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Soil microorganisms: A turf manager’s guide

Your soil is teeming with trillions of organisms essential to plant health. Get to know this subterranean cast, how the various microbes keep grass green and growing, and how you can support them.

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Filed to: Soil, Aerification

Soil microbes turfgrass
What lies beneath? Soil has five principal components: minerals, organic matter, air, water and soil organisms. One pound of soil can contain 930 billion organisms. Photo by Freestockcenter/Freepik


Editor’s note: The following article first appeared in the November 1996 issue of GCM. Because of its timeless insights for turf managers, we’ve dusted it off and are presenting it anew.

The successful superintendent realizes that soil management is critical to turfgrass quality and playability. Spiking, core cultivation, deep aeration, soil amendments and adequate drainage are but a few of the tools used to produce optimal soil conditions.

But grass plants are not the only organisms affected by such practices. Microorganisms abound in the soil:

  • 4,650,000,000,000,000 microbes may live on or beneath a 1,000-square-foot soil surface.
  • 1 pound of soil may contain 930 billion organisms (7).
  • The soil and thatch of a Kentucky bluegrass lawn can support an estimated 280 million bacteria, 6 million to 9 million actinomycetes, and up to 2.8 million fungi per gram of soil and thatch (2).

A few microbes cause diseases, establishing a bad reputation for the rest. Those that don’t are often ignored. But these organisms have many functions that are crucial to healthy turf growth and that can be encouraged through proper management techniques.

The soil environment

Soil is “the outer, loose portion of the earth’s surface that is distinctly different from the underlying bedrock” (1). It has five principal components: minerals, organic matter, air, water and soil organisms.

The mineral component of a soil consists of sand, silt and clay particles and usually makes up 45% to 50% of a soil by volume.

Organic matter is composed of plant and animal residues in varying stages of decomposition. The organic component of a soil is typically 2% to 5% of the total volume in native soils. Prepared root zone mixtures may include as much as 20% added organic matter by volume, which is about 2% to 4% by weight.

About 50% of soil volume should consist of pore space, with half the pores being relatively large and air-filled to provide adequate drainage. The remaining half of pore space should be smaller capillary pores that provide enough tension to hold water against the force of gravity for the plant’s use. Areas subject to substantial foot or vehicular traffic will tend to compact over time, resulting in a loss of larger air-filled pores and an excess of smaller water-holding pores. Compaction can create a soil system that holds excessive moisture and lacks sufficient oxygen for proper microbial activity and root growth. Adequate drainage and soil aeration are critical to both turfgrass growth and soil organisms.

Living organisms in soil

Organisms in the turf-soil system make up far less than 1% of the soil by volume, but sustaining healthy grass would be impossible without them. For instance, when they die, they serve as a nutrient source. One hundred pounds of dead microbes provides 10 pounds of nitrogen, 2 pounds of phosphorous and 1½ pounds of potassium.

The soil’s microorganism population consists primarily of bacteria, actinomycetes, fungi, algae and protozoa. In addition, larger organisms (macroorganisms) such as earthworms, nematodes and insects are common inhabitants of the turf-soil system.

Bacteria

Bacteria are the most numerous of soil microbes and often outnumber the other organisms combined. They may number 100 to 200 million per gram of fertile soil. They are extremely small (less than 1 millionth of an inch) and can multiply rapidly. Depending on the particular organism and environmental conditions, a population can double in 20 minutes.

Soil bacteria are extremely diverse, with more than 200 major types found in various soils. The predominance of any given type depends largely on soil conditions. Populations are usually greater in grasslands than in tilled soils because of higher root density, nutrient status and organic matter concentration. Bacterial numbers are also greater in warmer rather than cooler climates. Even so, populations as high as 1 million per gram of soil have been reported in Arctic soils, where temperatures never exceed 10 degrees C and the soil remains frozen nine to 10 months a year.

In a turf soil, bacteria are important in breaking down cellulose and other structural compounds to reduce thatch. The nitrobacteria (Nitrosomonas and Nitrobacter) are important in the process of nitrification — conversion of ammonium to nitrate for plant use. This essential process makes nitrogen available from soil organic matter, as well as from slow-release and many quick-release fertilizers. If not for the nitrobacteria, the golf course superintendent would have few fertilizers from which to choose.

The bacterium Thiobacillus is important in the conversion of sulfur to sulfate, an essential step in bringing alkali soils of the West back into production. Many biological control agents available for turfgrass applications are bacteria, including Bacillus papillae, Bacillus thuringiensis and Beauveria species. Bacteria also play an important role in the development of poisonous black layer and have been associated with C-5 decline of Toronto creeping bentgrass, the only known bacterial disease of turfgrass.

Actinomycetes

Actinomycetes are second in abundance to bacteria. They can reach populations of 100,000 to 100 million per gram of soil in temperate climates and normally make up 10% to 50% of the microbial community depending on environmental conditions. In high-pH soils, actinomycetes may account for 90% of the microbial pool.

Actinomycetes lie developmentally somewhere between the more primitive bacteria and the more advanced fungi. They produce slender, branched hyphae similar to those of fungi but that are much narrower. Actinomycetes show little activity when pH is less than 5, often amounting to less than 1% of the population in strongly acidic soils. They are most active at a pH of 6.5 to 8.

Actinomycetes are relatively slow-growing organisms. They are involved in fewer soil processes than bacteria are, but they are important in decaying the more resistant components of organic matter and thatch, such as lignin. They help reduce plant materials into humus and are quite prevalent in compost and clipping piles. The most common genus of actinomycetes is Streptomyces, which can make up as much as 70% to 90% of soil colonies. Streptomycetes produce a distinct musty smell that is often evident in newly plowed soils.

Fungi

Fungi are another important microbial inhabitant of turfgrass soils. Although they’re known for their ability to cause diseases, the vast majority of fungal species are, in fact, beneficial to turf soil. It’s difficult to estimate fungal populations, because each organism can produce mycelium that can also be broken to produce additional individuals capable of growth and reproduction. Fungi can produce 10 to 100 meters of hyphae per gram of soil, and 10,000 to 1 million propagules per gram of soil.

In most well-aerated agricultural soils, fungi are responsible for a large percentage of the soil’s biomass because of the extensive mycelia they produce. Individual mycelium are referred to as hyphae. Fungi are most dominant in acid environments but are well adapted to a variety of soil conditions. At pHs of less than 4, few bacteria and actinomycetes flourish, while most fungi can grow quite well.

Fungi have developed many specialized structures that allow them to survive periods of adverse environmental conditions. Conidia, sclerotia and other structures allow survival in a dormant state until moisture, temperature or other favorable conditions enable the fungus to break dormancy. Thus, the snow molds can survive over summer and Pythium species over winter from season to season.

Unlike green plants, fungi contain no chlorophyll and cannot manufacture their own food via photosynthesis. They obtain their nutrients from existing organic materials such as sugars, starch, cellulose and lignin. Thus, a major function of fungi is to degrade complex organic molecules and aid in organic matter decomposition. Fungi also improve soil aggregation and structure by physically binding together soil particles. Certain fungi are predators of nematodes and protozoa. Although turf managers often most closely associate fungi with disease problems, they shouldn’t underestimate other, beneficial functions of fungi.

Algae

When turf is mowed too short or is thinned out because of some other factor, algae may gain a competitive advantage, resulting in an algal bloom. Algal counts as high as 1 million per gram of soil have been noted in association with distinct algal blooms.

Algae are the dominant photosynthetic organisms in soil. Because light is necessary for their survival and growth, they exist primarily on or near the soil surface. The two most common types include blue-green algae and green algae. In temperate climates, green algae are most common. They are usually unicellular and prefer acidic soils (pH 5.5 to 6.8).

Blue-green algae predominate in soils with a pH of 7 to 8.5 and are seldom found when soil pH falls below 5. Alteration of pH alone is seldom sufficient to control algal outbreaks, however. Providing improved turfgrass growth conditions, minimizing light penetration and reducing surface soil moisture are essential cultural practices for controlling algae.

Soil microbes turf
Image by Jannoon028/Freepik


Algal infestations of turfgrass can result in a surface layer or crust that limits water movement and increases surface-moisture retention. Crusting can result in a smothering of developing turfgrass as well as a poor surface for play. Algae also competes with desirable grasses for nutrients and has been associated with the development of black layer. Chemical controls, including copper sulfate, hydrated lime, chlorothalonil and mancozeb, have been used with varying degrees of success as part of an overall algae-control program.

Protozoa

Protozoa are the simplest form of animal life in the soil and are the most abundant of soil invertebrates. They are primitive, unicellular organisms and are generally found in the upper 6 inches of soil and include organisms such as amoeba and paramecium. Their life cycle consists of an active stage and a resting (cyst) stage that allows them to resist adverse environmental conditions and survive for many years.

Protozoa are typically present at 10,000 to 100,000 per gram of soil. Protozoa feed on organic matter and other microbes, and some are cannibalistic. Some amoeba and other ciliates can divide several times a day and may consume several thousand bacteria per division.

Macroorganisms

When evaluating soil biological activity, it’s clear that the microorganisms are the most important (1). Microbes account for 60% to 80% of a soil’s metabolism and are critical to many essential processes. In addition to microorganisms, however, soils also contain macroorganisms such as earthworms, insects, mites, nematodes and other organisms.

Fertile soils may contain as many as 100 million macroorganisms per acre, with a live weight of 15 to 30 pounds beneath 1,000 square feet of soil surface. In most soils, the earthworm is probably the most important macroorganism. A fertile soil might hold 20 earthworm casts per square feet of surface, with a total population ranging from 10,000 to 20,000 per 1,000 square feet. As few as 10 earthworms per square foot of surface are sufficient to provide beneficial effects.

Earthworms can have an extremely positive effect on soil fertility. While inside the earthworm, organic matter and mineral soil are subject to digestive enzymes and grinding that increase soil fertility and nutrient availability. Earthworm casts are typically higher than native soil in bacteria, organic matter, nitrogen, phosphorus, potassium, calcium, magnesium, pH and cation exchange capacity. In addition to their positive effects on soil fertility, earthworms can also improve soil structure, aeration and drainage.

Earthworms mix and granulate soil by moving clippings and thatch down into their burrows. Research has confirmed the importance of earthworms in degrading thatch. After three months of observation, Kentucky bluegrass plots with abundant earthworms exhibited thatch that was broken apart and much better dispersed than thatch in plots without earthworms (6). Thatch accumulation was much lower and microbial activity was much higher when earthworms were present.

Environmental influences

Microorganisms are greatly affected by the chemical and physical properties of the soil. The principal factors influencing their populations include aeration, moisture, temperature, pH, organic matter content and depth.

Aeration

Microorganisms can be classified as either aerobic (requiring oxygen to survive), anaerobic (grow only in the absence of oxygen) or facultatively anaerobic (can grow with or without oxygen). Most microorganisms — and certainly those of greatest importance to turfgrasses — are aerobes. Thus, one of the best things a turfgrass manager can do is assure a well-aerated, non-compacted soil. Activity in poorly aerated soils tends to favor undesirable microbes often associated with unfavorable growing conditions.

Moisture

The moisture content of soil is important to microbe populations for several reasons. First, water is an essential component of the protoplasm of micro- and macroorganisms. Without adequate moisture, soil microbes either perish or go into a period of dormancy until moisture conditions become more favorable.

The optimum soil-moisture content for aerobic organisms is about 50% to 75% of a soil’s moisture-holding capacity (field capacity). Fungi and actinomycete populations are severely limited at soil moisture levels greater than 85% because of a lack of free oxygen, although they can survive desiccation by producing spores or by going dormant. Actinomycetes are better able than most other microorganisms at withstanding dry conditions.

Because they feed primarily on bacteria, protozoa are influenced by moisture fluctuations affecting bacteria. Moisture also affects the physiological function and locomotion of protozoa. Water is often a substantial limiting factor for the growth of algae. In general, algae proliferate under cool, wet conditions, although some species of blue-green and green algae have survived for as long as 10 years in dry soil.

Temperature

Soil microbes can be divided into three general types based on their temperature requirements: mesophiles (most active under moderate temperatures), psychrophiles (require low temperatures) and thermophiles (require high temperatures). In general, most microbes are most active at soil temperatures of 77 to 86 degrees F. Thermophiles are rare in soil, but are often found in abundance in rotting composts and clipping piles.

Soil microorganisms turf
Image by Adobe Stock


Soil microbial populations fluctuate significantly from season to season, mainly because of temperature and moisture variations. In temperate climates, there is often a burst of activity during spring as soils thaw and temperatures become favorable. The number of active bacteria, fungi and actinomycetes is usually greatest during spring and fall and declines during a hot, dry summer. Large fluctuations in moisture and surface temperature also tend to reduce microbial activity during summer.

Acidity

Soil pH is a measure of the hydrogen ion concentration in the soil solution and is measured on a scale of 0 to 14, with 7 being neutral, 3 being strongly acid and 10 being strongly alkaline. The optimum pH for most bacteria is near neutral; thus, liming acid soils will greatly benefit bacterial populations. The actinomycetes are most abundant at a pH of 6.5 to 8 and exhibit little activity as pH levels fall below 5.

The disease take-all patch is caused by a fungus (Gaeumannomyces graminis) that is particularly sensitive to pH. The disease is most prevalent on bentgrass species growing on wet, recently limed soils. The recommended control for take-all patch is to use acidifying fertilizers such as ammonium sulfate in an effort to reduce soil pH. At lower pH levels, other microorganisms effectively outcompete the take-all organism and reduce its ability to cause infection.

Organic matter

For microorganisms to function and grow properly, they need an adequate supply of essential nutrients. Of all the nutrients, carbon is the most important in the structure and function of biological organisms. Microorganism cells contain about 40% to 50% carbon on a dry-weight basis. Because microbes must acquire a large amount of carbon to assure their survival, soils high in carbon-containing materials such as humus and organic matter favor increased microbial populations.

Fungi, actinomycetes, protozoa and most bacteria are heterotrophic organisms — that is, they cannot manufacture their own food as green plants do. Heterotrophic organisms use food sources such as cellulose, lignin, starch, proteins and hydrocarbons to obtain their carbon. Microorganisms exist that can degrade organic matter (thatch), hydrocarbons (oil and plastic products), pesticides and other compounds. In each case, the organism is simply working to fulfill its need for readily available carbon present in the compound being degraded.

Following the addition of organic matter to a soil, bacteria and fungi usually dominate the population initially, with actinomycetes becoming active during the later stages of decomposition.

Depth

Although active bacteria and fungi have been found as deep as 60 to 80 inches below the turf surface, activity is greatest in the top several inches of the profile. Organic matter, root exudates, moisture, oxygen and nutrients in this zone help promote activity. Algae in particular are often restricted to the upper half-inch of the profile because of their need for sunlight and carbon dioxide. A 17-month study of a USGA putting green profile in Arizona revealed that the thatch layer contained 40 to 1,600 times as many bacteria as the soil, 500 to 600 times as many fungi, and up to 100 times as many actinomycetes (5). This increased thatch activity was associated with more favorable pH, moisture and nutrient availability.

Summary

Although often overlooked, the micro- and macroorganism population in a typical turf soil system is an extensive and dynamic population. Without a healthy and balanced complement of these organisms, many of the essential processes involved in plant growth could not be accomplished. Many environmental factors, including pH, moisture, temperature and nutrient status, affect the vitality of soil organisms. Fortunately, those environmental conditions favoring good turfgrass growth also tend to favor healthy microbial populations. By understanding the range of organisms present under a typical turf area, their functions and the conditions that favor their competitive nature, the golf course superintendent can better manage soil conditions for optimum turf quality.


Should you inoculate your soil?

Soil bacteria can be classified as either indigenous (true residents) or as invaders of a particular soil. Indigenous bacteria are those that have evolved over time so that they can flourish under the particular moisture, nutrient and pH conditions of a given soil. Invaders are non-native strains that can be introduced with precipitation, composts, topdressing, diseased tissue or via some other addition to the system.

These introduced strains rarely, if ever, contribute to the ecology and activity of a soil in a meaningful way. The likelihood that a single application of a given product will result in a permanent improvement in a soil’s microbial ecology is remote for several reasons:

  • Introduced organisms are likely to be overwhelmed and outcompeted by the much more numerous indigenous organisms.
  • There is no guarantee that introduced organisms will flourish under the existing moisture and pH conditions.
  • Surface application of materials exposes them to potentially harsh environmental conditions, including heat and drought stress.
  • Pesticide applications may have an adverse effect on the establishment of the introduced organisms.

Literature cited

  1. Alexander, M. 1977. Introduction to soil microbiology. 2nd ed. John Wiley and Sons Inc. New York.
  2. Cole, M.A., and A.J. Turgeon. 1978. Microbial activity in soil and litter underlying bandane- and calcium arsenate-treated turfgrass. Soil Biology and Biochemistry 10:181-186.
  3. Edwards, C.A., and P.J. Bohlen. 1996. Biology and ecology of earthworms. 3rd ed. Chapman & Hall, London.
  4. Hodges, C.F. 1993. The biology of algae in turf. Golf Course Management 61(8):44,48,52,54,56.
  5. Mancino, C.F., M. Barakat and A. Maricic. 1993. Soil and thatch microbial populations in an 80 percent sand:20 percent peat creeping bentgrass putting green. HortScience 28(3): 189-191.
  6. Potter, D.A., A.J. Powell and M.S. Smith. 1990. Degradation of turfgrass thatch by earthworms and other soil invertebrates. Journal of Economic Entomology 83:2362-2369.
  7. Roberts, E.C. 1989. The biology of soils. Golf Course Management 57(4):92-102.
  8. Vavrek, R.C. 1990. Beneficial turfgrass invertebrates. USGA Green Section Record 27(6):7-9.

Richard Cooper is associate professor of turfgrass science in the Department of Crop Science at North Carolina State University in Raleigh.