Impact of Leonardite on the Development of Soil Microorganisms
Currently, the effectiveness of using leonardite (humic and fulvic acids) on medium-loamy dark chestnut soil in southern Ukraine is beyond doubt. Researchers approach the explanation of the positive effect of humic substances on plant growth in different ways.
Literature data indicate that organic substances have a direct stimulating effect on plant vitality. At the same time, other scientists believe that the positive effect of humic substances on plant growth is explained by the influence of the products of soil microflora activity. Scientists note that the products of microbial activity play a decisive role in plant development. Studies have shown that the more humus there is, the more microorganisms are present in the soil, and the higher the content of biotic substances—vitamins B1, B2, B3, heteroauxin, and others. Plants contain more vitamins when grown in soil rich in microbes. The stimulating effect of soil solutions containing low-molecular-weight organic substances has led to the conclusion that replenishing the soil with fresh organic matter is important not only for restoring sources of mineral and carbon nutrition but also for the formation of stimulating substances by microorganisms.
Humic substances are complex products of microbial activity. On one hand, humic fertilizers affect the physical and physicochemical properties of the soil, and on the other, they directly influence the vitality of higher plants and microorganisms. According to our laboratory technicians, studies conducted on dark chestnut soils in the Kherson region showed that irrigating the soil with humic and fulvic acids from humus increased the content of various groups of microorganisms by 2-3 times, while simultaneously increasing the nutrient content in the soil.
Experiments conducted on low-humus dark chestnut soils in control fields of irrigated agriculture also indicate that leonardite significantly stimulates the development of soil microflora.
Experiments of 2013
Our first experiments with leonardite were conducted in 2013. In this experiment, leonardite was applied at a rate of 8 centners per hectare during pre-sowing cultivation for soybeans. During the growing season, samples were taken twice (June 15, 2013, and August 9, 2013) to monitor the development of the total number of microorganisms (on meat-peptone agar) and Azotobacter on soil plates.
| Experiment Variants | Horizon, cm | Number of Microorganisms (million per 1 g of soil) June 15, 2013 | Number of Microorganisms (million per 1 g of soil) August 9, 2013 |
|---|---|---|---|
| Control (no fertilizers) | 0-20 | 4.7 | 3.0 |
| Control (no fertilizers) | 20-40 | 2.2 | 1.2 |
| Leonardite 8 c/ha during pre-sowing cultivation | 0-20 | 18.2 | 4.2 |
| Leonardite 8 c/ha during pre-sowing cultivation | 20-40 | 2.8 | 1.6 |
The table data show that leonardite applied under cotton promoted enhanced development of soil microflora. As expected, the most significant positive effect of leonardite on microorganism development was observed in the upper soil layer (up to 20 cm). When samples were taken in early summer, the difference in the number of microorganisms in plots with leonardite exceeded the control by almost four times. By early August, the overall number of microorganisms in the soil decreased, but the patterns observed in the first sampling period persisted.
The number of Azotobacter was determined using Vinogradsky’s method on soil plates. In low-humus chestnut soils of the arid steppe of southern Ukraine, Azotobacter is scarce. It should also be noted that the Azotobacter present in these soils is inactive, as evidenced by the delayed appearance of its colonies on natural soil plates—on the fourth and fifth days. Our observations showed that leonardite significantly stimulated the development of Azotobacter on soil plates.
| Experiment Variants | Horizon, cm | Number of Azotobacter Colonies per 50 g of Soil, June 15, 2013 | Number of Azotobacter Colonies per 50 g of Soil, August 9, 2013 |
|---|---|---|---|
| Control (no fertilizers) | 0-20 | 224 | 116 |
| Control (no fertilizers) | 20-40 | 12 | 4 |
| Leonardite 8 c/ha during pre-sowing cultivation | 0-20 | 750 | 124 |
| Leonardite 8 c/ha during pre-sowing cultivation | 20-40 | 16 | 10 |
Thus, leonardite has a positive effect not only on the development of the total number of microorganisms but also promotes the enhanced development of beneficial soil microflora, such as Azotobacter.
Experiments of 2014-2015
Similar studies were conducted in 2014 and 2015. During this period, we tested the impact of leonardite on the development of other groups of microorganisms: ammonifying bacteria and anaerobic nitrogen fixers. In 2014, we monitored the development of soil microorganisms depending on leonardite application in vegetative pots with a capacity of up to 12 kg at optimal moisture (50% of full water capacity) and in a field experiment.
Leonardite in the amount of 50 g per pot was applied at a depth of 10 cm, mixed with the soil, and then corn was sown. When 3-4 leaves formed, the plants were removed from the pots, and soil samples were taken to determine the microorganisms.
| Experiment Variants | Number of Microorganisms (million per 1 g of soil) on Meat-Peptone Agar | Anaerobic Nitrogen Fixers | Ammonifiers | Number of Azotobacter Colonies per 50 g of Soil |
|---|---|---|---|---|
| Control (no fertilizers) | 6.7 | 0.025 (++) | 0.025 (++) | 378 |
| Organo-Mineral Mixture - 50 g per pot | 24.6 | 0.070 (++++) | 0.025 (++) | 440 |
| Leonardite 50 g per pot | 29.0 | 1.110 (++++) | 0.070 (+++) | 530 |
Note: Symbols: + - bacterial activity; ++ - weak activity; +++ - satisfactory activity; ++++ - good activity.
The data in Table 3 show that in vegetative pots, the application of 50 g of organo-mineral mixture increased the number of microorganisms grown on meat-peptone agar by 3.5 times compared to the control, while the same amount of leonardite increased it by more than 4 times. The organo-mineral mixture and especially leonardite have a significant positive effect on the activation of other groups of microorganisms—anaerobic nitrogen fixers, ammonifying bacteria, and Azotobacter.
An increase in the microbiological activity of the soil under corn due to the impact of leonardite was also observed in the field experiment.
| Experiment Variants | Horizon, cm | Total Number of Microorganisms (Meat-Peptone Agar) | Number of Ammonifiers | Number of Anaerobic Nitrogen Fixers |
|---|---|---|---|---|
| Control (no fertilizers) | 0-20 | 21.0 | 0.070 (++) | 0.025 (+++) |
| Control (no fertilizers) | 20-40 | 9.0 | 0.025 (++) | 0.006 (+++) |
| Leonardite 8 c/ha during pre-sowing cultivation | 0-20 | 28.0 | 0.070 (++++) | 0.060 (++++) |
| Leonardite 8 c/ha during pre-sowing cultivation | 20-40 | 31.0 | 1.110 (++++) | 0.025 (+++) |
As our observations showed, the application of leonardite significantly increased the number and activity of microorganisms in the soil in the root zone of corn. The improvement in soil microbiological activity also had a positive effect on the development and yield of corn. With leonardite, a corn grain yield of 89.4 c/ha was obtained, compared to 79.4 c/ha in the control.
Experiments of 2015
The experiment with leonardite application in 2015 was conducted under irrigated conditions. Leonardite was applied at a rate of 50 g per nest during corn sowing.
| Experiment Variants | Date | Total Number of Microorganisms (Meat-Peptone Agar) | Number of Ammonifiers | Number of Anaerobic Nitrogen Fixers |
|---|---|---|---|---|
| Control (no fertilizers) | June 11, 2015 | 9.7 | 0.250 (+) | 0.013 (+) |
| Control (no fertilizers) | July 23, 2015 | 6.3 | 1.100 (++) | 0.060 (+++) |
| Leonardite 50 g per nest during sowing | June 11, 2015 | 13.0 | 0.250 (+++) | 0.250 (+++) |
| Leonardite 50 g per nest during sowing | July 23, 2015 | 14.6 | 1.100 (++) | 0.250 (+++) |
The data in Table 5 indicate that applying 50 g of leonardite per nest had a positive effect on the development and activity of microorganisms in the soil of the corn rhizosphere.
| Experiment Variants | June 11, 2015 | July 23, 2015 |
|---|---|---|
| Control (no fertilizers) | 16 | 16 |
| Application of 50 g of leonardite per nest | Very many colonies | 496 |
During the first sampling period (June 11), the surface of soil plates in plots with leonardite was completely covered with Azotobacter colonies. During the second sampling, the difference in the number of Azotobacter between the leonardite plot and the control was more than 13 times.
Conclusions
The results of three years of research allow us to conclude that the use of leonardite significantly enhances the development of soil microflora, which, through its activity, enriches the soil with both nutrients and biotic substances that have a stimulating effect on plant development.