Drought – Aid from Humic Acid and Microbes

Presented to Grower Groups – Australia, South Africa, United States

By Gary Murdoch-Brown

It has been well documented that any worthwhile drought management strategy employs the following factors:

  • Capture a high percentage of rainfall (infiltration)

  • Maximise water storage in the soil (water holding capacity)

  • Efficiently recover the water (plant roots)

It is in this context that I want to introduce to you the benefits of humic substances and microbes as well as a brief background into what humic acids are where they come from.

There is much conjecture surrounding humic substances, and contrary to popular belief, there is an extensive library of scientific evidence proving both the positive and negatives of the subject. Dr William R. Jackson collated over 70,000 pages of documents, by more than 1,500 authors, from universities and researchers all over the world into his reference book; Humic, Fulvic and Microbial Balance: Organic Soil Conditioning. This agricultural text developed over nine years is considered the definitive works on the organic component of soils and illuminates the student to all its benefits. Much of the scientific data and references in this talk is from his book.

What are humic substances?

When plants and animals die and decompose, the final residue of decay is humus which contains a concentration of organic acid radicals that hold enormous amounts of energy (5,000 calories per gram, (20)). These soluble energy packets are characterised into functional groups, determined mainly by molecular weight and solubility, and are termed humic acids, fulvic acids, humins and a smaller, but not unimportant, group of ulmic acids, amino acids, and trace elements.

Humic Acids are confusingly defined as the collective name for all the acid radicals found in humic matter (humic, fulvic, ulmic, etc) and also the radical fraction that is soluble only in alkali. Commercially, the term in Australia usually refers to the collective.

Humic acids are found in all soils containing organic matter, in all oceans and many rivers. Extraction can be from compost, animal manures, peat and brown coals. Many researchers have proven that not all humic acids provide value and promote soil fertility and it is the raw material and method of extraction that will determine the beneficial effect of humic substances (1,2,3). Leonardite ore has proven to be the most beneficial raw material (4), and combination pH extraction followed by purification and concentration the most valuable extraction method (5).

Whilst the commercial claims of humic acids can be questionable, scientific research over the past 100 years has categorically proven some of the claims to be true. Still some the exact mechanisms for the results are still under research and debate, however the fact remains that when soil is enriched with a high quality humic substance, at the right rates and timing, both direct and indirect beneficial effects are evident in plant growth (12,13,14,15). As a consequence the following points are accepted as fact by the wider scientific community (16,17,18,19):

  • Promotes desirable structure, texture, and looseness or friability and crumbliness, of particular importance in tight clay soils,
  • Improves water holding capacity
  •  Delivers a more favourable medium for plant root system development

  •  Aids effective development of plant circulatory, respiration and

    transpiration systems

  •  Promotes microbial activity

  •  Active disintegration of soil rock, releases additional plant nutrients

  •  Aids heat absorption

  •  Buffers against rapid changes in soil pH

  •  Nutrient chelation

  •  P2O5 lock up reduction

  •  Liberates CO2 from CaCO3 to allow root uptake for carbohydrate synthesis

  •  Neutralises plant toxic elements

  •  High ion exchange capacity

  •  Stores nutrients, N P S and traces.

  •  Releases stored energy to plants

  •  Retards pathogens

  •  Stimulation of plant cellular growth and division from auxin type reactions

  • Decreases plant stress and premature deterioration

As a result:

  • Improved seed germination

  • Greater growth of fibrous roots

  • Increases in legume root nodule formation

  • Greater resistance to insects

  • Greater resistance to drought and effects of frost damage


The breakdown of all biomass (plant and animal matter) is accomplished by micro-organisms and the enzymes they produce. This action involves complex carbon compounds being digested and disintegrated into simple forms. These simple forms are then remanufactured, transformed, resynthesised, and recombined into all together new and different combinations and sequences of now new complex polymers. As the original material is broken down and new combinations are assembled, the new multiple combinations are forming humic molecules.

Apart from this “formation” of humus (6,7,8), micro-organisms are responsible for nutrient
cycling and availability to plants (8), adhesion, protection against dehydration, ion exchange and selection, tolerance to metals, and recognition and immunisation protection against predators (9).

Jackson (10) reviewed observations that soil micro-organisms not only stimulate the rebuilding of soils, but do so at a more rapid rate than other known methods such as crop rotation and mulching. Whilst only about 1% of microbial diversity has been studied, the credible research to date is unquestionable that under favourable conditions (diversity, temperature, pH, moisture) soil micro-organisms will provide (11):

How does this help our drought?

As we mentioned earlier, infiltration, water holding capacity and plant roots systems are the primary considerations in building drought resistant soils. These factors are influenced by soil texture, soil aggregation, organic matter and surface cover.

Soil texture refers to the proportion of sand, silt and clay which in turn influences the pore sizes in soil (combined with aggregation) and therefore the soils infiltration rate and holding capacity. Clays have a greater total pore space, but a portion of these pores are so small that water is held too tightly for deep infiltration and for plant roots to be able to extract it.

Sands on the other hand have relatively few pore spaces that are comparatively large. Water readily infiltrates but drains rapidly, most of the water that is held is available to plant roots. The most desirable soil, a sandy clay loam, is the best of both worlds – pores spaces of varying size that allow rapid infiltration, high holding capacity and plant availability. It should also be noted that texture is the one innate characteristic of a soil that cannot be changed through agronomic practice. The following table shows the effect of texture on moisture:

Table 1: Soil texture effects on soil moisture (21)

Aggregation is the characteristic that relates to how the soil particles come together to form large, water stable granules (aggregates). It is again well documented that well aggregated soils have increased water entry at the surface, aeration and water holding capacity (22). Plant roots systems are able to penetrate more readily and the increased root area allows for increased uptake of water and nutrients.

The level of organic matter, and its biological activity has the greatest influence on aggregation. Microbes produce the gluey substances that bind the components into stable aggregates. Unlike texture, aggregation is directly controllable by agronomic practice. Any means of promoting organic matter and reducing the impact of machinery will aid aggregation.

The benefits?

The benefits of well aggregates soils to plant production are numerous. As previously mentioned the increased penetration of root systems promotes growth, surface crusting is less evident as water stable aggregates are less likely break apart when rainfall hits them, this in turn reduces run off from subsequent watering. Aggregates are less prone to erosion as they are heavier than their particle components.

The microbial content of organic matter we have determined helps drought proof soils due to the production of the binding glues for aggregation (24). The water retaining capacity of many soils is proportionally relative to the humus content (26). Organic matter itself also determines the water holding capacity of soils in a direct manner. Every 1% of organic matter can hold 0.5ML/ha/m (23). The growth and proliferation of desirable soil microbes and microflora is at its best in the presence of high quality humic substances


Humic acids have a beneficial effect on soil, relating to the production and formation of organic matter and soil structure, which helps drought proofing. They increase water holding capacity of soils (28,29) and thus soils treated with humic substances resist droughts more effectively and produce better yields when water supply is more plentiful. Humic substances improve tilth, or the workability, of the soil (30,31). Soils are more friable and suitably sized particles are formed in the aggregate (31). Friability is maintained as the humic substances form colloidal mineral complexes that assist in aeration and the prevention of large clods and stratification (32).

At permanent wilting point in the soil, water is held too tightly to soil particles (220psi) for plants to utilise. Humic substances reduce this pressure due to there electrochemistry making more water available to plants (10).

Humic substances have also proven to have a direct effect in plant water conservation and utilisation. The increased seed germination (33,34) and rooting of plants due to humic acids (35,36,37,38,39,40) also increases the uptake of water and nutrients by simply establishing a greater mass (surface area) and reach to collect with. In addition, humic substances promote plants root exudates which are highly hydrated and function to prevent dehydration of the roots (25). Humic substances assist the balance of water under drought conditions by increasing plant respiration, helping the plant breath better under stress (more oxygen).

This balancing phenomenon also produces a reduction in the amount of water required. Many researchers have confirmed that humic substances enhance the permeability of the cell membrane (27) facilitating more efficient water and nutrient uptake (13). The changes to carbohydrate metabolism increase soluble sugars and increase osmotic pressure inside the cell wall. This enables a plant to withstand wilting when the relative humidity of the atmosphere decreases (14).

Surface ground covers are an obvious benefit in most cropping situations. Water efficient cover crops are preferable to reduce evaporation although not necessarily economical except when a green manuring program is being employed. Using stubble as a cover is a great alternative that reduces evaporation and also increases organic matter and biological activity. Tillage also has its effect with no-till systems significantly improving water infiltration, retention and organic matter content. The subject of tillage is a very large one and best left to other experts on another day.

I hope this information I’ve passed onto to you helps illuminate you to the possible benefits of humic acids, microbes and organic matter overall. The science is there for all to review and our experience has taught us that economical benefits can be achieved using high quality products based on these findings. EcoCatalysts has walked many of the hard yards to understanding the benefits, and producing products and programs to best achieve as many of them as possible. Whilst these tools are proving valuable, organic matter management is still only one tool in a dynamic system that requires a balance of fertility, nutrition, pest and weed control, farm management, weather and luck.



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