3.1. Soil Measurements and Analyses
The application of the fertiliser with humic acids significantly changed some physicochemical properties of the soil. In the HBG × 4 variant, the soil pH increased to 7.0, as compared with pH = 6.4 in the control variant (
Table 3). Akinci et al. [
27], Katkat et al. [
28] and Zydlik and Zydlik [
17] also observed that the application of humic acids reduced soil acidity. Soil salinity decreased from 0.30 (soil fertilisation) to 0.26 in the HBG × 3 variant and to 0.18 g NaCl dm
−3 in the HBG × 4 variant. This application had little effect on the content of soil macronutrients. In comparison with the soil fertilisation variant, only the content of N-NO
3 (HBG × 2) in the soil increased significantly (
Table 3).
The content of soil macronutrients did not change after the plants had been sprayed three or four times. In comparison with soil fertilisation, the application of humic acids did not cause a change in the content of two soil micronutrients, i.e., Zn and Cu. The content of Mn and Fe decreased by about 20% (
Table 4).
The microbiological properties of soil, including the species diversity of microorganisms, are a measure of its fertility. Soil microorganisms significantly influence soil productivity through the mineralisation of organic matter. Soil respiration is a measure of the activity of microorganisms [
29]. As their activity increases, so does soil respiration. In our experiment, soil respiration increased significantly. In comparison with the variants with soil fertilisation, the soil respiratory activity in variants HBG × 3 and HBG × 4 increased from 20.54 to 29.04 and 26.21 mg kg
−1 24 h
−1, respectively (
Table 5). Thus, these observations confirmed the findings of earlier studies, which indicated the positive effect of humic acids on soil respiration [
17,
30].
Another measure of soil bioactivity is its enzyme activity [
31,
32]. The higher the soil enzyme activity is, the higher the rate of mineralisation of organic compounds is, and thus the amount of macro- and micronutrients available to plants. Dehydrogenases, belonging to the group of oxidoreductases and hydrolases, as well as proteases, are some of the most important soil enzymes. In our experiment, the application of humic acids significantly increased the activity of soil enzymes. In comparison to the variants with soil fertilisation, the content of both dehydrogenases and proteases in the HBG × 2 variant doubled from 0.24 to 0.49 cm
3 H
2 24 h
−1 kg
−1 d.m. and from 1.49 to 2.93 mg tyrosine h
−1 kg
−1 d.m., respectively (
Table 5). The greater the number of treatments was, the higher the activity of both soil enzymes was. The quadruple application of Humi Brown Gold was the most effective. In comparison with the variant where double spraying was applied, the activity of soil dehydrogenases in the HBG × 4 variant increased by about 27%, whereas the activity of proteases increased by about 35% (
Table 5). Schoebitz et al. [
33] observed that the application of humic acids in a blueberry plantation increased the activity of soil enzymes.
3.2. Strawberry Leaf Analyses and Measurements
Depending on the experimental variant, the area of strawberry leaves ranged from 86.84 cm
2 (soil fertilisation) to 145 cm
2 (HBG × 2) (
Figure 1). The treatment of strawberry plants with the preparation containing humic acids increased the area of leaf blades. In the HBG × 2 variant, it was over 60% greater than in the variant with soil fertilisation (145.34 vs. 86.84 cm
2, respectively). However, when the number of treatments increased, there was no further increase in the leaf area than in the HBG × 2 variant. Rzepka-Plerens et al. [
34] and Derkowska et al. [
35] also observed an increase in the strawberry leaf area under the influence of biostimulants, including humic acids.
The analysis of the average leaf weight showed that the application of the preparation containing humic acids positively influenced the growth of strawberries. The weight of the leaves of the plants which had been treated with the fertiliser twice was almost two times greater than the weight of those treated with the soil fertiliser (19.46 vs. 10.77 g, respectively) (
Figure 1). However, the average weight of strawberry leaves did not increase with the number of treatments. There were similar results of the experiment conducted by Soltaniband et al. [
36], who found that the treatment of strawberries with several types of biostimulant (mycorrhizal and
Trichoderma fungi, seaweed extracts, bacteria) significantly increased their biomass.
The experiment did not reveal differences in the degree of dehydration of strawberry leaves, measured with the RWC (relative water content) and WSD (relative water deficit) indicators. The variants where multiple foliar treatments with the fertiliser had been applied did not differ significantly in either of the two indicators from the variants which had received soil fertilisation (
Figure 1). This may have resulted from the fact that there was sufficient soil moisture for normal growth of the plants, as evidenced by the measurements made with tensiometers (
Appendix B).
The chemical composition of strawberry leaves and fruits depends on the cultivar, climatic conditions, and the degree of fruit maturity. In our experiment, the content of macronutrients in the strawberry leaves was as follows (% d.m.): N—2.11–2.18%; P—0.24–0.28%; K—1.65–1.70%; Ca—1.79–2.27%; Mg—0.31–0.35% (
Table 6).
According to Wójcik [
37], who published the limit content of components in strawberry leaves, the N content in our experiment was low and the P and K content was optimal, whereas the Mg content was high. These values were lower (P, K) or higher (Mg, Ca) than the content of components in strawberry leaves according to Sas-Paszt et al. [
38]. In our experiment, the content of micronutrients in the strawberry leaves was as follows (% d.m.): Zn—9.77–12.0%, Cu—3.17–3.56%, Mn—47.0–70.3%, Fe—137–175%, B—35.28–36.72% (
Table 7). According to the limit content of micronutrients in strawberry leaves published by Wójcik [
37], the Zn and Cu content in our experiment was low, whereas the Mn, Fe, and B content was optimal.
The treatment of strawberry plants with the fertiliser containing humic acids did not have significant effect on the content of macro- and micronutrients in their leaves. Regardless of the number of treatments, the strawberry leaves which had received the fertiliser containing humic acids did not differ significantly in the content of each of the five macronutrients under analysis from the control variant with the soil fertiliser (
Table 6). Our research finding is similar to the observations made by the authors of other studies, who found that the biostimulants they had applied did not have effect on the content of N, P, Ca, or Mg in strawberry leaves [
19,
36,
39]. The fertilisation of strawberries with humic acids also did not significantly influence the content of micronutrients in their leaves. The content of three out of five micronutrients (Zn, Cu, B) in the strawberry leaves fertilised with the preparation containing humic acids did not differ significantly from the content of these elements in the control variant (
Table 7).
3.3. Flowering and Yield of Strawberry Fruit
The application of the fertiliser with humic acids (HBG × 3 and HBG × 4) positively influenced the flowering intensity of strawberries and the setting of fruit. For example, the number of flowers per plant in the HBG × 3 variant was significantly greater (by about 50%) than in the variant with soil fertilisation (
Table 8). Also, the average number of fruits on the plants in the HBG × 3 variant (21 pcs.) was more than two times greater than in the control variant (10 pcs.). This effect was not observed in the HBG × 2 variant. Nevertheless, the percentage of strawberry fruits set in this variant (about 70%) was much higher than in the control variant (50%) (
Table 8). In the HBG × 4 variant, the percentage of fruits set amounted to 79%.
The profitability of cultivation depends on the yield of crops. The application of the fertiliser with humic acids had a positive effect on the yield of strawberries. The yield from the plants in variants HBG × 3 and HBG × 4 was about 60% higher than from the plants where the soil fertiliser had been applied (
Figure 2). The yield of strawberries in the HBG × 2 variant (13.76 t ha
−1) was slightly higher than in the variant with soil fertilization (10.08 t ha
−1). However, the differences were not statistically significant.
The result of our experiment is in line with the findings of other studies. For example, Shehat et al. [
39] observed that the strawberries treated with humic acids gave a higher yield than those treated with a mineral fertiliser. Bogunovic et al. [
40] also observed that the yield of strawberries treated with biostimulants increased by several dozen per cent.
The increase in the yield of the strawberries treated with humic acids may have been caused by their easier uptake of macro- and micronutrients from the soil [
18,
41]. This uptake could have been facilitated by a well-developed root system of the plants, which grew better under the influence of humic acids [
42]. This conclusion was drawn from the observations of agricultural crops, such as, sunflowers [
27], vegetables and berries, e.g., blueberries [
35,
42]. However, our experiment did not confirm the hypothesis of the easier uptake of nutrients. The content of macro- and micronutrients in the strawberry leaves treated with the fertiliser containing humic acids did not differ significantly from the content of these elements in the leaves of the plants which had received the soil fertiliser (
Table 6 and
Table 7). The high yield of the strawberries in the variants treated with humic acids may also have been caused by the fact that the roots of these plants retained more water [
43]. However, this effect is the most noticeable when there is water shortage in the substrate [
44]. In our experiment, during the growing season, the moisture content in the soil was optimal or elevated (
Appendix B).
3.4. Strawberry Fruit Quality
Due to the increasing market requirements, producers are forced to pay more attention to the quality of fruit. It is usually determined by such parameters as weight, firmness, extract content, and the percentage of acids. It is easier to sell large fruits, which is crucial for the profitability of production. In our experiment, the strawberries harvested from the plants treated with the soil fertiliser had the lowest weight. The application of the Humi Brown Gold fertiliser had a positive influence on the weight of the strawberries. The weight of the fruits in the HBG × 2 variant was over 30% higher than in the variant where the soil fertiliser had been applied (22.3 g vs. 16.39 g, respectively) (
Figure 3).
The number of fertilisation treatments did not have significant effect on the average weight of strawberries. The authors of other experiments on the use of biostimulants, including humic acids, observed their positive effect on the weight of not only strawberries [
45] but also apples [
46,
47]. However, the experiments conducted by Soldaniband et al. [
36] did not reveal any significant effect of the biostimulants on the weight of strawberries. The effects of treatment of plants with humic acids may depend on various factors, such as the climatic and soil conditions, the way they are brought in or the species cultivated. For example, the treatment of blue huckleberry plants with humic acids did not improve the growth of shoots, yield, or fruit quality [
15,
20].
The suitability of soft fruits for transport is largely determined by their firmness. Firm fruits are more attractive to consumers [
48] and less susceptible to damage caused by
Botrytis cinerea. In our experiment, the strawberries from the variant with soil fertilization were characterised by the lowest firmness (158.8 g mm
−2). The firmness of the fruit harvested from the bushes in the HBG × 2 variant (198.0 g mm
−2) was significantly higher than that of the fruit harvested from the plants treated with the soil fertiliser (
Table 9). The fruit firmness increased along with the number of spray treatments, i.e., up to 206.7 g mm
−2 (HBG × 2) and 212.5 g mm
−2 (HBG × 3). Farahi et al. [
49] also observed an increase in the firmness of strawberries harvested from plants treated with humic acids.
The high content of extract (TSS) in fruit increases its dessert value. The strawberries harvested from the plants treated with the soil fertiliser had the lowest TSS content (8.54%). The highest TSS content in the strawberries was 10.2% (
Table 9). It was lower than the values noted in the experiments conducted by Wysocki et al. [
50] (13.23%) and Mikiciuk et al. [
45] (12.37%). The application of humic acids had a positive effect on the content of the extract in strawberries. The double treatment of the plants with the fertiliser containing humic acids caused a significant increase in the TSS content, i.e., up to 9.28%. The TSS content in the strawberries increased along with the number of spray treatments, i.e., by 14% in the HBG × 3 variant and by 17% in the HBG × 4 variant (
Table 9).
The application of humic acids had positive influence on the quality of strawberries both after harvesting and after storage. The stored fruits from the variants with HBG fertilisation were firmer than those harvested from the plants treated with the soil fertiliser (
Table 9). There were significant differences observed in the fruits from the HBG × 2 variant. The firmness of stored strawberries increased along with the number of treatments up to 192.13 g mm
−2. Also, the content of the extract in the stored strawberries harvested from the plants in all variants with humic acids was significantly higher than in the fruits harvested from the plants treated with the soil fertiliser (
Table 9).
The taste of fruits is also affected by the amount of sugars and organic acids dissolved in them [
51]. Citric and malic acids are the most important for the taste of strawberries. The fertilisation of strawberry plants with humic acids had a relatively small effect on the hydrolytic acidity of the fruit juice, expressed as citric acid. The variants with soil fertilisation (0.55%) and the HBG × 2 and HBG × 3 variants (0.54%) did not differ significantly in the hydrolytic acidity. The acidity of strawberries decreased to 0.42% only in the HBG × 4 variant. This result is similar to the findings of the study conducted by Mikiciuk et al. [
45], who found no changes in the acidity of strawberries after the treatment with biostimulants.
The colour of fruits affects their commercial value. In our experiment, the colour intensity (parameter L) of the strawberries harvested from the plants which had received the HBG fertiliser was significantly lower (paler fruits) than the colour intensity of the fruits from the plants treated with the soil fertiliser (darker fruits). The number of treatments did not have significant effect on the colour of strawberries. Similarly to the colour intensity, the values of the a-coordinate (the colour range from green to red) and the b-coordinate (the colour range from blue to yellow) and the chromaticity of fruits treated with the fertiliser containing humic acids were significantly lower than in the fruits from the variant with soil fertilization (
Table 10).
The strawberries harvested from the plants treated with the fertiliser containing humic acids were more resistant to fungal diseases. In these variants, during the harvest the percentage of fruits infested by grey mould (Botrytis cinerea) ranged from 0.25% (HBG × 3) to 0.42% (HBG × 2), whereas in the soil fertilisation variant it was almost nine times greater (2.25%). The analysis of the condition of strawberries after storage led to a similar conclusion. The amount of stored fruits affected by grey mould increased significantly. There were particularly big differences in the soil fertilisation variant—2.25% (freshly harvested fruits) vs. 17.60% (stored fruits). By comparison, in the variants where humic acids had been applied, the percentage of strawberries infested by grey mould after storage did not exceed 5%.
Both the physical and biochemical characteristics of fruits largely depend on the course of weather conditions [
52]. They change depending on the growing season. The harvest date was another parameter affecting all the qualitative parameters of strawberries. On the first harvest date, i.e., in early June, the fruits were characterised by the highest weight and the lowest hydrolytic acidity (
Table 11).
However, the large fruits harvested at that time had the lowest extract content—8.16%. The TSS content in the fruits harvested in the last period was 10.87%, which may have resulted from the high air temperature. Temperature is an important factor for both the yield and the content of various substances in strawberries [
53,
54]. In late June (the third harvest date), the air temperature was higher than at the beginning of the month (
Appendix A). The fruits from the second harvest date, i.e., mid-June, were characterised by the greatest firmness and the highest hydrolytic acidity (
Table 11).