The living earth

A fascinating but little-understood aspect of soil is its biological component. More living organisms occur in soil than in all other ecosystems combined. The living portion of soil is a diverse and dynamic collection of organisms, from types that you can easily see with an unaided eye down to creatures that you can observe only by using a high-powered microscope.

The primary activity of soil organisms is to break down carbon-containing material (organic matter). This function is essential because without decomposition, the earth would be littered with organic refuge. But soil organisms do more than just dispose of refuse. They:

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  • Improve soil aggregation, which influences water movement and aeration;

  • Enhance nutrient levels;

  • Develop soil organic matter;

  • Form symbiotic relationships with plants; and

  • Break down applied materials.

Growth and development of most aerobic soil organisms require temperatures between 50° and 96°F, adequate soil moisture and an available nutrient source. Soil microorganism activity decreases with decreasing soil temperature until near freezing, when most activity ceases. Also, soil aeration can determine the type of organism that inhabits a soil. In well-aerated soils, aerobic organisms use oxygen to oxidize or decompose organic matter. However, when oxygen is limited, anaerobic organisms predominate and rob plants of needed nitrates (NO3-), sulfates (SO4-2) and iron oxides. Soil acidity also influences microbial populations. Like turfgrass, most soil organisms flourish within a pH range of 5.5 to 8.0.

Kinds of soil organisms

• Bacteria

Bacteria are the most abundant of the soil organisms (>100 million per gram or teaspoon of soil) and the most important within the top 6 inches of soil. Bacteria are very small single-celled microorganisms that reproduce by cell division. Great diversity exists among bacteria: some species are aerobic, some anaerobic, some autotrophs and some heterotrophs. Compared to cultivated agricultural soils, bacteria populations are generally greater in grassland soils because of higher root densities, available nutrients and organic matter content. Like many soil organisms, bacteria are influenced by soil temperature, water content and pH. Soil bacteria populations, therefore, fluctuate with the season, with largest populations in the spring, early summer and fall. Due to a quick generation time (as fast as 20 minutes between cell divisions), bacteria can quickly colonize and exploit organic materials once conditions become favorable for growth.

Bacteria are responsible for many key biological reactions such as nitrogen fixation and sulfur oxidation, which make these nutrients available to plants. Also, bacteria are instrumental in the breakdown of cellulose and other structural components of thatch. In turfgrasses, heterotrophic bacteria are responsible for decomposition of organic materials and regulation of soil organic matter. However, not all bacteria are beneficial; some are pathogenic to plants and animals. The only known bacterial problem in turfgrass is C-5 Decline of Toronto creeping bentgrass. However, other bacteria species are being investigated as biological control agents for weeds (Xanthomonas campestris pathovar poannua), insects (Bacillus popillae and B. thuringensis) and nematodes (Beauvaria bassiana and Pasteuria sp.).

• Fungi

Fungi are not the most abundant of the soil organisms, but they account for the greatest amount of living mass in soil. (Scientists now believe the largest living things are soil-inhabiting fungi. There are reports of individual fungi that extend through several acres of soil.)

Fungi are multicellular and do not contain chlorophyll; therefore, they cannot manufacture their own food. Familiar examples include mildews, molds, rusts and mushrooms. Although fungi can be pathogenic to turfgrasses, they can also benefit turf by improving soil aggregation and decomposing complex organic residues. Some fungi enter into a mutually beneficial (symbiotic) relationship with roots of plants. This is well-documented with mycorrhizal fungi and trees. Scientists believe mycorrhizal fungi obtain carbohydrates from the plant and the plant uses fungi for greater nutrient and water accumulation. Mycorrhizal relationships in turfgrasses are not well understood, but high numbers of mycorrhizae have been reported in turfgrass soils. Mycorrhizae also may provide some resistance to various stresses by increasing available soil volume for water and nutrient acquisition.

A better-understood symbiotic relationship occurs between turfgrasses and fungi known as endophytes. Without doing harm, endophytes colonize the interior of roots. Like mycorrhizal fungi, endophytes obtain nutrients from the grass and, in return, produce toxic chemicals called alkaloids that protect the turfgrass plant from certain insects, pathogenic fungi and grazing mammals. Endophyte-infected turfgrasses tend to be greater in size and exhibit improved drought tolerance. Endophytic fungi are transmitted from one generation to the next via seed. Therefore, the use of endophyte-enhanced tall fescues and perennial ryegrasses are an excellent practical means to incorporate insect resistance into these varieties.

• Actinomycetes

Actinomycetes are believed to be an evolutionary transition between bacteria and fungi because they have characteristics of both organisms. Actinomycetes are single-celled microorganisms lacking chlorophyll. Based on numbers, they are second to bacteria in abundance in soils. Actinomycetes are considered to be slow growing, “late colonizing” organisms responsible for degradation of complex molecules like chitin, lignin, cellulose and phospholipids. Actinomycetes produce antibiotics and volatile substances that give soil a sweet, rich, “earthy” aroma. They are especially sensitive to pH changes, with populations being greatest at pH values above 6.0 and almost nonexistent at pH 5.0. Their exact roles in turfgrass soil biosphere are poorly understood.

• Algae

Algae are multicellular organisms with chlorophyll and, therefore, are autotrophic in their nutritional requirements. Thus, they must reside near the soil surface where light for photosynthesis is abundant. Examples include cyanobacteria (formally known as blue-green algae), green algae and diatoms. Algae reproduce by simple cell division and have cell walls similar to higher plants. Considered “early colonizing” organisms in natural habitats, algae are important in initial stages of soil formation. However, algae also can form crusts or mats that seal the soil surface, effecting water infiltration, water retention and soil/air gas exchange and influencing (mostly preventing) seed germination. Buried algae mats have also been associated with the physical, chemical and biological condition called black layer (see photo, below).

Algae especially can become a problem on golf putting greens where water is readily available and heights of cut are very low, allowing sunlight to reach the soil surface. Algae are a difficult problem to overcome on greens once established. The best control is prevention. Sound agronomic practices that maintain a dense turf cover — mowing at proper heights, judicious use of water and improving air circulation and drainage — aid in eliminating the environmental conditions conducive to algae growth. Practices that improve soil conditions such as aeration, topdressing, vertical mowing, spiking/slicing and air injection can, therefore, assist in controlling algae problems.

• Protozoa

Generally found in the upper 6 inches of soil, protozoa are the most abundant of the soil-inhabiting organisms that can be considered “animal” life. Protozoans include amoeba and paramecium, which feed on organic matter and other soil microbes. Little is known of soil protozoa, but they are thought to regulate bacterial populations through predation and competition. There are no known diseases of turfgrass as a result of protozoa.

• Nematodes

Nematodes are microscopic roundworms that can comprise as much as 90 percent of the multicellular invertebrates in soil. Millions of nematodes can inhabit a few square feet of soil. Nematodes reproduce by eggs and are most prevalent in warm, moist, sandy soils. They are essential in the soil food web, especially in the recycling of soil nutrients.

Like many other microorganisms, nematodes can positively affect soil. However, some species can be detrimental to plants. Of the thousands of nematode species so far identified, only about 50 feed on turfgrass roots. Sting, ring, stubby root, lance, root-knot, spiral and others are important parasitic nematodes on turfgrass roots. Detrimental populations are most likely found in irregular patches of the rootzone. This is to be expected, because most parasitic nematodes are root feeders and require plant roots to thrive and reproduce.

Few effective nematicides are available, so control is difficult. The best option is to fumigate prior to establishment, if possible. Biological control may hold some potential for nematode control. Current products being investigated are chitin/urea mixtures, sesame-based products, predacious bacteria and fungal byproducts that act as nematicides. Perhaps in the future, such products will provide practical effective control.

• Viruses

Viruses are sub-microscopic organisms with a relatively simple structure, a DNA or RNA (ribonucleic acid) core and an encasing protein coat. Viruses are intracellular parasites that do not respire or metabolize and must have a biological host to reproduce. Soils may harbor many viruses, but it is believed that they do not directly affect soil characteristics.

Little is known about pathogenic turfgrass viruses. However St. Augustinegrass decline and centipedegrass mosaic have been attributed to panicum mosaic virus. The virus is spread through sap on mower blades. Mowing when the turfgrass is dry has proven to reduce the spread of the virus. To best control this pathogen, use resistant St. Augustinegrass varieties. Fortunately, many of the improved varieties currently on the market are resistant.

• Larger organisms

Larger soil-inhabiting animals and insects include earthworms, grubs, slugs, mole crickets, ants and mites. These organisms mostly affect turf by churning the soil while feeding, which aids aggregation, water movement, aeration and thatch degradation.

Not only is the soil composed of sand, silt, clay, air and water, it is also a living, “breathing,” dynamic environment containing a broad diversity of organisms. These serve vital functions in the rootzone — most are beneficial but some are detrimental. Our understanding of soil micorbiology is in the early stages. As we learn more, there's little doubt we'll be able to put this knowledge to use to grow better turf.

Clint Waltz is a graduate assistant, H. “Skip” Skipper is a professor of soil microbiology and Bert McCarty is a professor of turfgrass science, all at Clemson University (Clemson, S.C.).

GLOSSARY

  • Aerobes — organisms that require free oxygen (O2) for respiration.

  • Anaerobes — organisms that do not require free oxygen, but use oxygen derived from various compounds for respiration. Anaerobes are associated with saturated soil conditions.

  • Autotrophs — organisms (plants and other types) that use the sun to produce their own energy and obtain carbon from atmospheric carbon dioxide (CO2) or the breakdown of inorganic substances.

  • Endophytes — root-inhabiting fungi that form beneficial relationships with plants; have been shown to improve stress tolerance and insect resistance.

  • Facultative anaerobes — organisms that can survive in the presence or absence of oxygen.

  • Facultative parasites — organisms that can survive as saprophytes but may attack plants if a suitable host is available.

  • Heterotrophs — organisms that obtain energy by consuming soil organic materials.

  • Parasites — disease-causing organisms that feed on living plants.

  • Saprophytes — organisms that live off of dead organic matter.

  • Symbiosis — a relationship in which soil organisms and plants each benefit from the presence of the other.

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