Integrated Environmental Biotechnology - Environmental Biotechnology: Theory and Application - page 249

for some time that fine filtration and sterilisation by heat treatment, significantly
reduce the extract's effectiveness (Trankner, 1992).
Compost teas are prepared for use by either aerated or fermented extraction
methods. So-called 'fermented' extraction was the original, first developed in
Germany and it is not, in fact, a fermentative process at all. Actually an infusion
method, this requires a suspension of compost in water to be made, in a ratio
typically around 1 : 6 by volume. The resultant mixture is allowed to stand for
a given period, usually between three and seven days, then coarsely filtered
prior to being used. The second method, which came out of research in Austria
and the US, is more active and, with a typical cycle period of around 10-12
hours, derives the product in a much shorter time. The acceleration is achieved
by increasing the oxygen transfer to the extract during formation, initially by
passing water through compost, collecting the resultant liquor and recirculating
it many times to concentrate and aerate. Once prepared by either method, the
finished product is used as a foliar drench, typically applied to commercial crops
at a rate of around 1000 l/ha (100 gal/acre).
The abilities of properly prepared biowaste composts themselves to suppress
and control soil-borne plant diseases, especially where mature compost is directly
mixed with the soil itself have been established (Serra-Wittling, Houot and
Alabouvette, 1996). The efficacy of the protection given by this kind of incor-
poration with soil known to be conducive to plant pathogens has also been
demonstrated. In particular, the deleterious effects of various species of Phythium,
Phytophthora and Fusarium as well as Rhizoctonia solani , which is a major
threat to many kinds of young plants, are largely suppressed or controlled. For
many years the horticultural industry had been aware, anecdotally and from expe-
rience, that composted tree bark suppressed root rots caused by Phytophthora spp.
Investigations of this revealed that plants which require the presence of vasic-
ular arbuscular mycorrhizae (VAM), small fungi around the roots, which are
intimately involved in nutrient and water uptake, grow better in blends of soil
and bark compost than in methyl bromide treated soil alone. Various bacteria,
fungi and yeast, largely indigenous to the original compost, have subsequently
been indicated as active agents in the overall effect. The biological nature of
the control has been established in laboratory trials involving heat treatment or
microwave exposure of the compost. This significantly altered the microbial bal-
ance, resulting in progressive inhibition of suppression and led to 100% plant
mortality in some instances. On this basis, the need to ensure that compost is not
subjected to extreme temperatures during storage or transport becomes a very
obvious problem to be avoided in any intended practical application.
The agricultural usage of biowaste-derived products has considerable poten-
tial, but public acceptability and quality assurance issues must be major concerns,
probably more here than in any other comparable sector. In the UK alone, farm-
ers have not been slow to learn the terrible consequences of consumer anxiety.
Bovine spongiform encephalopathy (BSE) and the passions raised by genetically
modified (GM) crops, or animals reared on them, entering the human food chain
has focused attention ever more clearly on supply chain issues. The ramifications,