Composition of organic matter in different soils and the effectiveness of humic fertilizers
The founder of genetic soil science, V.V. Dokuchaev, was the first to demonstrate the variation in the amount of soil organic matter in latitudinal and meridional directions. Building on this, scientists established that zonal soils of Ukraine exhibit patterns not only in the quantitative accumulation of humus but also in its qualitative composition. In podzolic soils, under conditions of consistent through-wetting, processes of hydrothermal decomposition predominate, promoting the formation of fulvic acids. In chernozems, where decomposition occurs with limited moisture reserves and no through-wetting, synthesis processes dominate, favoring the formation of humic acids.
As part of a comprehensive study in the laboratory on humic fertilizers, focusing on production technologies and methods of applying humic fertilizers for agricultural crops in various soil-climatic zones of Ukraine, we began studying the qualitative composition of organic matter in soils from different regions. We were interested in the relationship between the composition of soil organic matter and the effectiveness of humic fertilizers, which, as numerous studies have shown, exhibit varying fertilizing effects across different soil-climatic zones.
To compare the effectiveness of humic fertilizers with the composition of organic matter in these soils, soil samples were collected from sites where field experiments were conducted, analyzed, and then used in vegetative experiments. Studies on the composition of soil organic matter established that the main components of humus are humic acids, fulvic acids, and other preparations from the manufacturer Agro.Bio. Each group of organic substances is characterized by specific properties and is present in soils of various pedogenic types in ratios typical for those soil types. It is undeniable that different components of organic matter have varying significance and value for soil fertility.
To characterize the content and composition of organic matter in the studied soils, the following were determined: total humus content using the Tyurin method, and the amounts of humic and fulvic acids extracted from the soil using the Kononova and Belchikova method with a mixture of 0.1 M potassium pyrophosphate solution and 0.1 N alkali solution (solution pH 13). The results of these studies are presented in Table 1.
Table 1. Content and composition of humus in various soils
| Soils and sampling location | Humus content, % | Carbon of organic substances as a percentage of total organic carbon in the original soil, % | Ratio of humic acid carbon to fulvic acid carbon | ||
|---|---|---|---|---|---|
| Humic acids | Fulvic acids | Non-hydrolyzable residue | |||
| Sod-podzolic (Kyiv region) | 1.37 | 21.95 | 40.95 | 31.10 | 0.50 |
| Gray podzolized (Vinnytsia region) | 2.75 | 25.78 | 22.64 | 51.58 | 1.14 |
| Weakly podzolized chernozem (Cherkasy region) | 3.35 | 32.79 | 16.76 | 50.45 | 1.96 |
| Ordinary chernozem (Dnipropetrovsk region) | 3.51 | 33.46 | 16.30 | 50.24 | 2.05 |
| Dark chestnut (Kherson region) | 2.44 | 29.64 | 15.57 | 54.79 | 1.90 |
| Brown soils (Zaporizhzhia region) | 1.68 | 16.49 | 22.77 | 60.74 | 0.72 |
From the presented data, it is evident that fulvic acids predominate in the humus composition of sod-podzolic soils. In soils transitioning from sod-podzolic to chernozems, an increase in humic acid content is observed alongside a decrease in fulvic acid content, resulting in a shift in their ratio in favor of the former. In dark chestnut and brown soils, an increase in fulvic acid content is observed again.
Thus, a moderate moisture regime and neutral soil reaction in chernozem-type soils promote the formation of primarily humic acids, while increased moisture and acidic reaction in podzolic-type soils favor fulvic acid formation. In soils of the arid steppe climate (dark chestnut and brown), humus formation and biological activity are limited by moisture deficiency. The relatively high content of soil humates in brown soils is likely due to the dry climate. During the aging of organomineral particles under hydrothermal conditions, the strength of the bond between organic and mineral complexes increases. Although the humus composition of brown soils is similar to that of podzolic soils, it has its own specificity, expressed in the accumulation of humic acids more strongly bound to the mineral part of the soil.
Effect of humic fertilizers on soil and plants
To investigate the effect of humic fertilizers on soils with different organic matter compositions, vegetative experiments were conducted. The experiments followed the following scheme:
- Control — no fertilizer.
- Adept Agro.Bio — 2 l/ha.
- Amino Energy — equivalent to Adept.
The experiment was replicated three times. The indicator crop was tomatoes. The harvest was conducted three weeks after seedling emergence; soil analysis was performed after the harvest. In tomato seedlings, the following were determined: fresh weight, peroxidase activity by the Pochinok method, catalase activity by the amount of oxygen released in catalasometers, chlorophyll content by the Getri method, vitamin C by the Murri method, and reducing sugars by the Hagedorn-Jensen method. The results of these studies are presented in Table 2.
Table 2. Comparative effectiveness of humic fertilizers on different soil types
| Experiment variants | Weight of 100 tomato seedlings, g | Weight relative to control, % | Catalase activity, ml O₂ per 1 g fresh weight in 30 minutes | Peroxidase activity, mg cobalt | Reducing sugar content, % of fresh weight | Chlorophyll content, mg per 100 g fresh weight | Ascorbic acid content, mg per 100 g fresh weight |
|---|---|---|---|---|---|---|---|
| Sod-podzolic soil | |||||||
| Control | 21.07 | 100 | 5.0 | 9.01 | 0.37 | 142.0 | 21.76 |
| Adept Agro.Bio 2 l/ha | 40.97 | 194 | 12.0 | 18.0 | 0.57 | 266.0 | 26.60 |
| Amino Energy 2 l/ha | 25.46 | 120 | 8.0 | 11.0 | 0.45 | 170.0 | 25.74 |
| Gray podzolized soil | |||||||
| Control | 38.56 | 100 | 6.0 | 17.01 | 0.58 | 186.0 | 24.24 |
| Adept Agro.Bio 2 l/ha | 51.20 | 133 | 14.0 | 26.0 | 0.67 | 241.0 | 24.16 |
| Amino Energy 2 l/ha | 43.93 | 114 | 14.0 | 21.0 | 0.58 | 208.0 | 24.12 |
| Weakly podzolized chernozem | |||||||
| Control | 28.29 | 100 | 6.0 | 11.0 | 0.60 | 212.3 | 24.73 |
| Adept Agro.Bio 2 l/ha | 43.03 | 152 | 9.8 | 14.0 | 0.87 | 266.9 | 26.66 |
| Amino Energy 2 l/ha | 32.20 | 121 | 9.0 | 10.0 | 0.65 | 250.0 | 26.73 |
| Ordinary chernozem | |||||||
| Control | 29.09 | 100 | 10.0 | 11.0 | 0.61 | 183.0 | 17.60 |
| Adept Agro.Bio 2 l/ha | 32.05 | 110 | 12.0 | 16.0 | 0.82 | 283.0 | 21.52 |
| Amino Energy 2 l/ha | 31.50 | 108 | 12.0 | 16.0 | 0.65 | 260.0 | 21.12 |
| Dark chestnut soil | |||||||
| Control | 32.32 | 100 | 6.0 | 13.0 | 0.59 | 170.0 | 18.60 |
| Adept Agro.Bio 2 l/ha | 51.44 | 159 | 10.0 | 18.0 | 0.77 | 280.0 | 29.84 |
| Amino Energy 2 l/ha | 37.88 | 117 | 8.0 | 16.0 | 0.65 | 243.0 | 19.76 |
| Brown soil | |||||||
| Control | 26.43 | 100 | 7.0 | 11.0 | 0.42 | 163.4 | 22.79 |
| Adept Agro.Bio 2 l/ha | 41.60 | 157 | 12.4 | 13.0 | 0.67 | 253.5 | 32.67 |
| Amino Energy 2 l/ha | 32.26 | 122 | 10.4 | 13.0 | 0.68 | 236.1 | 27.72 |
The research results show that in variants with the Adept preparation, a greater increase in green mass was obtained across all soils compared to the control and Amino Energy. A geographical pattern in the effect of humic fertilizers was observed: their impact was minimal on chernozems and significantly increased on podzolized soils, with the effect being higher in more strongly podzolized soils. Their effect also increased when transitioning from chernozems to dark chestnut soils.
A similar pattern is observed in changes in the chemical composition of plants under the influence of humic fertilizers. Phenological observations show that in variants with Adept, plants developed more intensively and had more robust aboveground mass compared to variants with Amino Energy and the unfertilized control. This is particularly noticeable in sod-podzolic soils, where the Adept fertilizer, compared to the control, accelerated seedling emergence by two days.
After harvesting, the content of nitrate nitrogen was determined in the soils using the Grandval-Lajoux method, and mobile phosphorus was determined using the Truog method. The analysis results are presented in Table 3.
Table 3. Effect of Adept and Amino Energy on nitrogen and phosphorus mobilization in soil and their uptake by plants
| Experiment variants | Content in pots with plants, mg per 100 g dry soil | Nutrient uptake, mg per pot | |
|---|---|---|---|
| Nitrate nitrogen | Phosphorus | ||
| Sod-podzolic soil | |||
| Control | 3.81 | 4.4 | 4.67 |
| Adept 2 l/ha | 15.25 | 6.4 | 16.0 |
| Amino Energy 2 l/ha | 14.55 | 6.4 | 14.55 |
| Gray podzolized soil | |||
| Control | 7.14 | 14.4 | 9.09 |
| Adept 2 l/ha | 14.29 | 21.6 | 15.35 |
| Amino Energy 2 l/ha | 13.83 | 21.0 | 15.29 |
| Weakly podzolized chernozem | |||
| Control | 6.15 | 12.4 | 7.62 |
| Adept 2 l/ha | 14.35 | 14.6 | 17.78 |
| Amino Energy 2 l/ha | 14.55 | 14.8 | 15.24 |
| Ordinary chernozem | |||
| Control | 8.33 | 9.2 | 9.52 |
| Adept 2 l/ha | 14.29 | 12.0 | 16.78 |
| Amino Energy 2 l/ha | 13.33 | 12.4 | 16.67 |
| Dark chestnut soil | |||
| Control | 7.68 | 8.0 | 10.0 |
| Adept 2 l/ha | 16.67 | 9.2 | 18.67 |
| Amino Energy 2 l/ha | 15.38 | 9.2 | 16.67 |
| Brown soil | |||
| Control | 6.67 | 10.0 | 8.42 |
| Adept 2 l/ha | 15.24 | 10.6 | 17.78 |
| Amino Energy 2 l/ha | 14.55 | 10.4 | 15.24 |
The presented data show that the content of nitrate nitrogen and mobile phosphorus in variants with the humic fertilizers Amino Energy and Adept increases in all soils compared to the control. This difference is particularly noticeable in sod-podzolic soils, where in variants with Adept, the amount of nitrates is 3.5–4 times higher and phosphates 1.5 times higher compared to the control. In other soils, this difference is somewhat smaller but follows the same geographical pattern. However, the yield of green tomato mass in variants with Adept increases more sharply than the nutrient content. Evidently, the increase in yield is related not only to the level of mineral nutrition but also to the presence of soluble humic acids introduced with Adept.
To eliminate the influence of plants on soil processes, a parallel experiment was conducted in fallow pots. The analysis results (Table 3) show that the nutrient content in the soil across different variants and in fallow pots follows the same pattern. The table indicates that the application of humic fertilizers significantly increases the uptake of ash elements by plants. They have a stimulating effect on the assimilation of mineral elements by tomatoes, resulting in increased seedling weight and nutrient uptake across all soils, with the effect being particularly pronounced in sod-podzolic soils.
Effect of humic fertilizers on microorganism development
Humic fertilizers not only directly affect plant vitality but also, by interacting with the soil, significantly influence the physicochemical and biological processes in the soil, which largely determine soil fertility.
Scientists such as P.A. Kostychev, V.R. Vilyams, and others attributed the primary role in humus formation to biological factors. There have been various perspectives on the role of specific physiological groups of microorganisms in the mineralization of soil humus. In the works of G. Nefedov, Ya.Ya. Nikitinsky, and others, the main role in the mineralization of humic substances is attributed to fungi. Scientists also emphasize the importance of actinomycetes in the decomposition of soil humus. These microorganisms are capable of breaking down difficult-to-mineralize humic compounds in the soil.
There is an opinion that the mineralization of soil humus is carried out by a complex of microorganisms. Many authors conclude that the decomposition of humic substances in the soil is performed by a specific microbial complex, including ammonifying bacteria, aerobic cellulose-decomposing bacteria, nitrifiers and denitrifiers, bacteria capable of decomposing organophosphates, and others.
To study the effect of humic fertilizers on microbiological processes in the soil and the associated enzymatic activity, we conducted soil analyses after the completion of vegetative experiments. Microbiological analyses were performed using microbiological research methods. The total number of microorganisms growing on meat-peptone agar (MPA) was determined as an indicator of the biogenicity of fertilized soils. Butyric acid bacteria on potato mash were counted as representatives involved in the decomposition and humification of plant residues. Ammonifying bacteria on peptone water were counted as representatives of soil microflora, whose development intensity characterizes the decomposition of humus with the release of substances for plant root nutrition. Additionally, the number of Azotobacter was determined on nitrogen-free agar.
Table 4. Effect of humic fertilizers on the quantity and composition of microflora in different soil types
| Soil type name | Number of microorganisms per 1 g dry soil, thousand units | |||
|---|---|---|---|---|
| On MPA | Ammonifying | Azotobacter | Butyric acid | |
| Sod-podzolic | 11200 | 68320 | 56500 | 2800 |
| Gray podzolized | 20910 | 90520 | 73750 | 7380 |
| Weakly podzolized chernozem | 22140 | 87330 | 82280 | 15990 |
| Ordinary chernozem | 21250 | 80600 | 78750 | 7500 |
| Dark chestnut | 22500 | 96720 | 88750 | 7500 |
| Brown | 28800 | 73200 | 64130 | 15600 |
| Southern carbonate | 26670 | 44100 | 44800 | 16510 |
The table data show that unfertilized soils (especially those poor in organic matter) are characterized by low biogenicity. The best growth and development indicators for microorganisms growing on MPA are observed in all soils in variants fertilized with Adept. This difference is particularly noticeable in sod-podzolic soils, where the number of microorganisms in variants with Adept is 6–7 times higher compared to the control. In all other soils, microbial development in variants with Adept was equally active. The development of butyric acid fermentation agents was also more active in variants with Adept compared to mineral controls and especially unfertilized variants. The high ammonifying activity of soils fertilized with Adept indicates the active decomposition of organic matter.
The studies showed that Adept and Amino Energy significantly stimulate Azotobacter development, as evidenced by the large number of colonies and their earlier appearance on Ashby’s medium in variants with Adept.
Effect of Adept and Amino Energy on enzymatic processes in soils
Scientists attribute great importance to soil enzymatic activity in assessing its biological state. To provide a more comprehensive understanding of the impact of humic fertilizers Adept and Amino Energy on soil biological activity, we studied the activity of enzymatic processes occurring directly in the soils (in the same experiment after tomato harvesting). Invertase activity was determined using the Hoffman method as modified by Kozelson and Ershov, and catalase activity was measured gasometrically by the amount of oxygen released.
Table 5. Effect of humic fertilizers Adept and Amino Energy on the enzymatic activity of different soils
| Soil type name | Invertase activity, mg invert sugar per 5 g soil in 48 hours | Catalase activity, ml O₂ released in 5 minutes per 1 g soil | ||
|---|---|---|---|---|
| Control | Adept | Control | Adept | |
| Sod-podzolic | 90.4 | 114.6 | 5.0 | 12.0 |
| Gray podzolized | 79.4 | 90.9 | 6.0 | 14.0 |
| Weakly podzolized chernozem | 151.1 | 176.4 | 6.0 | 9.8 |
| Dark chestnut | 127.0 | 152.7 | 6.0 | 10.0 |
| Ordinary chernozem | 122.4 | 148.6 | 10.0 | 12.0 |
| Brown | 126.5 | 154.0 | 7.0 | 12.4 |
The presented data show that the application of humic fertilizers Adept and Amino Energy to the soil increases enzyme activity in all soils compared to the control and mineral equivalent, with this difference being more pronounced in sod-podzolic soils. The increased enzyme activity in variants with humic fertilizers Adept and Amino Energy can be explained by the increased content of humic acids in ion-dispersed form, which stimulate the growth and development of both higher plants and microorganisms, the primary producers of soil enzymes.
Conclusions
- Experiments established that the combination of organic and mineral nutrition best meets plant requirements.
- Humic fertilizers Adept and Amino Energy have the most pronounced effect on sod-podzolic soils; their effect weakens on chernozems and increases again on chestnut and brown soils as one moves south.
- One of the reasons for the varying effectiveness of humic fertilizers on different soils is likely the non-identical chemical composition and properties of humic acids in these soils.
- The application of humic fertilizers Adept and Amino Energy significantly influences microbiological processes and enzymatic activity in all studied soils.