3-EvaluatingMicrobio.. - US Composting Council

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Evaluating Microbiology
Of Compost
Vicki Bess
HE 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
T
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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
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84
COMPOST QUALITY
compost quality and to identify the minimum
functional groups such as the actinomycetes
amount of nitrogen substrate necessary to bring
which he associates with disease suppression, as
the compost up to the standards they required.
well as thatch reduction. Dinelli also uses the miDennis Kilmer of Desert Compost in Tucson,
crobiological analysis of compost to assist in deArizona, uses microbiological, maturity, and nutermining what substrates will enhance particular
trient testing for making compost decisions for
groups of organisms, as well as determining optiregular production cycles, as well as research for
mal timing to maintain microbial integrity in
the special needs of some of his clients. Kilmer
compost tea production.
chiefly uses the maturity index as a tool to know
Don Cranford of Cranford, Inc., a compost prowhen his product is ready to sell. Some of his
ducer in Spreckels, California, says, “There is no
clients, such as nurseries, are very sensitive to comsecret to making compost.” He uses the microbial
post quality and it is essential to be able to assure
analysis to quantify what he thinks he already has
them that his compost complies with their appliand to prove to growers that his compost is “good.”
cation needs.
Cranford also uses both traditional NPK, trace elThe pathogen inhibition assay has been useful in
ement and carbon to nitrogen ratio analysis along
screening different composts for the best potential
with microbial analysis and maturity index analyto suppress particular plant diseases. One grower
sis for a complete characterization of his compost.
He finds the maturity assay is specifically useful in
separating inferior comTable 1. Criteria for Evaluating Microorganism
post from quality comConcentrations in Finished Compost
post. According to him,
“it is a great tool to show
Functional Group Interpretation Of Compost Bioassay
the grower that we are inHeterotrophic
Finished compost should have 100 million to 10 billion (108 - 1010)
terested and intent on
Bacteria (Aerobic)
Colony Forming Units/gram dryweight (CFU/gdw).
making quality material.”
Compost with less than 100 million CFU/gdw will not perform as well as
Testing Different
Substrates
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
soil inoculants and may not be effective in suppressing plant diseases.
Anaerobic
Bacteria
Ratio of Aerobes to Anaerobes in the compost should be at least 10:1 or
greater.
An overgrowth of anaerobes indicates the compost was not turned with
sufficient frequency. It is important that anaerobic by products in the
compost be degraded prior to use with plants or germinating seeds.
Yeasts and
Molds (Fungi)
Finished compost should have between 1 and 10 thousand CFU/gdw
(1 x 103 - 1 x 104 CFU/gdw).
These organisms are important for breaking down organic compounds, soil
nutrient cycling, stabilizing soil aggregates, and controlling plant disease.
Actinomycetes
Finished compost should have at least 1 million to 100 million
CFU/gdw (106 - 108 CFU/gdw). Compost made with woody materials
may have more.
These organisms are important for many functions including the break
down and nutrient cycling of complex chemical substances such as chitin
and cellulose, improving soil crumb structure, and assisting in the reduction
of plant pathogen pressures. They are particularly efficient in alkaline soils.
Pseudomonads
Finished compost concentrations should be between1 thousand and
1 million CFU/gdw (103 - 106 CFU/gdw). Depending on starting materials,
this number could be lower, but is rarely higher.
Pseudomonads are important in nutrient cycling, assisting plants with
phosphorus availability, and some have been linked to the biological
control of plant pathogens.
Nitrogen-Fixing
Bacteria
The number of free living nitrogen-fixing bacteria in compost varies a lot
depending on the available nitrogen concentration but may be in the range
of 1thousand to 1 million CFU/gdw (103- 106 CFU/gdw).
Populations of these free living nitrogen-fixing bacteria will proliferate as
the available nitrogen in the compost decreases. As a consequence, there is
typically an inverse relationship between biologically available nitrogen in
the compost and the concentration of free living nitrogen fixing bacteria.
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EVALUATING MICROBIOLOGY OF COMPOST
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.
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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.
Vicki Bess is founder of BBC Laboratories based in
Tempe, Arizona.
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