Focusing on the technical issues related to composting and compost utilization for several different waste streams can improve value-added marketing opportunities.

Enforcement of U.S. regulations governing the concentrations of priority pollutants in environmental samples has had a major effect on the development of rational solid waste management practices for recycling solid wastes through composting. Diversion of priority pollutants from the solid waste stream has allowed composting to become the preferred alternative process for treatment of an ever-increasing diversity of solid wastes.

Composting of commingled municipal solid wastes (MSW) has all but been abandoned in the U.S. because the quality of the compost produced cannot consistently meet the expectations of the compost users. Landscapers have been unwilling to accept risks associated with the distribution of plastics and glass in such contaminated products to homeowners even though some batches of MSW composts can meet U.S. Environmental Protection Agency (Washington) guidelines governing the concentrations of priority pollutants. As a result, waste separation became a widely adopted strategy for recycling MSW in the U.S. and yard debris composting now is practiced widely.

Furthermore, composting of food and restaurant wastes is increasing rapidly. Municipal, paper mill and other industrial wastewater sludges have been composted for decades in the U.S., a trend that is increasing as priority pollutants are removed from these waste streams and where land application of the raw material poses unacceptable problems.

As a result of quality control procedures adopted by the composting infrastructure in the U.S., composts increasingly can be sold into value-added markets. The most value added aspect of high quality composts generally is the peat-substitute value in potting mixes and in landscapes for planting purposes. This particularly applies when blends of composted materials are marketed.

The second most valuable aspect is the plant disease suppressive nature of composts. Utilization of composts has facilitated the phase-out of methyl bromide and also significantly curtailed soil fungicide use. Biological control-agent-fortified composts, which provide more predictable disease control than natural composts, promise to further increase the value of composts.

Municipal and Industrial Solid Wastes
Recent estimates suggest that the U.S. generates at least 400 million tons of municipal solid wastes (MSW) each year. The estimated amount of MSW recycled grew from only eight percent in 1990 to 32 percent in 2001. Composting of mixed MSW in the U.S. lost the momentum it had in the early 1990s due, primarily, to product quality.

Compost produced from mixed MSW cannot consistently meet EPA 503 rules governing concentrations of heavy metals and other priority pollutants. Glass, other sharps that cannot be removed by separation strategies available for composted products and plastics in composted MSW product pose hidden problems. These materials typically are not recognized until the organic matter has decomposed in the field several years later. In as sense, application of MSW compost to soil is like converting fields into landfills over a long period of time. Thus, up from waste separation is critical to compost quality and value-added marketing.

Yard debris (leaves, brush, grass clippings, sod, etc.) represent a substantial portion of the composted MSW. More than 3,000 such composting facilities exist in the U.S. and the total tonnage of yard wastes composted in the U.S. exceeds 38 million tons per year. Even though the yard debris industry produces composts from separated wastes, great care must be taken to remove treated lumber from the waste stream before grinding to avoid contamination of the product with unacceptable concentrations of dioxins and heavy metals.

The system of composting used for yard debris varies widely. Windrow systems produce excellent quality composts, but frequent turning can result in significant reduction of particle size. Since profits associated with sales of composted mulch are greater than those associated with fine particle size composts, block-and-wedge systems have been developed in which the material is not turned with mechanical turners.

Block-and-Wedge Systems
Block-and-wedge systems allow production of excellent quality mulch products within five to eight months with total nitrogen content in the 1.7-percent to two-percent range. Odor production under challenging weather conditions by these systems seem to be lower, or at least no worse than frequently turned windrow systems.

The composted mulch produced performs like forest litter in the landscape. Subsoil utilized in landscapes develops a duff-like forest horizon appearance within months after out planting of nursery stock if mulched with a two- to three-inch deep layer of composted yard wastes. In conclusion, the block-and-wedge composting systems can yield excellent quality mulches as long as they are operated appropriately to avoid fires.

Adding Food Wastes to the Mix
A growing trend in U.S. composting plants is to blend food wastes with yard debris, and a few plants are composting food industry wastes in addition to agricultural wastes. Food wastes introduce special issues that must be addressed to produce value-added products consistently.

Pre-consumer byproducts, such as vegetable matter, decompose readily during composting and typically raise potash concentration to high levels-a nutrient that must be considered in utilization programs from a salinity, as well as nutrition, point of view. Post-consumer food wastes can be high in salt content, which may negatively affect the value of composts.

Yard debris composted with pre-consumer food wastes make excellent products for inclusion into container medium formulations. Readily degradable materials, such as food wastes, produce fulvic acids during their decomposition. Thus, organic matter transformations during composting of these products are similar to those of composted manures and sewage sludges.

Tree bark, a widely used commodity in container media, does not contain large quantities of readily degradable materials. Thus, wood industry-based byproducts benefit from amendment with composted manures and low-in-salinity composted food wastes. Fulvic acids maintain iron and other trace elements in solution, and are available to the plant event at pH values of 7.0 and higher. Particularly in regions where irrigation water is high in carbonate content, these amendments allow production of a greater diversity of nursery stock and at higher growth rates than in the bark/peat mixes by themselves. The latter media must be amended with expensive trace element formulations to provide such effects.

Impact of Compost Quality on Value-Added Marketing
Properties of composts that must be controlled for value-added utilization in almost any market area include the raw materials used as feedstock for composting, the sizing of materials during and after composting, the composting process itself, the degree to which the compost has been stabilized during the process and curing (maturity/stability), and the biological, chemical and physical properties of the end product.

The decomposition level of the organic fraction in composts (maturity/stability) critically affects many functions. For example, most fresh wood residues release glucose and other sugars during the decomposition process. These soluble nutrients directly stimulate plant pathogen growth, which can kill seedlings and even mature trees. Biocontrol agents of these pathogens reach high populations in fresh woody materials as they begin to decompose. Unfortunately, they do not provide disease control. Hence, mulches prepared from high-in-wood-content products do not suppress pathogens that cause root rots.

Apart from stimulating diseases, fresh ground wood residues also cause nitrogen immobilization, even when incorporated with high nitrogen content swine manure. Other fresh products that are more biodegradable inhibit plant growth due to oxygen deficiency. Finally, immature composts may be phytotoxic due to the presence of volatile organic acids, alcohols (for high C/N composts) or the release of high concentrations of ammoniacal nitrogen compounds (for low C/N composts). Thus, adequate stability is critical to value-added marketing.

Excessively stabilized composts that have been charred or pyrolyzed during the process also do not support value-added marketing. Charred particles are produced when compost temperatures exceed 70 degrees Celsius for long periods of time. The concentration of microbial biomass in these products is too low for biological control, as the material does not support the activity of biocontrol agents. Finally, excessively humified organic matter, such as in highly decomposed Sphagnum peat, does not provide control. It is not good enough for a compost to be dark or black in color; substrate chemistry matters!

The longevity of the suppressive effect of properly produced and cured composts depends on many factors. Stabilized lignocellulosic materials in composts, the chemistry of which resembles particulate organic matter (POM) in soil, seem to form the basis for long-term control. Generally, compost-amended container media become conducive to root rot within 18 to 36 months after potting, but this varies with the materials used and the climate. The rate of hydrolysis of fluorescein diacetate, combined with the concentration of microbial biomass supported by the substrate, seem to best predict root-rot severity.

The concentration of nitrate versus ammoniacal nitrogen in composts plays a major role in value-added marketing. They type of bedding blended with a particular type of manure not only affects the composting process but also affects the length of composting time required to reach an appropriate degree of stability relative to nitrogen nutrition in plants produced in the compost. Manures must be composted and cured until toxic concentrations of ammonia have been converted to nitrate to avoid crop failure immediately after planted due to ammonia toxicity, particularly if it is utilized on low-CEC soils or in container media.

Another issue is nitrate versus ammonium-based nutrition impacts on the incidence and severity of Fusarium wilt diseases, which affect many crops (basil, celery, cyclamen, tomato, etc.). High nitrate nitrogen and low ammonium nitrogen suppress these diseases. In contrast, high ammonium with low nitrate nitrogen nutrition severely aggravates these problems.

Low-C/N composts mimic these effects, particularly if high-in-ammonium-content composts are used or if potting mixes are amended with excessive volumetric rations of these materials. Therefore, this variable must be considered in the compost quality/maturity decision process because Fusarium wilt resistance is not available in several plant species. Thus, industry always is on the look out for composts that suppress rather than aggravate these diseases. The U.S. Composting Council (Holbrook, New York) will need to expand compost utilization guidelines to fully meet these industry needs.

Salinity is another property of composts that must be considered in compost quality control. For example, a high-in-salt-content source of compost was tested over four growing seasons. The compost severely increased root rot of soybeans and decreased yields by 30 percent or more when applied directly before planting. If the compost was applied to soybeans in the fall so that salts leached out, a yield increase was obtained after application of the compost that could be linked directly to root-rot control.

Several types of additional treatments in these experiments clearly established that sodium chloride was the culprit. Since numerous crops are susceptible to root rots, salt must be avoided, if possible, or a lower value product will be produced that will need to be marketed in the same low-value class as composted high-salt-content manures.

Manures can vary considerably in salinity, however. Farms in dry regions typically produce high-in-salinity composted manures. In contrast, composts produced on farms that dispose of liquid manures separately tend to be low in salinity. Thus, amendment rates for these manure products may range from five percent to 40 percent, depending upon salinity, physical properties of container media and crop needs.

Value-Added Properties for Container Media
The quality of composts produced from a particular material varies widely and depends often on marketing opportunities. Several factors contribute to this effect.

Apart from location effects, the "peat substitute" aspect of composts used in potting mixes typically is the most valuable property. Pine bark, rice hulls, shredded coconut fiber (coir) and other materials that decompose slowly over time are the most sought after composted products for use in container media. They may be used up to 60 percent to 80 percent by volume in potting mixes and these materials can maintain desirable physical property of container media related to aeration and drainage over several years if the right ratio choices of ingredients are made.

Although a significant amount of information is available on the degree of stability required for a particular type of compost to avoid generating major utilization problems, quantitative guidelines for their utilization in soils, as mulches or as components in potting mixes are just now being characterized. Rational standards for utilization of composted manures are becoming available, but information still is lacking for many applications.

To avoid stability problems high in microbial biomass, composts, in general, must be stabilized to reach a stability level of at least 1.0 mg CO2-Cg-1 dw d-1. For utilization in container media, a higher stability level (0.5 mg CO2-Cg-1 dw d-1) may be required. Presently, composts typically are used based on past experiences with specific products from given suppliers as opposed to standard physical, chemical and/or biological properties of the products that define quality.

Cost savings associated with reduced pesticide use has become the second most value-added benefit of compost utilization for crops susceptible to root rots. Readily biodegradable materials, such as composted biosolids, food wastes and manures, are major sources of essential major and minor nutrients. Such composts serve as substitutes for slow-release fertilizers. This property of composts prepared from sewage sludge and manures is not supplied by recalcitrant materials, such as composted tree bark. It represents the third most valuable property of composts used in container media. In field soils and in the landscape, other factors-such as impacts on erosion, water use, soil compaction, winter and drought injury protection-must be considered.

Physical properties of composts play a major role in market penetration and profitability. For example, properties of container media relative to aeration and drainage must be ideal for plant growth and biological control of root rots. This requirement typically determines the volumetric ratio of major recalcitrant ingredients used in container media.

Yard debris can be processed into value-added products for use in such media at rates of 25 percent or higher. The air-filled pore space after saturation and drainage must exceed 25 percent in 20-centimeter tall container medium columns for most crops. When this value drops below 15 percent or 20 percent, growth of many species is impaired and root rot begins to develop. In taller containers, values can be different due to the taller water column. Root rot becomes a major issue even in compost-amended media when the airspace declines to below 15 percent in 20-centimeter tall pots.

Water must be added during formulation of compost-amended container media prepared with high temperature ingredients, which typically are dry to adjust the total moisture content of the mix within range of 50 percent to 55 percent on a weight basis. This allows colonization of the mix by beneficial microorganisms, which include fungi, bacteria and actinomycetes that induce microbiostasis within days after formulation. Without added water, dry potting mixes do not become colonized adequately and the high temperature product medium remains conducive to root rot.

When all these precautions are taken, microbiostasis is induced, but this natural process still does not result in consistent suppression of pathogens suppressed by parasitism. These natural media also do not consistently suppress foliar disease of plants.

Specific microbial inoculants have been developed for utilization in compost-amended media that induce suppression of Rhizoctonia and other root plant pathogens, as well as to some foliar diseases and stress diseases of plants. The mechanisms by which these biocontrol agents induce growth and defense-related activities involves plant hormone metabolism. Clearly, healthy soils offer the potential to maintain healthy plants.

Future Outlook
Several new technologies promise to significantly increase utilization of disease suppressive composts in the U.S. A novel method for production of plants known as the "pot-in-pot system" allows trees to be produced in containers buried in soil. Large trees can now be produced as effectively in these systems as in field soil.

Because the pot-in-pot system is being adopted rapidly across the U.S., the quantity of organic matter required for such systems is exceeding the supply of bark and rice hulls. Thus, the nursery industry increasingly is testing alternatives for these basic ingredients of potting mixes. Composted yard debris and other types of compost high in recalcitrant materials are beginning to fill this market.

Because pots used in these systems tend to be deep, water retention and aeration requirements are different as well. Larger quantities of composts that contain predominantly small particles can be utilized successfully in these media, which means composted yard debris suit this high-value market in container media.

It is too early to predict the role that microbial inoculants will play in disease control; however, based on impacts of recent epidemics caused by Phytophthora ramorum on nursery stock in the U.S., coupled with the desire of the industry to decreased pesticide use to increased costs, re-entry regulations and environmental issues for some pesticides, it would seem that ISR-active inoculants will increasingly be used by growers in the future.

The future for utilization of composts as substitutes for peat and pesticides seems bright. However, the quality of composts marketed as high-value products must be consistently high. Specific microbial inoculants that add to the spectrum of plant diseases controlled undoubtedly will play a greater role in quality control in the future.

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