The use of composts and compost extracts in plant disease control - Disease Control in Crops - page 94

principles of composting and the factors affecting the composting process are widely
described in the literature (e.g. Epstein, 1997; Zibilske, 1998).

5.2.2 Manures

Manures can be defi ned as animal excrement, which may contain large amounts of
bedding. It is important to distinguish between true composts (which contain material
which has been composted) and fresh, stacked or stored manures which have not under-
gone aerobic composting. Limited research has shown variable effects of fresh or stacked
manures on the incidence and severity of plant disease. However, such studies are outside
the scope of this chapter.

5.2.3 Compost extracts and teas

The terms 'compost extract' and 'compost tea' have in the past been used interchangeably.
However, for the purposes of this review, they are treated as different products.
The term 'compost extract' has been used in the literature to defi ne water extracts
prepared using a wide range of different methods (Scheuerell & Mahaffee, 2002). The
terms 'compost extract', 'watery fermented compost extract', 'compost steepage',
'amended extract' and 'compost slurry' have all been used to refer to non-aerated
fermentations. The terms 'compost extract' (Weltzein, 1989), 'watery fermented
compost extract' (Weltzein, 1991) and 'steepages' (Hoitink et al ., 1997) are approxi-
mate synonyms defi ned as a 1:5 to 1:10 (v:v) ratio of compost to water that is fer-
mented without stirring at room temperature for a defi ned length of time. 'Amended
extracts' can be defi ned as compost extracts that have been fermented with the addition
of nutrients or microorganisms prior to application (Weltzein, 1991). The term 'com-
post slurry' has been used to describe non-aerated compost teas before the fi ltration
process (Cronin et al ., 1996). Nowadays, it is common practice to refer to compost
extracts as the 'fi ltered products of compost mixed with any solvent (usually water),
but not fermented' (Scheuerell & Mahaffee, 2002). This convention will be adhered to
in this chapter.
Compost tea is the term given by an increasing number of commercial growers (in
the USA and Europe) to the fi ltered product of compost fermented in water (Brinton
et al., 1996; Scheuerell & Mahaffee, 2002; Litterick et al ., 2004). Compost teas are pro-
duced by re-circulating water through loose compost or a porous bag or box of compost
suspended over or within a tank with the intention of maintaining aerobic conditions
(Litterick et al. , 2004). The product of this method has also been termed 'aerated compost
tea' and 'organic tea' (Riggle, 1996). Several companies have developed machinery for
the preparation of compost teas in this way under highly aerated conditions (www.attra.
ncat.org; www.soilfoodweb.com).
The term 'compost tea' has not however, always been associated with an aerated
fermentation process (Brinton et al. , 1996). It is important to distinguish between com-
post teas prepared using aerated and non-aerated processes, therefore, the terms 'aerated
compost tea' (ACT) and 'non-aerated compost tea' (NCT) are used in this chapter to
refer to the two dominant compost fermentation methods. ACT will refer to any method
in which the water extract is actively aerated during the fermentation process. NCT will
refer to methods where the water extract is not aerated or receives minimal aeration
during fermentation apart from during the initial mixing.