Agriculture Reference
In-Depth Information
microbial biomass. In the same study, ACT produced from compost windrows older
than 42 days contained nitrate in agreement with the findings of other studies where
maximum nitrification occurred in the secondary mesophilic stage of composting
(Bishop and Godfrey
1983
; Diaz et al.
1993
). Again, the maturity of compost used
to prepare tea may be a factor in its effectiveness, which may or may not be related
to the form and availability of substrates in compost tea for pathogen metabolism.
9.6
Food Safety Issues
Some jurisdictions set standards that require commercially prepared composts to
achieve conditions equivalent to pasteurisation for elimination of vegetative hu-
man and plant pathogens. In Australia, for example, application of the Australian
Standard for production of compost, soil conditioners and mulches (Anonymous
2012
) is required by law. Despite these standards, retailers of fresh food can be
concerned about the risk of human pathogens in poor quality compost applied to
horticultural crops. An additional concern is re-establishment and growth of human
pathogenic bacteria in compost tea, especially when nutrients are added during ex-
traction (Duffy et al.
2004
).
There are few peer-reviewed reports on human pathogen presence and growth in
compost tea prepared from mature, commercially available composts. Results from
in vitro assays involving inoculation of ACT with
Escherichia coli
and/or
Salmo-
nella enterica
revealed no significant increase in populations of these human patho-
gens (Duffy et al.
2004
; Kannangara et al.
2006
). However, such studies revealed a
strong positive correlation between the concentration of molasses and kelp in ACT
amended with these nutrients and
E. coli
numbers. Moreover,
E. coli
and
S. enterica
populations can proliferate in water-based solutions of fish hydrolysate, kelp, sea-
weed and/or humic acids, in the absence of ACT (Ingram and Millner
2007
). To
ascertain whether or not results from in vitro studies are relevant to commercial pro-
duction conditions, Ingram and Millner (
2007
) inoculated commercially-available
compost prepared and sold to produce ACT, or ACT amended with nutrients, with
human pathogens. There was a significant increase in the numbers of
E. coli
,
S. en-
terica
and total fecal coliforms in ACT treatments supplemented with either a com-
mercially available compost tea 'supplement' consisting of a mixture of molasses,
bat guano, sea bird guano, powdered soluble kelp, citric acid, Epsom salts, “ancient
seabed minerals” and calcium carbonate or any one of the following: powdered
soluble kelp, liquid humic acids, and rock dust. These results suggest that practitio-
ners adopting ACT need to be especially cautious when adding nutrients to compost
extracts to prevent contamination of ACT with human pathogens during production,
particularly if conditions during or after production enable pathogens to proliferate.
In general, practitioners have been using ACT on the assumption that human
pathogenic bacteria, if present, do not proliferate in the presence of abundant micro-
flora that outcompete human pathogenic bacteria for nutrients. Palmer et al. (
2010a
)
prepared ACT using compost sampled from commercial open windrows during the
