Page 17 - ALG Issue 2 2025

COMPOSTING
Composting
Bulking agents
Compost bins and heaps that have turned
wet and smelly are often given as the
reason inexperienced home composters
stop. The problem is caused by an
incorrect carbon: nitrogen ratio with too
many greens being added to the mix. This
often occurs when a high proportion of
the waste consists of food waste or grass.
The problems can be easily rectified when
things go wrong and avoided altogether by
getting the Green: Brown ratio right.
The unpleasant smells are usually
generated by anaerobic bacteria and often
contains hydrogen sulphide gas which
smells of bad eggs. It is toxic in enclosed
spaces but merely unpleasant when
home composting outdoors. The main
environment impact is that the greenhouse
gas methane is also produced, and it is
possible that the anaerobic bacteria will,
during the initial stages of composting,
inhibit the formation of the desirable
aerobic microbes resulting in inferior
compost. A high moisture content will also
result in more nutrient leaching from the
compost and may limit the temperature
rise required in hot composting. It is,
therefore, important to maintain a good
supply of air to all parts of the composting
materials. Bulking agents play a significant
role in maintaining air distribution.
The normal advice given when we
start composting is that we should aim
for a carbon: nitrogen ratio of 1:30 or, for
us simpler souls, about equal volume of
Greens and Browns. Bulking agents are
Browns which are usually added when
composting materials high in nitrogen,
which tend to compost rapidly producing
significant amounts of water filling the air
spaces between the composting particles.
The bulking agent should create and
maintain air spaces in the materials while
reducing the moisture level by absorbing
water while helping to maintain bulk and an
aerobic environment. In addition, the use of
bulking agents will reduce the unpleasant
odours, reduce the emission of greenhouse
gases and, at a practical level, provide a
means of returning a wet, smelly fly infested
bin to a pleasant smelling aerobic condition.
Woodchip
I tend to use woodchip as my main bulking
agent as it is available free on the allotment.
It is slow to decompose, so it is advisable to
leave fresh woodchip for about four months
to start decomposition before being added
to the compost.
Sawdust and Wood shavings
Sawdust is often used by home composters,
although it may mat if not well distributed
but is very effective at absorbing water and
it helps absorb odours. Wood shavings have
larger particles and add bulk, creating air
spaces with less likelihood of mating.
Cardboard and paper
Cardboard and paper, shredded if possible,
or torn and crumbled, is good at absorbing
liquid but may become too wet and mat if
not distributed throughout the feedstock.
Straw
Often used as a normal brown, it may mat
unless shredded. Mixed straw and manure
from a stable, farmyard, or chicken bedding
is very effective and includes its own greens
in the form of the manure.
Volume of Bulking Agent
Suggested volumes range from 50% - 66
% bulking agent in addition to the normal
Browns depending on the moisture content.
When using a single bin such as a Hotbin or
Green Johanna, the feedstock Greens and
Browns and the bulking agent can be mixed
in the top of the bin using an aeration toll or
manure fork. Hotbin suggest using a caddy
of waste plus a half a caddy of shredded
paper plus a fifth of a caddy of woodchip.
In a larger New Zealand or pallet bin, the
Greens and Browns can be layered, in the
normal way, with additional layers of bulking
agent and then mixed when the contents
are turned as part of the aeration process.
Rod Weston, Master Composter
Allotment and Leisure Gardener | Issue 2 2025 | 17