Compost is a dark, soil-like material made of decomposed organic
matter. Many gardeners depend on composting as a means of using
garden refuse to maintain organic matter levels in their soils. On a
larger scale there is interest in the use of composted town refuse
horticultural purposes. Many councils are now collecting ‘green waste’
and supplying composting equipment to encourage householders to
recycle organic matter, as well as paper, glass and metals. Horticulturists
are increasingly concerned with the recycling of wastes and attention
is being given to modern composting methods. It is fundamental to
successful organic growing.
For successful composting, conditions must be favourable for the
decomposers. The material must be moist and well aerated throughout.
As the heap is built, separate layers of lime and nitrogen are added as
necessary to ensure the correct pH and C:N ratio. Organic waste brought
together in large enough quantities under ideal conditions and turned
regularly can be composted in two to three months. It is an exothermic
process (heat is given off in the reactions) so enough heat can be
generated to take the temperature to over 70°C within seven days, with
the advantage of killing harmful organisms and weed seeds. The high
temperatures can lead to a loss of ammonia (nitrogen).
|Table 18.1 Compost ingredients
In order to achieve a mix that allows adequate aeration, it is convenient
to distinguish between ‘green’ (leafy or ‘tender’) and ‘brown’ (fibrous
or ‘tough’) materials and combine in approximately equal measure (see
Table 18.1). Note that shredded cardboard and paper can be added as
‘brown’ which is useful for recycling waste, but inks should be kept to
a minimum. Decomposition is quicker if the ingredients are shredded to
increase availability to organisms.
When very large quantities are available the ingredients can
be heaped up on a concreted base. This makes it easy to use
power equipment to turn the ingredients to maintain good
aeration and to mix in the cooler outer layers to ensure all
parts heat up and decompose rapidly.
Most gardeners will not be able to obtain enough
components at any one time to create the ideal composting
process. An alternative approach is to build the heap over
time. This is normally done in a slatted bin with a front
that opens for access (see Figure 18.7). There should be
an open base over soil to allow organisms and air in. A
suitable cover is needed to keep some warmth in and rain
off once the process has started. This method can produce
good compost, but tends to take many months or even years
to complete. Because it does not heat up adequately, care
should be taken with regard to weeds, pests and diseases
which are not killed in the process.
|Figure 18.7 Compost bins. A typical set of bins large enough
composting, slatted to allow in air and to
allow easy access to add new material
and to turn the
As much material as possible should be
collected and prepared for composting. It
should be chopped or shredded, ‘green’ and
‘brown’ mixed and water added to the heap.
It is difficult to be successful with batches
of less than one cubic metre at a time (when
less than this the cooling at the surface
is greater than the heating at the centre
where decomposition is proceeding). It is
advantageous to have a second bin alongside
so the compost heap can be turned and
loosened more easily on a regular basis to
maintain good aeration.
These are containers that can be rotated on
an axis to provide an easy method of turning
small batches to create compost in a relatively
short time. Batches can heat up sufficiently to
kill off weeds and diseases and the enclosed container deters vermin. The
compost ingredients should be gathered together and the tumbler filled in
a short space of time. Nothing is added until the batch is completed.
Worm composting and wormeries
lends itself to handling small quantities which can
be added as they arise, such as kitchen waste, especially over the winter
period when there is little plant material to accumulate. Compost worms
), also known as brandling or tiger worms, feed on
organic matter. Whilst these can be purchased they are readily found in
rotting vegetation such as compost heaps.
The container can be a plastic dustbin, usually equipped with a tap to
drain off liquids which can be diluted and used as liquid feed for plants.
Smaller containers, wide rather than narrow, can be made out wood,
ideally with some insulation to maintain temperatures. Put in a 10 cm
layer of sand and cover with a polythene sheet. Bedding material, such
as well rotted compost or farmyard manure, may be needed for the
worms to live in until the system gets going. Spread chopped waste to
a depth of 5 cm. Add 100 or so worms and cover with wet newspaper to
keep out the light and maintain moisture levels. A lid is needed to keep
out the rain. Ideally, temperatures should be maintained between 20°C
and 25°C and the pH kept between 6 and 8; lime can be added if the
compost becomes too acid. The worms eat the vegetation as it starts to
rot which means that once in balance there is no smell.
The compost is removed when ready; the decomposing top layer is
separated off and used to start the next run. The compost is spread out to
dry in the sun and the worms are recovered by placing a wet newspaper
on the compost where they will congregate under it.
On a larger scale, wormeries are used to compost farmyard manure with
continuous systems available that separate the composted material from
the worms which can be recycled, with surpluses being available as