Written by Mike Hamilton, CCA & President of Turf Dietitian
Increasing Microbial Populations by Incorporating Organic Carbon Sources
If you’re looking to increase your soil’s microbial activity, you must develop or maintain a cultural program that produces an optimal environment. Or at least one more favorable in terms of aeration, moisture, pH, and organic production needed to fuel the population. We know that the abundance of microbes in soil is directly proportional to the organic matter content or added organic compounds.
Thus, soils receiving large amounts of organic residues and carbon supplements support a larger microbial population.
When adding organic compounds to soil, there is an explosion in microbial numbers.
Sustaining elevated microbial populations depends on the organic source’s complexity and the application frequency. Eventually, microbial activity levels lower once the carbon sources get consumed. The normal state of affairs in sandy soils that do not receive carbon regularly is a deficient microbial population that performs limited duties in the soil and metabolizes very slowly.
Regrettably, adding simple carbon materials such as molasses and other carbohydrates will only elevate populations for a short period.
Soil microbes quickly use up carbohydrate substrates, and little, if any, lasting effects are noticeable.
Superintendents should incorporate organic and amino acids to manage high populations of soil microbes for extended periods.
Another potentially harmful effect on microbial populations is the long-term use of ammonia-based fertilizers. Soil acidification due to ammonia oxidation by nitrifying bacteria can drop pH after prolonged use of ammonia-based fertilizers.
Soil pH’s below 6.0 can cause marked reductions in populations of bacteria and actinomycetes and simultaneous increases in the relative abundance of fungi.
Incorporation of Beneficial Species of Microbes into the Soil
As the scientific knowledge of soil microbial biodiversity grows and the factors that control populations increase, microbiologists will find ways to incorporate non-indigenous species of microbial populations into indigenous environments. Thus, each soil is endowed with a stable community of microbes uniquely selected by and adapted to that soil’s prevailing physical, chemical, and biological conditions. Although it’s a fact that non-indigenous species can survive in existing populations, it can take several seasons for them to become dominant enough to impact the soil.
Because of this slow establishment, biological alternatives get met with reluctance. A more significant comfort factor is in using a chemical formulation that delivers more consistent results when applied as directed.
However, as research progresses and as we gain a clearer understanding of the characteristics that make an organism successful in the soil or rhizosphere environment, we will likely see the development of useful microbial products. These will serve several purposes, such as increasing plant growth, reducing organic matter, improving nutrient mineralization, and protecting crops from disease.
The Ideal Microbial Environment
Aerobic microbes need what all living things need to prosper: oxygen, water, food, and suitable habitats in which to live. Generally, soil microbes grow best in soils of near-neutral pH (7.0). Similarly, if soils become too acidic (down to pH 4 or 5), bacteria and actinomycetes usually decline, and fungi assume a more dominant position.
Aerobic microbes need an adequate supply of inorganic nutrients, a balance of air and water pores (ideal is 65% air and 35% water), and abundant organic substrates. When these parameters get too far beyond the normal range, some population segments will likely be stressed.
For example, aerobic bacteria will be disadvantaged when soil becomes saturated and available O2 depletes. Conversely, anaerobic organisms will weaken when soil moisture is at least 50% below field capacity and rich in oxygen. Anaerobic bacteria lead to unique problems such as the formation of a black layer caused, at least in part, by the anaerobic sulfate-reducing bacteria and the formation of surface algae.
The most important limiting factor for microbial growth in soil (assuming moisture is adequate) is the abundance of available organic carbon sources.
The vast majority of soil microbes require organic carbon compounds (organotrophs) to oxidize for energy and build the organic constituents of their cell bodies.
Of course, organic amendments may also contribute some usable carbon, but consider that amendments such as compost, which is essentially microbially decomposed organic materials, do not contain high levels of readily available carbon.
Carbohydrate Sources:
- Glucose
- Fructose
- Galactose
- Sucrose
- Lactose
- Cellulose
- Chitin
- Starch
- Yeast
- Xylose
- Maltose
Organic Acid Sources:
- Formic Acid
- Fulvic Acid
- Acetic Acid
- Propionic Acid
- Butyric Acid
- Caproic Acid
- Carbolic Acid
- Caprylic Acid
- Enantic Acid
- Humic Acid
- Pelargonic Acid
Amino Acid Sources:
- Alanine
- Arginine
- Asparagine
- Aspartic Acid
- Cysteine
- Glutamine
- Glutamic Acid
- Glycine
- Histidine
- Isoleucine
- Leucine
- Lysine
- Hionine
- Metphenylalanine
- Proline
- Serine
- Threonine
- Tryptophan
- Tyrosine
- Valine
Another factor of great importance for the decomposition of carbon in soil is the level of available nitrogen.
When adding large amounts of available carbon to soils low in N, nitrogen becomes tied up, or immobilized, in the cells of the degraders. The net effect here is to induce nitrogen deficiency for plant growth due to swamping the system with available carbon.
Pay careful attention to the carbon to nitrogen (C/N) ratio of organic materials added to soils. The ideal ratio for most grass species is 20 to 1.