The benefits of BENEFICIALS
A fascinating, but often misunderstood, aspect of soil is its biological component. More living organisms occur in the soil than all other ecosystems combined. The biological component of soil is both diverse and dynamic. It ranges from organisms that are easily seen with the naked eye to minute organisms that require a microscope for observation.
Microbes are part of all natural and intensively managed landscapes. They may provide answers to some of your problems or they may be causing some problems that you need to address. In recent years, researchers have been investigating the use of microorganisms to positively affect the growth of plants and turfgrasses.
A subterranean menagerie
Soil organisms are classified by their carbon and oxygen requirements.
Autotrophs (literally, “self-feeding”) in most cases use the sun to manufacture their own energy and obtain carbon from atmospheric carbon dioxide. However, other autotrophs produce energy through oxidation (decomposition) of certain inorganic substances (e.g. sulfur, iron and ammonium). Examples include plants, algae and some bacteria.
Heterotrophs, by contrast, obtain their energy and carbon from the breakdown of organic materials such as humus, plant debris (thatch) and other organisms. Examples include bacteria, fungi, protozoa and soil animals.
Aerobes are organisms requiring free atmospheric oxygen (O
2) for respiration. These organisms are associated with soils that have good drainage and aeration.
Anaerobes do not need free oxygen. They get their oxygen from various compounds such as nitrate (NO
3 -) and sulfate (SO 4 -2). Facultative anaerobes are adaptable and can survive in the presence or absence of free oxygen. Anaerobes are generally associated with saturated (wet) conditions.
Most soil organisms are beneficial saprophytes, which means that they consume dead organic matter and do not pose a threat to living plants. However, a few organisms are disease-causing parasites that are destructive to living plants.
The primary benefit of soil organisms is decomposition of carbon-containing material such as organic matter. This function is essential; without decomposition, the earth would be littered with organic refuse. In addition, microbes interact with soil and plants in the following ways:
Microbes improve soil aggregation, which influences soil-water movement and aeration.
They enhance nutrient levels through nutrient cycling and organic-matter decomposition.
They are essential in developing soil organic matter.
Microbes degrade, mineralize and immobilize materials (organic matter, pesticides, fertilizers) that turf managers apply to the soil.
Microbes also form symbiotic (mutually beneficial) relationships with plants.
The good guys
Many products are available that contain beneficial microorganisms or a mixture of microorganisms and growth-promotion substances. Researchers have used several strains of Bacillus thuringiensis (a soil bacterium) and some species of nematodes to combat destructive insects. Also, some fungi, bacteria and nematodes feed on plant pathogens or compete with pathogens for energy sources and growing space.
While some beneficial microorganisms may produce noticeable benefits such as those just mentioned, others have more complex and obscure effects. These microorganisms include Rhizobia spp., the endophytic and ectophytic mycorrhizae fungi (a group of fungi that live in or on plant roots) and free-living soil organisms.
Rhizobia spp. are bacteria that infect the roots of various leguminous species such as soybeans and clover. These bacteria are able to sequester or “fix” atmospheric nitrogen (78 percent of the air we breath is nitrogen gas) and transform it to ammonia nitrogen, which can be taken up by plants. The benefits of this group of bacteria for agriculture crops and forages are well documented. However, researchers have not found Rhizobia associations with turfgrasses.
Mycorrhizal fungal associations with plants are also well documented. In natural ecosystems, mycorrhizae are believed necessary for the survival and competition of many plants. However, their role and benefits in intensively managed turfs and landscapes are less clear. Marketers claim that these products can increase absorption of minerals and nutrients, disease resistance, drought tolerance and tolerance to salts. There may be merit to these claims, but independent research is sparse in this area.
Researchers at the University of Rhode Island investigated mycorrhizae in bentgrass putting greens and found that nearly every one of the 200 samples taken from the putting greens contained at least one species of mycorrhizae. In greenhouse trials, researchers inoculated bentgrass with mycorrhizae and did recognize positive benefits including enhanced establishment and more chlorophyll content in bentgrass plants. Increased drought tolerance and rapid recovery from wilting were the most pronounced effects.
Other, endophytic, fungi have been intensively studied in turfgrasses. These fungi form complex relationships within plants and have been shown to increase heat and drought tolerance in the grasses that they infect. They also successfully deter many feeding insects. These endophytes are transferred via seed, rather than inoculation. Breeders have produced many varieties of such species as perennial ryegrass, tall fescue and fine fescues that contain endophytes.
Free-living soil organisms are commercially available as inoculants. They can be classified into three broad categories:
Products that improve soil organic matter and release soil-bound nutrients to plants.
Products that produce better yields, especially during times of drought.
Microbes that fix atmospheric nitrogen, thus increasing plant-available nitrogen in the soil.
The first use of bacteria as an insecticide dates back to 1938. In subsequent years, several strains of Bacillus thuringiensis were discovered. However, turfgrass researchers have just begun to investigate the effects of bacteria on plants and their environment. Whether bacteria will become a widespread management tool remains to be seen. At least one commercial preparation is available now. However, bacteria have the potential to be effective non-chemical alternatives for disease control and plant growth for two reasons. First, bacteria are largely unaffected by most turfgrass fungicides. Thus, they can be integrated with fungicide programs. Second, turfgrass has shown resistance to most bacterial pathogens, as only one reported bacterial disease occurs in turfgrass.
Beneficial bacteria and fungi are frequently packaged with non-nutritional, growth-promoting compounds as well as energy sources for the microbes. These biostimulants are not subject to the same labeling and regulatory requirements as pesticides. Therefore, producers can make claims based on minimal research.
Also, results are confounded by the use of additives. In other words, it's difficult to tell whether any apparent benefit to turf was the result of the microbes or some other component of the mix. Often, observed growth responses are the result of nutrients such as nitrogen, iron or sulfur.
Many products contain diverse ingredients such as enzymes, nutrients, microbes, plant extracts and sugars. Testimonials abound as to the benefits of applying fructose, honey, molasses, sugar and syrup to stressed turfgrass. Turfgrass managers should carefully evaluate extraordinary claims of the benefits of biostimulants. Check to see if unbiased research is available for the product. It's also helpful to check with other turf managers that you know and trust to see what their experiences with biostimulants have been like. Fortunately, most of these products are not detrimental to turf. A small test with a limited amount of product typically puts you at risk of nothing more than losing the purchase price. Better yet, see if the vendor will supply you with a sample adequate to conduct a field test at your site.
In the complex biosphere of turfgrass soils, competition, antagonism and predation by natural species may limit the success of introduced organisms. It may be difficult to build up a population of introduced organisms to the extent that their benefits become apparent. Also, many of these organisms are sensitive to unfavorable environmental conditions such as exposure to ultraviolet (UV) light and desiccation.
A potential solution is to apply microorganisms during nightly irrigations. Some products already are available for use in this manner. Check application recommendations before you apply these products to avoid any procedures that may limit your success with them.
Manufacturers must properly formulate, ship and store these products to ensure survival. Upon receipt, store them in a cool place until application. This step is critical because most microorganisms are sensitive to excessive heat, drying and sunlight.
Other products may contain dormant or spore stages of microorganisms that can withstand environmental extremes until watered or tilled in.
Although several products are available that contain microbes and substances that contribute to microbial growth, research supporting positive benefits in well-maintained turfgrasses is not abundant. Without a doubt, however, many microbes do modify soils in positive ways. In fact, the most productive soils in the world were developed under grassland habitats that support large numbers of microbes. The characteristics of these soils — high nutrient-holding capacity, high nitrogen content, high organic-matter content and good soil structure and aggregation — are partly attributable to the soil biological component.
Soil is a dynamic living system teeming with many organisms. Most of them are beneficial or at least do not adversely affect plant health. Soil organisms are essential to turfgrass growth and development, even if their particular roles are not fully understood and appreciated. This field of research is in its infancy but is likely to uncover significant benefits of microbials that can be exploited in turf management. Some currently available commercial preparations may provide benefits, but ask for objective evidence.
Clint Waltz is a research assistant, H. “Skip” Skipper is a soil microbiologist, and Bert McCarty is a turfgrass scientist, all at Clemson University (Clemson, S.C.).
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