Effects of Humic Substances on Soil Microbes

By Richard Lamar, PhD
Senior Director of Humic Research
Bio Huma Netics, Inc.

Most of the work on agricultural applications of humic substances (HS) has focused on their biostimulant effects on plants. Far less work has been conducted on the effects of HS on soil microbial populations. It’s not surprising to learn, from the few studies that have been published, that HS also stimulate the growth of soil bacteria, even the bacteria that inhabit earthworm digestive tracts. One of the most important discoveries is that many species of soil bacteria are able to grow on humic acid (HA) as their sole carbon source (Tikhonov et al., 2010).

These findings have important implications for the roles played by soil bacterial communities—including those residing in the guts of soil fauna, such as earthworms—in the humification process (i.e., the process of conversion of dead plant tissues to humic substances). This means that these bacteria are consuming HS and modifying HS by metabolizing humic molecules and using the metabolized molecules to produce proteins, fats, and other types of molecules. When the bacteria die, they are in turn consumed by other microbes and those molecules created from metabolized humic molecules wind up being included as HS.

The other important piece of information that has come out of the work on bacterial-HS interactions is that, in addition to being potential carbon sources, HS can also act as soil bacterial growth stimulants or growth regulators (Tiknonov et al., 2010). This was demonstrated in a study in which a number of soil isolated bacterial species were grown on a medium that contained glucose as the carbon source (10 mg/ml) and humic acid (1 mg/ml). Thus, the humic acid was 10X lower than the glucose. Growth of the bacteria on this medium was compared with the growth of bacteria on a medium that did not contain the humic acid. The growth of 41% percent of the bacterial species (these were isolated from earthworm digestive tracts) were stimulated by the inclusion of the humic acid. The authors of the study concluded that, because the concentration of glucose was so high and the increase in available carbon from the addition of 1 mg/ml humic acid was insignificant, the humic acid acted as a growth stimulant to the 41% of bacteria whose growth was increased.

These types of studies have demonstrated that HS can stimulate the growth of plant-growth-promoting rhizobacteria (aka PGPR bacteria, for which the “rhizo” stands for rhizosphere or the area of soil that is intimately associated with plant roots). One of the most well-known PGPR bacteria are Pseudomonads, strains of which have been found to be able to solubilize phosphate, produce siderophores (important for Fe uptake), ammonia, and the plant-growth-regulator auxin (Gupta, 2008; Selvakumar et al., 2009).

REFERENCES

Gupta, A. and M. Gopal. 2008. Siderophore production by plant growth promoting rhizobacteria. Indian J. Agric. Res. 42(2):153–156.

Salvakumar, G., P. Joshi, S. Nazim, P. K. Mishra, J. K. Bisht and H. S. Gupta. 2009. Phosphate solubilization and growth promotion by Pseudomonas fragi CS11RH1 (MTCC8984), a psychrotolerant bacterium isolated from a high-altitude Himalayan rhizosphere. Biologia, 64(2)239-245

Tikhonov, V. V., A. V. Yakushev, Y. A. Zavgorodnyaya, B. A. Byzov, and V. V. Demin. 2010. Effect of humic acids on the growth of bacteria.  European Soil Science, 43 (3):305–313.

8 Simple Steps to Healthy Crop Soil

by Larry Cooper

8 SIMPLE STEPS TO HEALTHY CROP SOIL: A PREVIEW

Of course, you'll need to read this complete article to understand how and why each of these steps is essential for creating healthy crop soil, but here's a preview.

  • Understand that soil is a living system.
  • Measure and document your soil characteristics.
  • Disturb the soil structure as little as possible.
  • Bring plant diversity to the soil.
  • Keep soil covered at all times.
  • Keep living roots in the soil all year round.
  • Build soil organic matter.
  • Have a soil health plan: Review and revise it regularly.

Decline in soil health is one of the most potentially devastating world-wide crises of the 21st century, but the average person who does not farm probably never gives farm soil a second’s thought: The supermarkets are fully stocked—everything must be okay, right?

It’s not. A Reuters news headline from 2014 stated, “Only 60 Years of Farming Left If Soil Degradation Continues.” The article quoted the United Nations Food and Agriculture Organization (FAO) as saying that about a third of the world’s soil has already been degraded from chemical-heavy farming techniques, deforestation, and global warming. It was predicted that in 2050 the amount of agricultural land, in particular, would be only a quarter of the amount available in 1960—yet we will have 2 billion more people to feed.

What can be done about it? Quite a bit, actually; though reversing soil degradation and improving soil health is going to require changes in thinking and changes in some very hard-wired cultural practices. The 8 Simple Steps to Healthy Crop Soil that we’re about to discuss are culled from a variety of farming philosophies, some as old as time itself. You can find them in modern-era discussions of Regenerative Agriculture, Restoration Agriculture, and Conservation Agriculture, but they also draw from many aspects of “conventional” farming that were in place long before the concept of “conventional” included chemical fertilizers and pesticides. Most important, these steps are modeled on the practices followed by Nature itself in every undisturbed forest floor, unplowed prairie, and pristine mountain meadow.

The very best thing about the 8 Simple Steps to Healthy Crop Soil is that these practices can be profitably applied with good results by commercial farmers (conventional and organic), hobby farmers, community gardeners, even the “square-foot” backyard gardeners in the middle of a city. And while geography, soil type, and soil history certainly influence how the 8 Simple Steps to Healthy Crop Soil are implemented, implementing them all (and it has to be all of them) will lead to good results in all soil-based plant-growing situations.

What are the benefits of healthy soil?

Better Yields. Healthy soil produces more abundant crops of higher quality that are less susceptible to pests and diseases, more drought resistant, and better tolerant of wind, heavy rain, hail, heat, and all the other mayhem that keeps farmers up at night.

Economic Return. In addition to better yields, crops will require less chemical input in terms of fertilizers and pesticides. This won’t happen overnight, or even necessarily in the first year or two. But in the long term, growers will find their input expenditures greatly reduced.

A Farm for the Kids. Restoring health to the soil in a sustainable way means that growers will leave their kids a productive, profitable farm that the kids, in turn, can also leave in good shape for the grandkids.

Saved Planet. Though not necessarily our immediate goal, following the 8 Simple Steps to Healthy Crop Soil will increase the amount of carbon sequestered in the soil and lower CO2 levels in the atmosphere, which will help reduce global warming and give our grandkids’ grandkids a decent world in which they can live and prosper.

With these impressive benefits in mind, let’s dig deeper into how to create healthy crop soil.

Download/read the complete White Paper here.

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