By Bruce Suddeth, CSFM
The turfgrass soil system is a highly complex and infinitely changing habitat. Although studies of soil microbiology have been conducted for decades, scientists have recently made considerable progress understanding microorganisms and their function in soils supporting turfgrass growth. The following will examine basic soil properties and effects of soil microbes, basic review of microbe types and their functions, as well as how we can improve soil microbial activity and improve turfgrass vigor.
Soil supports plant life and contains organic substances required for plant and animal growth. Soil is defined as a mantle of weathered rock, which in addition to organic matter, contains mineral and nutrients capable of supporting plant growth. Soil is a collection of five major components: minerals, organic matter, water, air, and living organisms.
Minerals contribute less than 50 percent volume of a soil. Minerals are derived from the disintegration and decomposition of rocks. Nutrient availability, aeration and water retention are influenced by the mineral content. Stones, gravel, sand, silt and clay are formed from minerals. The clay portion of a soil often has the greatest influence on fertility and drainage because the chemical properties are influenced by surface area. Clay particles also have the greatest influence on microbial activity. Silt and sand have a lesser influence on soil chemistry and biological activity. Of all the soil particles, clay has the highest cation exchange capacity (CEC), which is the ability of the soil particle to hold positive ions. The CEC can alter soil physical properties, it also affects soil pH and fertility. Most plants obtain K+, Ca2+, Mg2+ from exchangeable sites. The exchange capacity of a soil is determined by the type and amount of clay and organic matter in the soil. The CEC of organic matter far exceeds clay. Organic matter is composed primarily of carbon, hydrogen, and oxygen, and is chemically heterogenous. In organic matter, hydrogen (H+) ions are strongly held under acid conditions and are not easily replaced by other cations. As the pH increases, the H+ ions are gradually replaced by other cations. Therefore, the CEC of organic matter is pH dependent. The exchange sites of a soil are important because they can attract or repel anions and cations but can also attract and repel charged organic molecules. Because the surfaces of microorganisms are composed of organic molecules, which are positively or negatively charged depending on the soil pH, soils can attract or repel microorganisms. This means soil microorganisms can chemically adsorb or bind to soil particles. How this is done is not fully understood. At typical soil pH values (5-8), soil microorganism is negatively charged. There likely are a variety of mechanisms responsible for microorganisms chemically adsorbing or binding to clay and organic matter.
Typically, organic matter makes up 3 to 6 percent of the volume of plant and animal materials in the soil. Humus is derived from organic matter, and is the substance left when organisms have modified the organic matter. Humus is the colloidal remains of organic matter, and contains the organic carbon and nitrogen needed for microbial development. Humus is the major food reservoir for microorganisms, and is composed of plant material modified by microbes. It contains C, H, O, P, S, N and small amounts of other elements, as well as various polymerized long chain organics. Humus is found in the “A” horizon of a soil profile. It is composed of a heterogeneous group of substances of unknown parentage and chemical structure.
Air and water
Air and water account for approximately 50 percent of the soil volume in an ideal soil. Compaction will reduce the percentage of air and water space, therefore, these percentages can change with varying traffic and soil types. Pore spaces regulate the volume of a bulk soil not occupied with soil particles.
Living organisms make up the fifth, and often the most underemphasized, component. Generally, less than 1 percent of soil by volume is living organisms. Soil organisms are critical for turfgrass growth and many important processes. The microbes can be divided into two major groups: microflora, which are plants; and macrofauna, which are animal parts.
When evaluating soil biological activity, it’s clear that microflora (bacteria, actinomycetes, fungi, and algae) are most important. Microflora contribute 60 to 80 percent of the total soil metabolism, and are irreplaceable in many essential processes. Each organism contributes to the whole soil life cycle.
Differences in moisture, organic matter, and pH can affect populations of organisms. Estimates have been shown there are approximately 100 to 200 million bacteria in each gram of soil. Other estimates conclude there may be an average of 930 billion organisms present in one pound of soil. The weight of microbes total 70 pounds per 1,000 ft2 of area. This large quantity of soil microbes serves many essential functions in their lifetime. When dead, the microorganisms serve as a nutrient source. One hundred pounds of dead microorganisms can provide 10 pounds of nitrogen, five pounds of phosphate, and two pounds of potash.
Bacteria are the most numerous of all the microbial populations in the soil. Bacteria have the largest capacity for rapid growth and vigorous decomposition of many natural materials.
Bacteria can be categorized as indigenous or invaders. Indigenous bacteria are primarily responsible for the biochemical function of the soil community. Invaders enter the soil through diseased tissue, manure and sewage sludge, and do not contribute significantly to the ecological activities of the community. A segment of the bacteria population grows readily when organic nutrients are added to the soil. They decline once the food has been exhausted. Other bacteria feed on less available nutrients such as soil organic matter, plant residues or components of other microbial cells. These types of bacteria usually don’t deplete their food sources, grow slowly, and don’t fluctuate in population as much.
Bacteria can also be classified as aerobic (needing oxygen to survive) or anaerobic, (grow in the absence of oxygen). Bacteria are rarely found in free soil water. They are held adsorbed to clay and humus particles and in slimy excretions. The number and type of bacteria are many and dependent on soil type and cultivation practices. There are almost 200 different types of bacteria in the soil. Bacterial numbers are usually greater in grasslands than in farmland because of high root density, plant debris, and useable organic matter.
Fungi provide a significant portion of biomass because of their large-diameter hyphae in most well aerated cultivated soils. The individual hyphae strands are referred to as mycelium. Fungi are dominant in acid soils, but can adapt to different habitats. Fungi do well at a pH of less than 4, primarily due to less competition for food. One major function of fungi is their ability to degrade cellulose, pectin, lignin and starches. This degradation aids in humus formation. Especially important in turf is the fact that some fungi can cause plant disease. This is only by a very small percentage of fungi that this occurs.
A form of fungi that is specialized and is of benefit to turfgrass is mycorrhizae. Mycorrhizae are specialized soil-inhabiting fungi that exist symbiotically with turfgrass roots to benefit both. This fungus is usually only found in the immediate local of a root. The fungus can either be ectotropic (penetrate between root cells) or endotropic (directly penetrating host cells). In turfgrass, endomycorrhizae are the most important. This fungus allows nutrient and hormone exchange between it and the host plant. Even though the fungus is parasitic, it is not pathogenic. Endomycorrhizae reproduce by spores, and can only complete their life cycle by infecting live roots. In some plants, an increase in drought tolerance has been associated with mycorrhizae. This was related to the increased uptake of water and nutrients.
Actinomycetes are microorganisms that produce slender, branched filaments that develop mycelium. Individual hyphae are similar to fungi but are narrower. Actinomycetes participate in the decay of resistant plant and animal tissue, formation of humus and humus type compounds, and the rotting and heating of hay, composts, and manures. They are second in abundance only to bacteria. Actinomycetes are more common in dry soils and with pH greater than 5. Populations of actinomycetes are greater in warmer climates.
Algae is common in habitats where there is sufficient moisture and light. Soil algae include green algae, blue-green algae, diatoms, and yellow-green algae. In temperate climates, green algae and diatoms are most dominant. Green algae are usually unicellular rather than filamentous and dominate acid soils. Algae do not contribute considerably to soil biochemistry and fertility but do contribute to the soil structure.
Soil environment and organisms
Microorganism are greatly affected by environmental conditions. The principal factors include; moisture, temperature, aeration, pH, and organic matter content. Soil water is important for several reasons. Water is a key component of the protoplasm of microorganisms. Without sufficient moisture, soil organisms either die or go into a period of dormancy. The optimum moisture content for aerobic organism is about 50 to 75 percent of the soil moisture holding capacity. Fungi and actinomycetes are infrequent at soil water contents greater than 85 percent, due to the lack of free oxygen. Algae do best under cool, wet conditions.
Microorganisms are diverse and exist over a wide range of soil temperatures. Each has an optimum temperature for growth. Microbes are most active at soil temperatures of 74 to 95 degrees Fahrenheit. Soil microbial populations fluctuate from season to season because of temperature and moisture variations throughout the year. The number of active bacteria, fungi, and actinomycetes is usually greatest during spring and fall. They decline during the hot dry summer period. Most microorganisms and those important to growing turf, are strict aerobes. They are most abundant under well aerated conditions.[Second-level subhead, italic] pH
The optimum pH for most bacteria is near neutral. Liming acid soils will greatly increase bacterial populations. The actinomycetes population is most abundant at pH 6.5 – 8.0. Fungi tend to dominate low pH environments because they out compete other organisms, not because these low pH’s are optimum.
Soils rich in carbon-containing materials such as humus and organic matter favor increased populations of microorganisms. After the addition of organic matter, bacteria and fungal flora usually dominate initially, with the actinomycetes becoming active during the latter stages of decay. Fungi must obtain energy from oxidizing organic materials. Increasing organic matter content increases the size of the fungal population and alters the composition.
Active bacteria and fungi can occur as deep as 60 to 80 inches below the turf surface, but most of microbial activity takes place in the top several inches.
Benefits of soil organisms
A prime function of soil organisms is to cause decay or oxidation of organic matter from the earth’s surface. During this decay process, several beneficial reactions take place:
1) Nonsymbiotic nitrogen fixation by organisms living in soil independent of other organisms. These organisms fix 10 to 20 pounds of nitrogen per acre per year from the atmosphere.
2) Symbiotic nitrogen fixation by organisms living in nodules of legumes fix nitrogen from the atmosphere and transfer it to the soil when the plant dies.
3) Ammonification where, during breakdown of organic matter, certain organisms free ammonium to the soil. This is important because it is how all organic nitrogen is released to the soil.
4) Nitrification whereby organisms convert ammonium to nitrate. Excess ammonium can be harmful to some plants. Some of the hydrogen ions from the ammonium are left in the soil solution and reduce the soil pH.
5) Phosphorous mineralization where soil organisms convert organic phosphorous to orthophosphates.
6) Sulfur conversions as soil sulfur is held in organic forms. Organisms convert sulfur to sulfate that can be used by plants.
7) Other reactions. Organisms during the decay process release other elements to the soil, such as Ca, Mg, K and micronutrients.
In order for microorganisms to function and grow properly, they must have an adequate supply of nutrients. Of all the requirements, the need for carbon is the single most important consideration in the structure and function of biological organisms. Plant tissues and microbial cells are about 40 to 50 percent carbon on a dry weight basis. Both plants and microbes must acquire a substantial amount of carbon for their survival.
Organisms can be divided into two categories based on how they obtain energy and carbon compounds. They are either autotrophs or heterotrophs. Autotrophs obtain energy from sunlight or oxidation of inorganic compounds and convert carbon dioxide (CO2) to carbon for use. Algae, green plants, and a few bacteria use sunlight to generate carbon and energy. The glucose formed during photosynthesis serves as a source of carbon and energy released due to microbial activity and/or respiration.
Heterotrophic organisms require pre-existing sources of organic nutrients to provide carbon and energy. Fungi, actinomycetes, protozoa, and most bacteria are heterotrophic. They use existing energy sources such as cellulose, lignin, starch, sugars, proteins and hydrocarbons to obtain their carbon. Degradation of these materials releases energy, of which some is used to build microbial protoplasm. The energy released can be used to fuel reactions needing energy and may also be given off as heat.
The most important function of microbes is usually considered to be the breakdown of organic matter and the replenishment of carbon dioxide (CO2) for photosynthesis. There are many sources of organic matter in a turfgrass soil system. Above-ground plant parts, roots, rhizomes, stolons, animal tissue and microorganisms themselves are all available sources. Plants are composed mainly of cellulose, hemicellulose and lignin. The older plants get, the less water, fats, sugars, proteins and other mineral elements they possess.
Microbes contain approximately 50 percent carbon. Twenty to 40 percent of the carbon from decomposition is used by microbes, and the rest is released as CO2 or other wastes. As microbes take up carbon into their cells, they also take up other nutrients in a process called immobilization. These mineral nutrients are unavailable to plants until the microbe dies.
Organic matter decomposition can be divided into three processes:
1) Plant tissue reduced by microbial enzymes.
2) Plant ingredients can be used to synthesize new microbes.
3) Final products of the reducing process are excreted into the soil.
A few conditions favoring rapid decomposition of plant residue and growth of microorganisms are as follows:
1) Low lignin content and small particle size.
2) Adequate nitrogen available or residue with low C:N ratio.
3) Near neutral soil pH for diverse microbial populations.
4) Adequate soil moisture and aeration.
5) Warm soil temperature with an optimum of 86 to 113 degrees Fahrenheit.
Nitrogen is essential for proper microbial growth and organic matter decomposition. The amount of nitrogen varies among plant tissues. Nitrogen-rich materials (low C:N ratio) decompose rapidly, while those with high C:N ratios (>30:1) decompose slowly because a large amount of carbon has been added without a sufficient supply of nitrogen to propel microbial activity. Microbes may use soil nitrogen to decompose high C:N materials and result in a nitrogen deficiency, which requires the addition of fertilizers. This problem usually does not exist in turfgrass, except when soil amendments are incorporated prior to establishment. For example, sawdust can have a C:N ratio as high as 200:1, meaning some nitrogen needs to be added to it for proper breakdown by soil organisms. Turfgrass clippings, with their high nitrogen content (4 to 6 percent) compared to other crops, should be unaffected by the C:N ratio.
Enhancing soil organism activity
Microbial growth starts off slowly then increases rapidly. The amount of time for a population to double is known as the generation time. Generation times can vary with the organisms and environment. Microbe growth occurs only a portion of the time, because, at some point, nutrients become limiting or waste products accumulate to toxic levels. The death of existing organisms equals the production of new organisms and the population holds steady. Temperature and pH are the most influential factors affecting microbial growth outside of the food source.
What should a turfgrass manager do to enhance microbial growth? What products or methods can be used? What to do next?
Biostimulant is a loose term for products that include; microbial inoculums, food for microbes, soil conditioners, plant hormones, and other non-nutritional growth-promoting substances. The group of biostimulants called plant hormones may contain one or more of the following: cytokinins (regulates rooting), gibberellins (regulates cell elongation/division), auxins (regulates cell elongation), abscisic acid (regulates stomatal activity), and ethylene (regulates seedhead production). Most physiological processes in plants involve an interaction of several hormones, and individual hormones have several functions. Normal hormone production can be influenced by environmental and cultural stress.
Other types of growth stimulants on the market contain humates or humic acid. These are naturally occurring compounds that are the end products of biological decomposition of organic matter. Humates or humic acids claim to increase CEC, increase microbial activity, and chelate micronutrients.
The most promising method of managing and enhancing the activity of soil microbes is with composted organic matter in wastes and other materials. Composts have been shown to add an active microbial component to soils and stimulate those microbes already present in the soil. Commonly used composts include brewery sludge, yard wastes, poultry litter, animal manure, and more. There have never been as many commercially available products to the turfgrass industry, so how do you choose the appropriate product for your application? First, start with the product label. Products registered with the EPA can legally justify claims of the product. Some products are marketed by independent research. Understand independent, scientific research that supports the products. Know who conducted the research, under what conditions, and the relevancy to turfgrass systems. Check to see if the results have been published in technical reports or journals. Conduct on-site testing that is relevant to your applications. Also, test at several locations representative of varying conditions and use untreated controls in side-by-side comparisons.
Turfgrass management is an infinitely evolving science. As we increase our understanding of the microbial community in turfgrass systems, more products will come to market. Some will be useful, some not. Independent research will be essential to the development of effective products.
The importance of a strong microbial community is without question. The effectiveness of various products available to stimulate microbial activity can always be of question. The turfgrass manager should become familiar with soil microbiology and processes, check for independent research to support product claims, and test products to make sure they are effective and are economically feasible. Even with these products, don’t forget the basics of turfgrass management: adequate sunlight, drainage, air circulation, proper fertility, good water management, traffic control and cultivation.
Bruce Suddeth, CSFM, is director building and landscape services at the University of South Carolina Upstate, Spartanburg, S.C.
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