Evaluating Microbiology of Compost

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Courtesy of BioCycle Magazine

The fact that not all compost is created equal has both producers and growers looking for ways to successfully evaluate compost quality. Traditional compost analysis has focused on NPK and micronutrient concentrations in an effort to mirror fertilizer analysis. Compost, however, is much more complex than fertilizer and its most significant value to the grower may be far more than its mineral contribution to the soil. Compost has an important microbiological component that impacts how it will perform as a soil inoculant and plant disease suppressant. This same living portion of the compost can determine what kind of nutrient cycling disposition the compost will add to the soil.

Both growers and compost producers have become aware of the importance of this microbiological compost feature and are using it as one of the methods to determine compost quality. At BBC Laboratories, an environmental microbiology laboratory in Tempe, Arizona, the microbiology of compost is evaluated by methods similar for evaluating soil microbiology. A standard analysis for microbiological content in compost is determined by the concentration of six functional groups of microorganisms: aerobic bacteria, anaerobic bacteria, fungi, actinomycetes, pseudomonads and nitrogen-fixing bacteria. Now there are ways to evaluate the concentrations of these organisms in finished compost and these serve as an interpretation guide to determine the quality of the compost as an inoculant of soil microorganisms (see Table 1).

Another important indicator of compost quality includes compost maturity which is a term used to express the phytotoxic (plant-toxic) compounds that are frequently associated with unfinished or poor quality compost. Growers having negative experiences with compost are usually the victims of immature compost, which can inhibit seed germination and cause rapid nitrogen depletion, root tissue damage or even plant death. The maturity testing includes establishing a maturity index for the compost used at two application rates as well as pH and electrical conductivity analysis.

Compost, compost teas and other biological products can be tested for the ability to inhibit the growth of plant pathogens. Although this analysis is only a primary screening mechanism, there have been very good correlations between the lab results and field testing, especially if the compost or biological product tested is applied with an understanding of how the pathogen grows, how the pathogen is transmitted, and when the plant is vulnerable. If the screening proves successful, work can be continued on the isolation of specific inhibitory organisms for purposes of enhancing the compost or biological product.

How Producers And Growers Improve Their Compost

Dan Dinelli of North Shore Country Club in Glenview, Illinois, is concerned about overall turf health and uses compost and compost tea as a part of his turf management program. He naturally is interested in the disease suppressive qualities of compost as he looks for sustainable approaches to turf management. Dinelli has both traditional nutrient analysis as well as microbiological analysis performed on the different composts he uses on the golf course. By comparing these analyses with his field evaluations, he is able to draw some conclusions about the biological vs. nutrient impact of the compost. This is a tool he uses in choosing the appropriate compost for his turf needs. He looks at compost for general richness of microorganisms along with specific richness in particular functional groups such as the actinomycetes which he associates with disease suppression, as well as thatch reduction. Dinelli also uses the microbiological analysis of compost to assist in determining what substrates will enhance particular groups of organisms, as well as determining optimal timing to maintain microbial integrity in compost tea production.
 
 Don Cranford of Cranford, Inc., a compost producer in Spreckels, California, says, “There is no secret to making compost.” He uses the microbial analysis to quantify what he thinks he already has and to prove to growers that his compost is “good.” Cranford also uses both traditional NPK, trace element and carbon to nitrogen ratio analysis along with microbial analysis and maturity index analysis for a complete characterization of his compost. He finds the maturity assay is specifically useful in separating inferior compost from quality compost. According to him, “it is a great tool to show the grower that we are interested and intent on making quality material.”

Other composting facilities, with the production of quality compost as their objective, use the microbiological characteristics of their compost to test the impact of different substrates.


As an example, one compost producer/grower had ready access to a substantial amount of carbon material, but the nitrogen substrate needed to balance the inputs was expensive to both acquire and transport to the composting site. Knowing the budget was limited, but having experience that indicated a quality compost was needed for performance, they used microbiological testing to determine how lowered nitrogen inputs impacted compost quality and to identify the minimum amount of nitrogen substrate necessary to bring the compost up to the standards they required.

Dennis Kilmer of Desert Compost in Tucson, Arizona, uses microbiological, maturity, and nutrient testing for making compost decisions for regular production cycles, as well as research for the special needs of some of his clients. Kilmer chiefly uses the maturity index as a tool to know when his product is ready to sell. Some of his clients, such as nurseries, are very sensitive to compost quality and it is essential to be able to assure them that his compost complies with their application needs.

The pathogen inhibition assay has been useful in screening different composts for the best potential to suppress particular plant diseases. One grower with a serious Phytophthora problem screened different composts for the potential to inhibit the specific organism isolated from his field.


None of the composts tested were inhibitory to this particular Phytophthora and the grower continued the pathogen inhibition assays with commercially available biological products. A product successful in the laboratory testing was found and applied to the field with subsequent testing indicating no detectable Phytophthora in the soil or plant roots.

Another grower having problems with Sclerotinia rot in lettuce (Sclerotinia minor) noticed the problem was greatly diminished when a specific compost was used. After testing the compost against Sclerotinia in a pathogen inhibition assay, it was determined there were specific organisms in the compost that caused inhibition. As a result, the grower was able to backtrack and investigate what was different in the compost production that caused the enhancement of the disease suppression.

Both compost producers and growers are becoming more aware of the need to test compost for human pathogens, such as E. coli and Salmonella. Properly made compost should not contain these pathogens and these tests are frequently a part of a compost quality testing program structured to give growers and consumers confidence. Compost producers are finding that growers of fruit crops such as strawberries are especially sensitive to the pathogen issue and are only using composts that test pathogen free.

The testing parameters for compost quality will continue to develop as more is learned about the various roles of the compost microorganisms. It is an exciting new frontier with tremendous potential for controlling and modifying the composting process to better serve the growers needs.

By Vicki Bess.

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