Parameters of radish phytomass (Raphanus sativus L.) determined by vermicompost and earthworms (Eisenia fetida)

With progressive days of the experiment, the weight of radish phytomass was being increased (Tables 2 and 3). On average, root weight increased 46.43 times (by 108.12 g per 10 individuals) between the first and second samplings, i.e. between the 27^th and 36^th days from plant germination (totally 9 days). The weight of leaves was increased by only 3.95 times (by 52.83 g per 10 individuals). Between the second and third samplings (totally 6 days) the root weight accumulated only 1.42 times (by 46.66 g per 10 individuals) and the weight of leaves 1.43 times (30.37 g per 10 individuals), meaning that the growth dynamics of both the aboveground and underground phytomass were almost equal. This fact indicates that radish roots increased their weight most intensively after 27 days from plant emergence, i.e. in the second half of the radish growing season. Roots and leaves grew and increased their weight up to the 42^nd day of the growing season, i.e. until the end of the experiment, when the highest weights of leaves and roots were recorded. From the 36^th day of the growing season the growth dynamics slowed down, while the leaves and roots growth had almost the equal dynamics, in spite of the fact that the absolute value of weight gain of roots was higher than the absolute value of weight gain of leaves.

During the first days of the radish growing season the ratio of leaves to roots weight (L:R) was higher than 6:1 in each treatment (Table 3). It is evident that at the beginning of the growing season the aboveground phytomass was formed faster compared with underground phytomass. After 27 days of the growing season the root growth was more intensive as a result of the transfer of photosynthates from leaves to roots and, consequently, leaf growth decline. On the 36^th day of the growing season the leaf weight was lower than the root weight. A faster root growth compared to leaves caused that in the stage of technological ripeness (3 and 9 May) the ratio of roots-to-total phytomass was higher than 50% and achieved the values of 60.96% and 60.85% (Tables 2 and 3). Similarly, the roots of sugar beet, fodder beet and turnip can grow more intensively than the leaves. The change in the ratio of roots-to-leaves growth rate occurs predominantly during the changing sunlight intensity and nutrition intensity (Suqiura and Tateno, 2011). Formation of leaves is supported heavily by nutrition, however, the light sufficiency has the opposite effect (Poorter et al., 2012). The change of leaves-to-roots weight ratio occurs probably only to a certain extent, and it is determined genetically, via optimisation of the growth rate of the whole plants (Osone and Tateno, 2003). Nowadays, attention is focused on the possibility to predict the yield quantity, based on the determination of the dependence between the roots-to-leaves weight ratio and yield.

According to Table 2, the addition of Vc into the soil (Treatments 2–5) resulted in both the fast radish growth and the increase in the roots-to-leaves weight ratio. The increase of aboveground and underground phytomass weights was measurable already on the 27^th day from the beginning of plant emergence. The speed of Vc effect was high, because of high content of available nutrients, as referred in Table 1. Many countries tend to state the nutrients content in their total forms, when declaring the parameters of organic fertilisers; however, these are not sufficient indicators of their quality. Therefore, in the process of declaration of the quality of organic fertilisers, it is advisable to declare also the content of available nutrients, which affect plant growth markedly.

On the 36^th day, the average weight of radish roots was 6.61 times higher and the average leaf weight was 5.48 times higher in the treatment with Vc (2–5), than in the control treatment without Vc (1). On the 42^nd day of the growing season in treatments with Vc (2–5), the average weight of radish roots was 6.38 times higher and the leaf weight was 6.74 higher, compared with the control treatment (without Vc).

The significant multiple increases of phytomass weight resulting from the Vc addition into the soil approved the knowledge that the achievement of high yields is not possible without the application of considerable quantities of easily available nutrients (Cooke, 1982; Cong et al., 2019). At the same time, recording more dynamic growth of roots weight compared to leaves weight in treatments with Vc confirmed the fact that fertilisation by nitrogen usually determines a more significant growth of roots than leaves (Suqiura and Tateno, 2011).

The L:R ratio in Treatment 5 (Table 3) was higher in all the three samples of plant material compared to the control treatment, and in contrast to the same ratio in Treatments 2–4, which means that the 50% proportion of used (high quality) Vc in the growing medium was ineffective. This must be taken into account, because the main utility part of radish during its cultivation is roots, not leaves. In Treatments 2–4 the L:R ratios were smaller, even comparable to the control L:R ratio. It signalises that 10%, 20% and 25% proportion of Vc resulted mainly in roots formation, or at least balanced roots-to-leaves formation. In many Asian countries, radish is grown not only for root consumption, but often predominantly for leaves consumption (Bhatnagar and Azhar, 2016). Thus, it is highly probable that growers of radish leafy varieties will like the information that the highest leaves-to-roots weight ratio was recorded in the treatment with the highest proportion (50%) of Vc.

Along with the rise of Vc proportion in the soil substrate (by decreasing the ratio of soil to Vc) to the ratio of 1 unit Vc to 4 units S (Treatments 2 and 3), the weights of radish roots and leaves were increased (Table 2). In terms of this ratio the highest weight of the radish aboveground and underground phytomass was achieved. The addition of Vc to S was above the ratio of 1:4, i.e. above 20% content Vc in the growing medium (Treatments 4 and 5) led to the gradual decreasing intensity of radish roots and leaves formation. This fact affirms that in order to achieve the highest yield of radish roots, the proportion of tested Vc in the soil with low contents of available N, P and K (Table 1) should not be higher than 20% (Treatment 3).

In the substrates with the ratio of Vc:S at the level 1:3 and 1:1, the smaller underground and aboveground phytomass were formed than in the treatment with the ratio of Vc:S at 1:4; however, it was still significantly higher than in the treatment without Vc. This finding offers the opportunity to use Vc in soils with lower contents of available nutrients (Table 1) in relatively high doses (25% and 50% proportion in substrate), mainly in the case of a surplus production of Vc without risk of yield decreases.

In Slovakia, a local surplus production of composts is a serious problem. It is a consequence of disproportion between the increasing production of composts and the limited possibility of their application. The rising production of composts results from the legal obligation of inhabitants in Slovakia to compost biologically degradable wastes. The diminishing opportunity of their application into soils is caused, on one hand, by continuous decrease of private house gardens used for gardening, where so far the compost has been used; on the other hand, it results from the increasing disbelief in the quality of composts produced by non-agricultural companies. Therefore, the findings about the addition of Vc at the level of 50% of the total weight of soil substrate (in the top-soil layer) into soil with low available nutrient contents show that yield can be increased, which is a significant practical knowledge. It is necessary to take into account that application of high doses of Vc into soils should be carried out only in justified cases, in particular, in areas where the quality of groundwater and parameters of the environment will not be worsened.

The average root thickness of radish cultivated in the treatment without Vc (Treatment 1) was lower than 2.0 cm (1.46 cm and 1.69 cm). For this reason, roots cultivated in the control variant are almost unmarketable (Table 4). In the treatments with applied Vc (Treatments 2–5), the average root thickness was bigger than 2.0 cm, which satisfies the requirement of a majority of multiple stores. On the 36^th day of the growing season the root thickness varied in the interval of 2.96 cm to 3.36 cm, and on the 42^nd day of the growing season it varied from 3.08 cm to 3.72 cm.

The effect of Vc on root thickness corresponded with its impact on root weight. The highest root thickness was achieved in the treatment with S:Vc ratio of 4:1 (Treatment 3). The decrease of S:Vc ratio under 4:1, i.e. the increase of Vc proportion in the soil substrate above 20% (Treatments 4 and 5) led to the gradual decrease of radish root thickness. Analyses of Treatments 2–5 showed the smallest root thickness in Treatment 5.

This information proves that according to the normal distribution curve of the functional relationship of between the yield and N concentration in plants, the enhancement of N concentration in plants increases the yield up to a so-called breaking point, from which additional increases of N doses have a depressive impact on yield formation. The breaking point is determined by several factors, e.g. plant requirements for nutrients and soil nutrient reserves. In this sense, even low doses of fertilisers can result in decrease of yields in the case of their application into soils, which are supplied sufficiently by nutrients, or if the crops with very low requirements for nutrients are cultivated, as it was recorded by Berti et al. (2009); Liao et al. (2009); Li et al. (2015); and Adekiya et al. (2019). Disproportional and excessive plant nutrition is not only uneconomic, but also non-ecological (Chaudhuri et al., 2016).

The EW affected mostly negatively formation of radish aboveground and underground phytomass during the whole experiment (Treatments 6–13 vs Treatments 2–5).

The influence of EW on root and leaf weights was not identical during the growing season. The percentual drop of radish roots weight as a result of presented EW in soil substrates was decreased in the course of the growing season. On the contrary, the negative impact of EW on leaves weight was higher in the 2^nd and 3^rd samplings (3 and 9 May) in comparison to the first sampling (Table 2).

On the 27^th, 36^th and 42^nd days of the growing season, the roots weight treated with EW was lower (averages in Treatments 6–13) by 25.2%, 22.0% and 18, 3%, i.e. by 0.73 g, 30.42 g and 34.9 g per 10 individuals, respectively, than the roots weight not treated with EW (averages in Treatments 2–5). Similarly, also the leaves weight in the treatments with EWs (average Treatments 6–13) was lower on the 27^th, 36^th and 42^nd days of the growing season by 15.57%, 21.1% and 20.9%, i.e. by 3.16 g, 18.54 g and 26.2 g per 10 individuals, respectively, than the leaves weight cultivated in the treatments without EW (average Treatments 2–5). The submitted findings are in contradiction with the data published by Xiao et al. (2018), who, based on the meta-analyses, argue that EW in soil substrate significantly elevated plant aboveground biomass by 16%, underground biomass by 29% and total biomass by 22%. On the contrary, they support the opinion that roots are one of the main contributors of the principal foodsource of EW and may act well as an attractant to them (Brown et al., 2000). The idea that root hairs can be attacked by EW in the initial period of plant growing was presented by Kováčik et al. (2018b). The gradual decrease of roots weight from the 27^th to the 42^nd days of the growing season caused by EW (Table 2) corresponds with the given view.

As a result of the presented EW, radish roots weight (overall of the second and third samplings) was decreased by 65.32 g per 10 individuals and leaves weight decreased by 44.74 g per 10 individuals. It is evident that EW had a more negative impact on the roots weight than leaves weight (Table 2). Therefore, the finding can be more significant for growers of radish root varieties.

The following comparisons, such as Treatment 2 vs Treatments 6 and 7, Treatment 3 vs Treatments 8 and 9, Treatment 4 vs Treatments 10 and 11, Treatment 5 vs Treatment 12 and 13 indicate that the percentage of roots weight decline cultivated in treatments with EW (in all three samplings) was the highest compared with treatments without EW, where the growing medium contained the least quantity of Vc (10%). The root weight decreases between Treatment 2 and Treatments (6+7)/2 were at the level of 38.59%, 32.14% and 27.2% on the 27^th, 36^th and 42^nd days of the growing season, respectively. On the contrary, EW caused the least percentual fall of root weight cultivated in treatments, where the substrate contained the maximum, i.e. 50% of Vc (Treatment 5 vs. 12 and 13). The drop of root weight between Treatment 5 and Treatments (12+13)/2 was on average 2.47% in all the three samplings. Based on these facts, it is possible to state the hypothesis that if red worms have enough appropriate fodder (compost, Vc, crop residues, etc.) in its environment, it will not attack the root hairs of young plants. The yield of cultivated crops will be higher.

EWs affected the root thickness (diameter) similar to the root weight (Treatment 2 vs Treatments 6 and 7, Treatment 3 vs Treatments 8 and 9, Treatment 4 vs Treatments 10 and 11, Treatment 5. vs Treatments 12 and 13). They decreased the root thickness in the second as well as in the third sampling. The only exception was Treatment 12 in the third sampling, where the drop in root weight was non-significant in relation to the treatment without EW (Treatment 5) (Table 4). The average root thickness in the treatments with Vc and without EW (Treatments 2–5) increased in comparison with radish average root thicknesses cultivated in the treatments with Vc and EW (Treatments 6–13) on the 36^th day of radish growing season by 0.46 cm and on the 42^nd day by 0.51 cm, respectively.

Not only the presence of EW in the substrate, but also their number determined the formation of radish phytomass (Treatment 6 vs Treatment 7, Treatment 8 vs. Treatment 9, Treatment 10 vs. Treatment 11, Treatment 12 vs. Treatment 13) (Table 2). The average root weight cultivated in the treatment with 20 individual EW per pot was on the 27^th, 36^th and 42^nd days of the growing season, lower by 7.96%, 3.21% and 3.52% compared with the weight of roots in the variants with 10 individual EWs per pot. Nurhidayati et al. (2016) recorded both increase and decrease in cabbage yield with the increased number of EW of specie P. corethrurus in Vc. The composition of Vc and its origin decide what number of the EW will increase, and what number will decrease the crop yield.

In the first sampling in the treatment with 20 EWs, a significant decrease in the root weights was detected in two treatments in comparison to the radish root weights cultivated in the treatments with 10 numbers of EWs, in particular in the treatment with the S:Vc ratio of 9:1 and in the treatment with the S:Vc ratio of 4:1. In the second and third samplings, there was a considerable decrease in the root weights in the treatments with 20 numbers of EW, only in the treatment with the least (10%) content of Vc (S:Vc = 9:1) compared with the treatment with 10 EWs. This fact approves the hypothesis that red worm (E. fetida) probably attacks the root hairs in case of fodder insufficiency.

The number of EWs did not influence the thickness (diameter) of the roots (Table 4). The difference between the pairs of treatments (6–7, 8–9, 10–11, 12–13) was insignificant. Similarly, the differences in the leaf weights were insignificant, which indicates that the number of EWs did not affect the leaf weights in a considerable way. The average decreases in leaf weights in the treatments with 20 EWs were lowered on the 27^th, 36^th and 42^nd days of the growing season by 4.50%, 1.40% and 2.65% compared with the treatments with 10 EWs.

The total chlorophyll content (on average for all variants of the experiment) was lesser only by 2.43% (8.17 mg · m⁻²) on the 42^nd day, comparing with that on the 36^th day of the growing season. Similarly, the ratio of chlorophylls a content to chlorophylls b content was lower on the 42^nd day, eventually similar to that on the 36^th day of the growing season, highlighting that chlorophyll a always dominated over chlorophyll b. A slight decrease in the total chlorophyll contents in the radish plant leaves harvested 6 days later related to a shorter time interval between the particular samplings and also to the fact that the radish seeds did not germinate and emerge on the same day concurrently, but in the course of 3 days, so the age of the analysed leaves was lesser than the stated 6 days. It is evident that the recorded values are consistent with those of Ebbs and Uchil (2008), who stated that in the process of leaf ageing, the decrease in the total chlorophyll contents can be measured. The degradation of chlorophyll during ripening and plant ageing is a natural process (Kumar et al., 2019).

The contents of chlorophylls a and b in radish leaves of both samplings increased progressively along with the increasing proportion of Vc in soil. Concurrently the chlorophyll a to b ratio was lowered, as a result of the fact that along with the increasing content of available N, Mg, S and other nutrients in soil, the content of the total chlorophyll in plants increases (Kováčik et al., 2014).

On average of variants 2–5, the increase of chlorophyll a, b, a+b contents was at the level of 38.78 mg · m⁻², 27.05 mg · m⁻² and 65.83 mg · m⁻² on the 36^th day of vegetation period compared to the contents in the control variant (Treatment 1). On the 42^nd day of the growing season the increase achieved 41.9 mg · m⁻², 27.35 mg · m⁻² and 69.25 mg · m⁻². In the relative expression, on the 36^th day of the growing season the increases were 19.57%, 33.69% and 23.64%, and on the 42^nd day of the growing season the increases were 22.15%, 35.02% and 25.91%.

As the plant leaves contain more chlorophyll a than chlorophyll b (Novichkova et al., 2006, Noman et al., 2018), the absolute increase in total chlorophyll content as a result of the improved plant nutrition was higher in chlorophyll a than in chlorophyll b; however, the relative increase was higher in chlorophyll b. Relatively higher increase of chlorophyll b than chlorophyll a content explains recorded decrease of chlorophyll a:b ratio, along with the rising proportion of Vc in soil. The Vc increased relatively more chlorophyll b than chlorophyll a content. The highest ratio of chlorophyll a:b was monitored in the control treatment.

On the 36^th day of the growing season, the differences in the chlorophyll contents between the particular treatments with (Treatments 2–5) and without Vc (Treatment 1) were significant. On the 42^nd day of the growing season, considerable differences occurred only in the treatments at 20%, 25% and 50% of the proportion of Vc in the soil. This finding is related to the decreasing positive impact of the added available nutrients on the formation of chlorophylls during running days of the growing season. The effect of the added nutrients depends on their doses, e.g. the lower the nutrient dose, the shorter is the effect.

A comparison of the average values of chlorophyll contents in the treatments with (Treatments 6–13) and without EW (Treatments 2–5) indicate that the lower contents of chlorophyll a, chlorophyll b and, consequently the total chlorophyll content, were obtained in the treatments with EW. At the same time the lower chlorophyll a:b ratios were measured as a consequence of the fact that the decrease in chlorophyll a content was higher than that of chlorophyll b. The decrease in chlorophyll a on the 36^th and 42^nd days of the growing season was at the level of 2.40% and 2.84%, and the decrease in chlorophyll b achieved 0.79% and 0.29%, respectively. The declines in chlorophylls a, b a+b contents, caused by the presence of EW in the soil, were not statistically significant in any treatment. Therefore, it can be stated that EW tend to decrease the chlorophyll content in radish leaves, or that the effect of the EW on the chlorophyll content was insignificant.

The differences in the chlorophyll contents found between the pairs of treatments (6 and 7, 8 and 9, 10 and 11, 12 and 13) were minimal and statistically insignificant. On average, in the treatments with 20 EWs (Treatments 6, 8, 10, 12) the chlorophyll a, b, a+b contents were lowered by 1.72%, 0.37% and 1.3% in comparison with the contents of chlorophylls in the treatments with 10 EWs (Treatments 7, 9, 11, 13). The lowest ratio of chlorophyll a:b was registered in the treatment with 20 individual EWs (Table 5). The findings of the experiment show a higher number of EWs tending to decrease the content of chlorophyll in radish leaves to more than a lower number; however, the drops were insignificant.

On the 36^th day of the growing season the content of vitamin C in radish roots was lesser by 18.1 mg · kg⁻¹ compared with its content on the 42^nd day of the growing season. On the contrary, the opposite tendency was recorded in the leaves (Table 6). On the 36^th day of the growing season the content of vitamin C in the leaves increased by 15.1 mg · kg⁻¹ in comparison to the content detected on the 42^nd day of the growing season. A higher content of vitamin C in older than younger leaves of lettuce and broccoli was shown by Omary et al. (2003) and Yamada et al. (2003). Similarly, Cheptoo et al. (2019) claim a higher content of vitamin C in older than younger leaves of amaranth plants. On the contrary, the decrease in the content of vitamin C in dill leaves in successive days of the growing season was measured by Lisiewska et al. (2006).

During both terms of radish harvesting the content of vitamin C in the leaves dominated over that in the roots in all treatments of the experiment. On average, in all the treatments the content of vitamin C was 2.50 times higher in the leaves than in the roots on the 36^th day of the growing season and 1.98 times higher on the 42^nd of the growing season.

Along with increasing proportions of Vc in the growing media (Treatments 2–5), the content of vitamin C decreased in the radish phytomass aboveground and underground (Table 6). The reason was that the increasing proportion of Vc in the soil (S+Vc = 9:1; 4:1, 3:1, 1:1) also enhanced the content of inorganic nitrogen (the Vc contained 33.7 times more N_min than the soil) (Table 1). The negative correlation was usually recorded between the inorganic nitrogen content in the soil and the vitamin C content in plants (Lisiewska and Kmiecik, 1996). The opposite effect of Vc on increase in the vitamin C content in cabbage leaves (Brassica oleracea) was presented by Nurhidayati et al. (2016).

The highest content of vitamin C in the roots and leaves was detected in the control treatment without Vc (Treatment 1). Conversely, the lowest content was recorded in the treatment with mixed Vc and S in the 1:1 ratio (Treatment 5), i.e. in the treatment where the highest quantity of Vc was used.

Along with the increasing content of Vc in the soil substrate, the roots-to-leaves vitamin C content ratio had increased (Table 6). It is evident that the decline in vitamin C content was more considerable in the leaves than in the roots as a result of higher applied doses of Vc. Similarly, the statistical significance of the decreases of vitamin C contents between Treatments 2–5 was more distinctive in the evaluation of the vitamin C contents in leaves compared with roots. These data prove the fact that Vc decreased the vitamin C content more heavily in leaves than in roots, as a result of more intensive ageing of radish leaves than roots.

The radish roots from the last harvest (9 May) had a higher content of vitamin C than roots from the previous harvest (3 May). On the contrary, in the leaves the content of vitamin C declined between 3 and 9 May. The older leaves contained less vitamin C than the younger ones.

Embedding of EW into the soil substrates containing Vc did not significantly affect the content of vitamin C in radish phytomass aboveground and underground (Treatment 2 vs Treatments 6 and 7, Treatment 3 vs Treatments 8 and 9, Treatment 4 vs Treatments 10 and 11, Treatment 5. vs Treatments 12 and 13) (Table 5). In both harvesting terms the vitamin C contents measured in the radish roots and leaves were the same in the variants 6–13, or statistically insignificantly higher than the contents of vitamin C measured in the variants 2–5. The average increases were 0.60 and 0.57 mg · kg⁻¹ (root) and 0.01 mg · kg⁻¹ and 3.52 mg · kg⁻¹ in the leaves. Xiang et al. (2016) published similar data; they also measured the statistically insignificant impact of EW of E. fetida specie on the vitamin C content in papaya fruits (Carica papaya L.).

The addition of EW to the treatment with 10% proportion of Vc of the total substrate weight tended to decrease the content of vitamin C in radish roots and leaves in comparison with the treatments containing more Vc (20%, 25% and 50%). The drops were insignificant. The probable reason for this insignificant difference was the insufficient quantity of fodder for red worms in the substrate with 10% proportion of Vc. The subsequent attack of root hairs by the EW induced a stress in radish plants. The positive effect of the EW (P. corethrurus) up to 25 individuals per m² on the vitamin C content in cabbage leaves was presented by Nurhidayati et al. (2016). On the contrary, along with the increasing number of EW in soil at above 25 individuals (50, 75 and 100 individuals per 1 m²) the content of vitamin C in cabbage leaves decreased proportionally.

The number of EW did not significantly influence the vitamin C content in radish roots and leaves (Treatment 6 vs Treatment 7, Treatment 8 vs. Treatment 9, Treatment 10 vs. Treatment 11, Treatment 12 vs Treatment 13 – Table 6). Nevertheless, it can be claimed that the content of vitamin C was slightly higher with 20 EWs added per pot than with 10 EWs added per pot. The similar positive tendency of the impact of a higher number of EWs on the vitamin C content was measured also in the leaves.

In the radish plants sampled during the first harvest (3 May), the nitrate content (NaNO₃) was higher than in plants sampled during the second harvest (9 May). The average decline of the nitrate contents in radish roots in the course of 6 days, i.e. between both harvest terms, was 197.57 mg · kg⁻¹ and 153.75 mg · kg⁻¹ in leaves, respectively – in all variants of the experiment (Table 7). The decrease in nitrate content in time (content of nitrates on 3 and 9 May) occurred in all the treatments. The decline in nitrate content with the following days of the growing season was also presented by Li et al. (2017). In their experiments the contents of nitrates in tobacco leaves measured from the 45^th to 75^th day after transplanting varied approximately from 100 mg · kg⁻¹ to 1,250 mg · kg⁻¹ NO₃⁻–N. On the contrary, Tabaglio et al. (2020) did not record the unambiguous dependence between the nitrate content in lettuce leaves and duration of lettuce growing season in the last 6 days before lettuce harvest.

In the experiment, a higher decrease of nitrates content in roots compared to leaves was the result of higher nitrates content in the roots compared to the leaves. In the roots there was on average 2.26 times more nitrates than in the leaves in both the samplings. The opposite effect, i.e. higher content of nitrates in vegetable leaves than in roots is stated by Santamaria et al. (1999). Similarly, Zhen and Leigh (1990) detected a higher content of nitrates in wheat leaves than in roots. Conversely, the higher content of nitrates recorded in roots than in leaves corresponds with the data of Anjana and Iqbal (2007), who explained that vegetables forming reserve organs have a different mechanism for nitrates storage than leafy vegetables. A higher content of nitrates in roots than in leaves of different plant species was presented by Black et al. (2002). The site of accumulation of nitrates in plants is determined by the localisation of mechanisms reducing nitrates (leaf, root), which depends on a plant species (Hucklesby et al., 1990).

Along with the increasing quantity of Vc in the soil substrate, the content of nitrates was increasing in radish roots and leaves (Table 7). In Treatments 2–5 the content of NaNO₃ in radish roots increased by 1,790.68 mg · kg⁻¹ and in leaves by 769.33 mg · kg⁻¹ on average in both sampling compared with the control variant 1. The increase in content in the roots was 4.22 times and in leaves it was 3.93 times higher. The increase of nitrates content in radish phytomass along with the increase of quantity of Vc in the substrate was a consequence of the well-known dependence of the content of nitrates in a plant on the content N_in in soil, on the dose of inorganic nitrogen applied into soil in the form of the mineral or organic N fertilisers (Yuan et al., 2014; Adekiya et al., 2019). Low N doses do not increase the nitrate contents in plants (Liao et al., 2009). The increase in nitrates content was accompanied by a decrease in the vitamin C contents (Tables 7 and 8). The increase in nitrate contents and the decrease in the vitamin C content along with the growth of application dose of Vc were presented by Papathanasiou et al. (2012).

The comparison of nitrate contents in Treatments 2–5 with Treatments 6–13 (Table 7) indicates that the EW in the growing media showed a tendency to decrease the nitrate contents in radish roots and leaves. On average, in Treatments 2–5 versus 6–13, the drop in the content was insignificant in the leaves, at the levels of 3.26% and 2.39% between both the harvest periods (3 and 9 May). The decrease in the nitrate contents in the roots was significant in two treatments (with 10% and 20% content of Vc) and was insignificant in two instances (treatment with 25% and 50% content of Vc). On average, in Treatments 2–5 vs 6–13 the decline in nitrates contents in radish roots was 6.09% and 4.16% during the harvest period on 3 and 9 May, respectively. Based on the information that EW increases the nitrogen mobility in soils and the quantity of washed-up N-NO₃ from soil (Domínguez et al., 2004; Marhan et al., 2015), one may expect in the treatments with EW, an increased content of nitrates in radish plants. This assumption, however, was not true.