Agriculture Reference
In-Depth Information
5.3
Production of composts and compost extracts/teas
5.3.1
Composts
Composting systems differ greatly in terms of their sophistication and therefore cost,
depending on the nature of the material being composted, and on the end use of the
compost being prepared. Relatively low-cost, open turned windrow systems are generally
used to compost low-hazard materials such as green or garden waste (termed yard waste
in the United States), whereas enclosed, or in-vessel systems are more suitable for food
wastes and wastes containing animal by-products. Several countries (including those in
the European Union) have legislation that ensures that the design and operation of the
composting process will guarantee safe, effective composting of all animal by-products
treated in this way.
In order for the composting process to proceed optimally, the input (or feedstock)
materials must be suffi ciently moist (40-60% moisture content), have a suffi ciently
open structure to allow air to penetrate the mass, and must have a suitable carbon to
nitrogen (C:N) ratio (around 25:1 to 40:1). Some materials, such as green waste or garden
waste have a suitable C:N ratio, whereas others such as straw (C:N ratio around 80:1) or
vegetable wastes (C:N around 12:1) must be mixed with other materials in order to obtain
a feedstock with a suitable C:N ratio. For example, the composted bark products that have
been used in so many documented studies generally have mineral nitrogen added prior to
composting in order to allow the process to proceed optimally.
Most of the composts used in studies reported here were made using some form of
simple, outdoor, turned windrow process. Feedstocks used vary widely, and for this
reason, are always defi ned where the effects of composts are discussed.
Vermicomposts are the digestion products of worms. There is little or no self-generated
heating during their production. Due to the differences in their manufacture and proper-
ties and the fact that few studies have examined their effect on plant disease, they are not
covered in this chapter.
5.3.2
Compost extracts/teas
The production of aerated and non-aerated compost teas involves compost being fermented
in water for a specifi c time period. Both methods require a fermentation vessel, compost,
water, incubation and fi ltration prior to application. Nutrients may be added before or
after fermentation and additives or adjuvants may be added prior to application.
There is continuing debate regarding the benefi ts of aeration during compost tea pro-
duction (Brinton
et al
., 1996; Ingham, 1999; Ingham & Alms, 1999). Aerated production
requires less time. Non-aerated methods are associated with low cost, low-energy input
and numerous reports of successful plant disease control (Weltzein, 1991). Several con-
sultants and scientists have suggested that NCTs can cause phytotoxicity and that the
production of NCTs provides an ideal environment for the growth and reproduction of
human pathogens, but there is little evidence to substantiate these claims at present.
The design of machinery which facilitates preparation of aerated compost tea varies.
Designs include:
•
open tanks (Riggle, 1996)
showers of recirculated water which fi lter through bags of compost suspended over
