Science in Action: Encouragement of Plant and Mycorrhizal Relationships

                     Two of the most important relationships between microorganisms and domesticated plants are the relationships between the nitrogen-fixing bacteria called Rhizobia and the mycorrhizal fungi called VAM fungi with vascular plants. The relationship between Rhizobia and the so called nitrogen fixing plants such as the mesquite and acacia can be appreciated mostly from the research done on Rhizobia and leguminous crop plants like the peas and beans.  If a soil contains the Rhizobial bacteria, then legumes planted into that soil can benefit from this bacteria’s ability to take inert nitrogen gas from the air and oxidize it, or fix it, into nitrate, a form of nitrogen usable by the plant. Soil devoid of Rhizobial bacteria need to be inoculated with a “starter” population so that legumes planted in it can take full advantage of this symbiotic relationship and the “free” nitrogen it provides to the plant. Of course in return the bacteria in this relationship receive nutrients and other benefits from the plant.

                     But the discovery of mycorrhizal fungi is relatively new to researchers when compared to the time when nitrogen fixing bacteria were discovered and explored. Unlike Rhizobial bacteria which live in close association with legumes, VAM type of fungi infect almost all higher plants. Also unlike Rhizobia, VAM fungi penetrate the living cells of plants and form the organs called vesicles and arbuscles in the root. By doing so, the fungi link the plant to the soil, assisting in the transport of mineral nutrients, particularly phosphorus, to the plant. In return, carbon compounds from the plant are released into the soil where soil microorganisms rely on these compounds as an energy source. The plants in both cases become home to these invading microorganisms and a relatively safe and necessary place for them to live.

                     It is a widely held belief among researchers that VAM mycorrhizae can play an important role in poor or eroded soils. Researchers believe that VAM fungi can play a major role in increased agricultural production particularly when soil nutrients have been depleted. For this reason we might be able to imply their importance when establishing landscape plants in poor soils. The known roles of VAM fungi are the increased absorption of minerals, stimulation of growth, increased production, enhancement of resistance to environmental stresses and soil diseases.

                     Although VAM fungi stimulate phosphorus uptake of plants from the soil, over application of phosphorus to the soil actually inhibits VAM growth. Concentrations of phosphate phosphorus over 50 ppm in the soil have been reported to severely inhibit VAM colonization. However, the application of what we would consider normally insoluble phosphates, such as rock phosphate and bone meal, are effective to maintain VAM fungi. VAM fungi aid in making the application of this type of phosphorus, normally mostly unavailable to plants, available to the plant as a slow release source. An additional advantage of insoluble phosphate with VAM then is the long- term availability of phosphorus, compared to more soluble phosphates.

                     Besides increasing the availability of phosphorus to plants, VAM fungi-infected plants have high a potential for resistance to soil and environmental stresses. According to fossil records, VAM fungi infected roots of Early Devonian land plants over 400 million years ago. The Early Devonian period is recognized as a time when plants invaded the land from the oceans. Researchers have speculated that VAM mycorrhizae may have played an essential role in assisting ocean inhabiting plants in their advance toward terrestrial living by helping them to adapt to a new stressful and continuously changing environment. 

                     Poor soils include soils naturally devoid of organic matter either naturally or through erosion, and those intensively cultivated or subjected to excessive use of fertilizers and agrochemicals. In the past to maintain economic levels of productivity, farmers were forced to increase the use of inputs such as agrochemicals. With the use of VAM mycorrhizae, agricultural production inputs such as fertilizers and agrochemicals could be reduced. In landscapes, decreased use of chemicals such as fertilizers and pesticides would probably be standard practice. Instead, more concentration on enhancing the soil environment to promote soil microorganisms would be recommended.

                     In general, the population of VAM spores in poor soils, such as the soils damaged by severe dryness or erosion, is very low. VAM has been known to develop well in close association with grasses. Grasses are very effective for the revival of soil with low a density of VAM fungi. VAM develop well in orchards where grass is used for a sod. However, some growers believe that a clean culture, bare soil, is best for high quality. This is true among growers of landscape plants as well. Thus our soil management system must be re-evaluated in light of this information.

                     When concentrating on enhancing the soil environment for VAM development it was mentioned above that the amount of agrochemicals and applied chemical fertilizers be reduced. Other agrochemical inputs would have to be addressed as well. The application of several kinds of fungicides, such as copper fungicides, iprodione, iprodione-like compounds, benomyl and benomyl-like compounds, have been shown to severely inhibit VAM growth. The application of herbicides may also inhibit the intensity of VAM infection by decreasing the population of specific so called weeds which strengthen the activity of VAM fungi.

                     Although several kinds of VAM fungi have been known to have the resistance to adverse pH conditions, most of the VAM fungi are vulnerable to high pH soils, preferring pH ranges less than 7.5. The growth of VAM fungi also is poor in anaerobic soils. Although VAM fungi infect the roots of aerenchima-developing plants such as mangrove and paddy rice plants the percentage of VAM infection is generally low

                     The application of uncomposted organic matter severely inhibits VAM development. Applications of composted organic matter, however, stimulates VAM formation and the growth of plants. One of the VAM stimulators is low concentrations of ethylene. Ethylene evolves from composting organic matter and, at very low concentrations, may be effective for VAM growth. About 0.5 ppm ethylene gas stimulates VAM hyphal growth and VAM development in association with plants. But ethylene at concentrations above 0.5 ppm can depress hyphal growth and mycorrhizal formation. In soils with poor aeration where uncomposted organic matter has been applied, ethylene is often detected at concentrations high enough to inhibit VAM growth.

                     Activated charcoal incorporated into soils has been shown to be very effective in VAM development. But excess applications of charcoal may inhibit plant growth. The reason for this inhibition is not known but is thought to be associated with changes in the soil chemistry. Therefore, an appropriate amount of charcoal to be applied is less than eight tons per acre which works out to be somewhere around one third of a pound per thousand square feet. Zeolite is a mineral soil amendment which shows some promise in assisting with the development of mycorrhizae without the problems of that are associated with charcoal. It absorbs antagonists which inhibit VAM growth without the problems associated with charcoal.