before choosing a compost method)
of nitrogen and other nutrients
can provide essential nutrients to soil. Of the major nutrients-nitrogen,
phosphorus, and potash- the nitrogen conservation is the most important
in most areas of the world because so often the shortage of nitrogen
limits the amount of food that is produced. Nitrogen is also more
difficult to conserve than phosphorus and potash, as are the
micronutrients, which, owing to the chemical condition in which
they are present,
lost only to leaching. Leaching may lose nitrogen, but the major
loss comes from escape of ammonia or other volatile nitrogenous
compost material to the atmosphere.
There has been much research and writing on conserving nitrogen
and other nutrients, particularly with respect to microbiology of
the soil. Results of investigations and studies on nitrogen utilization
in the basic biological processes provide fundamental information
on the control of nitrogen loss in composting.
Nitrogen loss as ammonia in aerobic composting is affected by the
C:N ratio, pH, moisture content, aeration, temperature, and the form
of nitrogen compounds at the start of the composting materials.
Since organisms use about 30 parts of carbon for each part of nitrogen,
a C:N ratio in the raw compostable material of around 30:1 is best
for good composting is satisfactory for tying up or binding nitrogen
in biological cell material, preventing its escape.
avoid nitrogen loss, optimum
ratios of C:N range from 26 to as high as 38 depending
on conditions. A ratio of available carbon to available nitrogen
of about 30 or more permits minimum loss of nitrogen, but the ratio
of carbon to nitrogen measured chemically is often not the ratio
of available carbon to available nitrogen. Since many feedstocks
contain considerable amounts of cellulose and lignins resistant to
biological decomposition, and since most of the nitrogen is usually
in a readily available form, an actual C:N ratio of 30 may be necessary
to provide maximum conservation of nitrogen. Also, studies indicated
that nitrogen conservation decreased rapidly as the C:N ratio increased
from 40 to 50. This rapid decrease is not entirely consistent with
the fundamental aspects of bacterial decomposition. Above a C:N ratio
of 50, nitrogen conservation remained uniform at about 70% of the
optimum. Basically there should be little drop in nitrogen conservation
below the maximum when the initial C:N ratio is above the ratio utilized
by the organisms. When carbon is higher than the ideal C:N ratio,
organisms will require all the nitrogen for decomposition of the
carbonaceous materials. University of California studies found Nitrogen
losses of around 50% when the C:N ratio was in the range 20 to 25.
From about 30 upward, nitrogen losses were very small.
NITROGEN CONSERVATION IN RELATION TO C:N RATIO
Initial C:N ratio
Final percentage of nitrogen
Nitrogen conservation %
table shows a few examples of nitrogen conservation for different
C:N ratios. In manure composts, nitrogen was conserved only when
the C:N ratio was adequate and when immediate decomposition set
in. This resulted in transformation of soluble forms of nitrogen
into insoluble forms. Whenever decomposition was delayed, owing
to too low or too high a temperature, losses of volatile forms
of nitrogen occurred. From 85% to 90%, and possibly 95%, of the
nitrogen in the raw materials can be conserved if the C:N ratio
is high and other avenues for nitrogen loss are controlled.
There are three phases in the relation of nitrogen supply and conservation
to available carbon in biological decomposition:
When more nitrogen is available than necessary for organisms
to use carbon, large quantities of ammonia and volatile forms
nitrogen are given off and lost;
When the requisite amount of nitrogen to carbon for bacterial
utilization is present, decomposition proceeds without appreciable
loss of nitrogen;
- When nitrogen is low in relation to carbon, some of the organisms
will die and their nitrogen will be recycled. Small additional
amounts of nitrogen may be picked up by nitrogen fixation when
In all three phases there is a tendency to reach the same final
amount of nitrogen, that which the bacteria can hold when the compost
is in a stabilized condition. In the first phase nitrogen is lost;
in the second, it is stabilized and conserved; and in the third,
it is recycled, conserved, and sometimes accumulated. This illustrates
that composting operations can be operated to conserve most nitrogen.
Ammonia escapes as ammonia hydroxide as the pH rises above 7.0.
In the later stages of composting the pH may rise to between 8.0
and 9.0. At this time there should not be an excessive amount of
nitrogen present as ammonia. Materials that contain large amounts
of ash will have a high pH and may be expected to lose more nitrogen.
Some compost operators add lime to improve composting. This should
be done only under rare circumstances, such as when raw material
has a high acidity due to acid wastes or contains materials that
give rise to highly acid conditions during composting. When the pH
remains above 4.0 to 4.5, lime should not be added. The pH will be
increased by biological action and nitrogen conserved.
The moisture content of compost affects nitrogen conservation less
than the C:N ratio and the pH. Ammonia escape is greater when the
moisture content is low. The water serves as a solvent and diluent
for the ammonia, thereby reducing vapor pressure and volatilization.
A moisture content range of 40% to 60%, satisfactory for other aspects
of composting, will assist in conserving nitrogen.
Aeration and turning adversely affect nitrogen conservation. If
ammonia is present, it will escape more easily when material is disturbed
and exposed to the atmosphere. However, if the initial C:N ratio
is high enough, nitrogen losses during turning will be small. Since
some ammonia may be present during the dynamic transitional phases
of active decomposition, turn only as often as necessary to maintain
aerobic conditions and control flies.
temperatures increase volatilization and escape of ammonia. Since
high temperatures are fundamental in aerobic composting and
destruction of pathogen, not much can be done about controlling temperatures
other than to avoid temperatures above 160° Fahrenheit, which retard
bacterial activity and permit ammonia accumulation. Since the greatest
ammonia loss occurs during early stages of active decomposition,
only little conservation of nitrogen will be gained by reducing temperatures
after the two turns or after the first 6 to 8 days of active decomposition.
The nitrogen initially present in the material may affect nitrogen
conservation. If large amounts of ammonia are present in raw materials,
some of this ammonia may be volatilized and lost before the organisms
have had sufficient time to utilize and stabilize it, even though
the C:N ratio is satisfactory for nitrogen conservation. This can
be an important factor since much of the nitrogen loss occurs during
the first few days of composting.
Some materials, such as cellulose and porous fibrous matter, have
the capacity to absorb or hold moisture and volatile substances,
thereby reducing the tendency to escape. Materials of this type play
a part in reducing nitrogen loss from compost, which contain accumulated
ammonia. Materials containing considerable quantities of horse or
cow manure exhibit less nitrogen loss at low C:N ratio than other
materials, and can be considered nitrogen carriers. This could have
been due to the form of nitrogen, to the absorptive of nitrogen holding
capacity, or to some other characteristic of the manures.
Also, addition of soil to compost with high ammonia content absorbed
some of the nitrogen.
Loss of nitrogen
by leaching may occur in rainy weather or if the composting material
has too high initial moisture content and excess
liquid drains away. Loss by leaching depends on the amount of soluble
nitrogen in the compost and on the amount of rainfall. Arranging
compost piles so that water can’t enter may minimize leaching.
The greatest nitrogen conservation may be accomplished by anaerobic
digestion in water when liquids as well as the solids are conserved.
In such cases, while nitrogen fixation would not be expected, there
should be almost no nitrogen loss, since ammonia in low concentration
in the liquid would not escape.
Conservation of phosphorus and potash in composting is not difficult
since about the only loss occurs through leaching during rainy weather.