H. A. J. Hoilink
M. J. Boehm
J. E. Heimlich
The 1993 ban on yard waste from landfills has encouraged government and private agencies to begin composting. This creates a subsequent problem: What can be done with the compost produced? One potential use for compost is as a potting medium.
Throughout the 1960s, the nursery industry explored the use of tree barks as peat substitutes in container media to reduce production costs. From these studies, composting technologies for tree bark developed. During the 1980s, similar processes were developed for food and agricultural wastes and municipal sludge. Currently, the large volume of yard waste, including grass clippings, leaves and brush, is spurring interest in exploring uses for yard waste compost.
In addition to the material utilization, improved plant growth and decreased plant losses attributed to root rots were side benefits observed during utilization of tree barks. In fact, the control of many diseases by compost is at least as effective as that provided by synthetic fungicides.
To be used as a potting mix component, yard waste compost must be of consistent quality and maturity. In fact, variability in quality related to maturity is the major problem limiting the use of compost. When compost is used in gardens or in agricultural fields, "maturity" of the material is less important, so long as there is sufficient time between compost application and planting to allow the material to stabilize (age or mature).
The technology for predicting compost maturity is available for several types of wastes. Also, the composting process can be controlled so that soil borne diseases (e.g., those caused by Fusarium spp., Phytophtora spp., Pythium spp., Rhizoctonia solani, and other pathogens) are suppressed. Specific parameters must be monitored to yield a compost that both controls disease and promotes plant growth.
As organic materials interact with moisture, air, bacteria and fungi, heat is generated. The composting process can be divided into three phases determined by temperature and heat output.
During the first phase, the initial 24-48 hours, temperatures gradually rise to 40-50 C (104-122 F). During this time, sugars and other easily biodegradable substances are metabolized mostly by bacteria and fungi.
During the second phase, which may occur over extended periods of time, temperatures between 40 and 65 degrees C (104-149 F) prevail. Cellulose and other more difficult substances to biodegrade are destroyed at that time. Lignins, the darker, woody components in plant tissues, break down even more slowly. During this high temperature phase, plant pathogens, weed seeds and biocontrol agents (excepting Bacillus spp.) are killed by the heat. Turning compost piles ensures uniformly high temperatures and helps produce a homogeneous product.
The third stage is the curing phase when the concentrations of materials that readily decompose decrease. The rates of decomposition, heat output and temperature decline during this phase. A micro-flora, similar to that found in soil, now colonizes the compost. Mature compost has a dark color, consists largely of lignins, humus and biomass and has a distinctive soil or "earthy" odor. This odor is attributed to the soil microflora present in the compost.
There are two mechanisms for biological control of diseases caused by soil-borne plant pathogens in compostamended substrates. The first is "general suppression," which controls "nutrient-dependent" pathogens. Many types of beneficial microorganisms contribute to this "general suppression." If propagules (reproductive bodies) of the pathogens are accidentally introduced into container media, they do not germinate in response to plant root exudates (secretions). The microflora introduced with the compost successfully competes with pathogens for these exudates. The "slow release" nature of the organic nutrients tied up in mature composts supports beneficial activity of this microflora and offers sustained biological control: the essence of "organic farming."
The second method of plant pathogen control is known as "specific suppression," where specific beneficial microorganisms eradicate specific pathogens. The specific microorganisms tend to randomly recolonize mature composts after the peak heat. Composts produced in the open near a forest (where many microbial species are present,) are likely to have higher suppressive properties than composts produced from similar materials on concrete pads or in partially enclosed facilities where there is less microorganism diversity.
Composts prepared from municipal sewage sludge are consistently conducive to plant pathogens because care is taken to kill fecal pathogens and parasites by heat exposure. This heating process, as mentioned earlier, kills beneficial microorganisms as well. Therefore, composts may have to be incubated for a month after peak heating to restore pathogen suppressive properties before they can be used beneficially. In field soil, several months may pass before the disease suppression qualities are fully induced. The best practice is to incorporate composts well ahead of planting.
The problem of variations in the suppressive qua] ties among composts can be solved by using specific microbial inoculants, or additives. The laboratories at Ohio State University have developed inoculants that can be introduced into compost after peak heating but before natural recolonization has reached significant levels. This inoculation results in consistent levels of disease suppression for general and specific pathogen suppression described above. Ohio State holds patents for this procedure of biological control.
Biological control does not occur in fresh, undercomposed organic matter. Only in the mature compost are pathogens killed by hyperparasites. Therefore, the degree of organic matter decomposition, or maturity, is important. To achieve this maturity level, composts must be stabilized to reach the level of decomposition where plant growth is stimulated.
In summary, considerable knowledge exists on the suppression of pathogens through biological control. Commercial nurseries have used the natural disease suppression phenomenon of compost-amended soils (general suppression) for years. Research suggests that the same benefits can be extended to the floriculture and vegetable industries with compost-amended potting media.
However, it will be necessary for these industries to inoculate with specific biocontrol agents to provide control against all major soil-borne pathogens. Those pathogens suppressed by "specific suppression" only are much more important to the floriculture and vegetable industries.
This fact sheet is one of a series based on composting research by The Ohio State University at the Ohio Agricultural Research and Development Center.
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Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.
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