Plant nutrients are present in inorganic plant-available forms to a markedly higher level in digested residue than in untreated waste [19], due to the mineralisation of large amounts of organically bound nutrients during the digestion process [20]. Table 5 shows the post-harvest values of some chemical characteristics of the soil (pH, available P, K, Ca, Mg) following the different treatments.

The soil was extracted according to Mehlich III (CH₃COOH, NH₄NO₃, NH₄F, HNO₃ and EDTA). The content of available P in the extract was determined colorimetrically and the content of available K, Mg and Ca using an AAS. The ion-selective electrode (ISE) was used to determine the pH value [17]. The content of N-NH₄⁺ in the soil was assessed colorimetrically; the N-NO₃^- content using ISE. The content of macrobiogenic elements in the plant biomass was assessed after wet mineralisation (H₂SO₄+H₂O₂). The N content was determined by Kjeldahl analysis, while colorimetric analysis was used to assess P, and the AAS method to assess K, Ca and Mg [18].

The results were processed statistically using variance analysis followed by testing according to Scheffe (P <0.05).

In the treatment with digestate and that with NPKMg fertiliser, the contents of P, K and Mg in the soil were found to have increased significantly after harvest compared with the other treatments (Table 5). Thus the soil was enriched with these nutrients after application of these two fertilisers despite the removal of nutrients in the harvested kohlrabi. Both treatments also produced considerably higher yields of bulbs (and hence also higher nutrient uptake) than the other treatments. Loria and Sawyer [21] reported that anaerobically digested liquid swine manure could provide similar plant-available N and P as would be expected from raw swine manure. In the present study, there were no differences between treatments in terms of soil pH (Table 5). Digestate is alkaline and increases the soil pH [22] making it suitable for acid soils. The reason for the reduction in soil K content in the N fertiliser treatment was its higher uptake by plants as a result of the synergistic effect of N fertilisation [23].

Nitrogen is the nutrient most subject to transformations affecting its availability to plants. These transformations include mineralisation, immobilisation, nitrification and denitrification, as well as leaching and ammonia volatilisation [24]. It is difficult to synchronise the supply of N from organic manures with the demand by crops for N [25]. Soil microbial activity leads to N release that is not necessarily in synchrony with plant nutrient demand [26]. The elevated NH₄⁺-N concentration in digested effluents suggests its potential suitability as a readily available N fertiliser source.

The post-harvest soil content of Nmin was lowest in the unfertilised treatment (Table 6). No appreciable differences in the total content of Nmin (6.12-7.57 mg/kg and 4.02-4.70 mg/kg respectively) were observed among the other treatments. However, a difference was observed in the forms of N present. The proportion of N-NH₄⁺ was greater than the N-NO₃^- only in the digestate treatment. Digestate has a large proportion of organically bound N (50-75%) which is available only after mineralisation [27]. The results suggest that, due to the short duration of the experiments, only a minor amount of this total N was nitrified. For the plants to utilise more N from the digestate, a longer period of crop growth and N uptake would be necessary.

One characteristic of kohlrabi is a high demand for N from the soil [28], and therefore deficiency of NO₃-N in the soil reduces yields [29]. Sharof and Weir [30] studied the minimum amount of N required by vegetable crops, including kohlrabi, in relation to N balance in the soil and found that N requirements were invariably lower than values indicated by field trials.

As early as the first stages of growth in this pot study, there was a visible difference between the fertilised treatments and the unfertilised control. The plants in the latter had a lighter colour and growth of the above-ground biomass was markedly slower. At harvest, symptoms of P deficiency (violet discolouration) were detected on bulbs of the control treatment, which was the result of low P supply to the soil and relatively unsuitable pH value for P uptake.

The weight of the unfertilised bulbs was 48.5% (2010) and 66.2% (2011) lower than those in the treatment with N fertiliser only (Table 7). This demonstrates that N is an important element in terms of yield [31], [28]. However the weight of single bulbs fertilised with the digestate and with NPKMg fertilisers was significantly higher, by 34.7-42.9% (digestate) and 37.2-38.2% (NPKMg fertilisers) respectively, than that of bulbs fertilised with N fertiliser only. The positive synergistic effect of additional nutrients (especially P, K, Mg) on yield formation was apparent for the digestate and NPKMg fertiliser treatments. However, no significant differences were found between these two treatments. Lošák et al. [39] found the same yield results with kohlrabi of a different variety (Segura F1).

In experiments lasting several years, Stinner et al. [3] also reported positive effects of three different types of digestate (fermented clover-grass mixture, cover crops and post-harvest residues) on wheat yields. Similarly, Bath and Elfstrand [7] reported that yields of leek were higher after digestate application than after fertilisation with compost. On soil with a low or satisfactory supply of available nutrients, Cigánek et al. [1] found that grain yield of winter wheat increased by 30.0-63.9% and seed yield of winter rape by 38.5–57.7% when fertilised with digestate compared with an unfertilised control.

Vitamin C, including ascorbic acid and dehydroascorbic acid, is one of the important nutritional quality factors in many horticultural crops and has many biological functions in the human body. The content of vitamin C in vegetables can be influenced by various factors such as genotype differences, pre-harvest climatic conditions and cultural practices, maturity and harvesting method, and post-harvest handling procedures [32]. Mozafar [33] reported that N fertilisers, especially at high rates, seem to decrease the concentration of vitamin C in many different vegetables. In contrast, Nilsson [34] reported that N fertilisation did not affect the content of vitamin C in cauliflower.

The lowest content of ascorbic acid in kohlrabi in this study was observed in the unfertilised control (Table 8). A number of authors have concluded that adequate nutrition and fertilisation helps to increase yields and quality parameters, e.g. vitamin content [31], [23]. Maurya et al. [35] showed that with higher N doses cauliflower contained significantly more vitamin C. No significant differences in the ascorbic acid content were detected between the fertilised treatments (Table 8).

The concentration of NO₃^- in plants is affected primarily by the vegetable species, level of N fertilisation, plant organ analysed, growth stage and the sulphur concentration in the tissues [36], [37]. Kohlrabi is prone to a higher risk of nitrate accumulation in tissues than some other vegetables [31]. The lowest nitrate content was observed in the unfertilised treatment and the second lowest nitrate content in the digestate treatment (Table 8). The same results were described by Lošák et al. [39] with kohlrabi variety Segura F1. The reason is probably that the digestate contains a significant proportion of organic N (25-50%), which is subject to mineralisation and subsequent nitrification only after a certain period [27].

It can be assumed that during the short growing period of kohlrabi (approx. 7 weeks), only part of the organically bound nitrogen was mineralised. Therefore mineral N-NH₄⁺ from the digestate (or after its nitrification N-NO₃^-) was available to the plants and was sufficient for yield formation, but did not increase the nitrate content in the bulbs. The nitrate content was highest in the two treatments fertilised with nitrogen in the form of urea (N and NPKMg fertiliser treatments), where it was threefold that in the digestate treatment (Table 8).

In terms of the content of macronutrients in the bulbs (Table 9), the differences were most marked in the case of nitrogen. The N content was highest in the urea-fertilised treatments (N and NPKMg fertiliser treatments) from which the plants could exploit the available forms, i.e. NH₄⁺ and NO₃^-.

In the digestate treatment, the N content in kohlrabi tissues was significantly lower as a result of limited mineralisation of organically bound N within the short time interval. Furthermore the addition of readily degradable C compounds may have led to immobilisation of mineral N in soil [26]. In terms of the other nutrients, there were mostly no significant differences between the treatments.

A low P content was detected in the tissues of plants where urea alone was applied (treatment 2), which was due to its low content in the soil and alkaline pH negatively affecting its uptake [23]. Möller and Stinner [38] reported that slurry digestion did not influence plant P and K uptake.