After the application of the digestate (treatment 3) and mineral fertilisers (treatment 4) the weight of single bulbs significantly increased by 27.9 and 29.2%, respectively, compared to the urea treatment (2). The content of ascorbic acid in the fertilised treatments did not differ (772–789 mg/kg) but it increased significantly compared to the unfertilised treatment (511 mg/kg). There were no significant differences between the two treatments fertilised with mineral fertilisers in the bulb nitrate content (678 and 641 mg NO3−/kg fresh matter, respectively). After digestate application their contents decreased significantly to 228 mg NO3−/kg fresh matter. Digestate treatment resulted in comparable or better yield and qualitative parameters compared to treatment with mineral fertilisers.

The effect of fertilisation with digestate on kohlrabi yields and quality

“…There was thus an obvious positive synergistic effect of additional nutrients in digestates (especially N and K) on yields. This confirms that N is the decisive element in terms of yields as reported previously by Feller and Fink (1997) and Lošák et al (2011). In experiments lasting several years Stinner et al (2008) also reported positive effects of three different types of digestate (fermented clover-grass mixture, cover crops and post-harvest residues) on wheat yields.…”

“…However, in a previous experiment with kohlrabi of the cv. Segura F1 (Lošák et al 2011) the content of ascorbic acid was found to be higher in all the fertilised treatments (772-789 mg/kg FM) as compared to the unfertilised treatment (511 mg/kg FM). Previous studies differ in their conclusions regarding the effect of nitrogenous fertilisation on the content of vitamin C. Mozafar (1993) reported Mean values of kohlrabi bulb nitrates (n = 4).…”

“…Segura F1 Lošák et al (2011) found out the similar results -the nitrate content in kohlrabi was the highest after the application of urea (678 mg/kg of FM), while it decreased significantly in the digestate treatment to 228 mg/kg of FM. Unfertilised control had the lowest nitrate content -41 mg/kg of FM (Lošák et al 2011). The reason could be that the digestate contains a specific proportion of organic N (25-50%) which is subject to mineralisation after a certain period (Kirchmann and Witter 1992).…”

“…Besides macronutrients, digestates contain micronutrients, too. The application of digestates can therefore save considerable costs for the purchase of mineral fertilisers (Lošák et al 2011).…”

“…Literature on the application of digestates as fertilisers for vegetables is very scarce. Expert opinion on the properties and possibilities for practical use of digestate as an organic fertiliser is divided (Odlare et al 2008, Kolář et al 2008, Möller et al 2011, Lošák et al 2011. Digestion is associated with large losses of organic carbon (C) (Möller 2009).…”