University of Arkansas - AgriScience Project
Composting Resource Sheet
adapted from Composting in Schools
by Cornell University
Temperature
The temperature is affected by
- The size of the pile
- Its moisture content
- Aeration
- Carbon/nitrogen ratio
- Ambient temperature
Optimum temperature is below 70C because hotter temperatures will kill off beneficial microbes.
Turning will help to dissipate the heat. The thermophilic stage (40- 60C) lasts for several
weeks to several months, depending on the size of the system and the composition of the
ingredients. Maintaining the thermophilic stage is important in controlling thermosensitive
pathogens, fly larvae, and weed seeds, and is the period of rapid decomposition. As the compost
cools turning the pile will result in a new temperature peak due to the replenished oxygen supply
and the exposure of organic matter not yet decomposed. The curing of the compost takes place
when the temperature drops and the mesophilic microbes take over the decomposition. This long
process is needed to make the organic matter more stable and suitable for use with plants.
Particle Size
Microbial activity takes place on the surface of the organic particles. Decreasing particles
increases the surface area. If the particle size is too small and compact, air circulation is decreased
and the O2 available to microorganisms withing the pile is limited and will adversely effect the
microbial activity. Optimum size for particles is 1.3 to 7.6 cm ( about .5 to 2 inches).
Aeration
If O2 levels fall and CO2 concentration increases, odor problems may arise. Aeration can be
accomplished by inclusion of air pipes, forced air flow, or mechanical mixing or turning.
Moisture
The optimum moisture content for composting is 50-60%. Too little moisture (<30%) inhibits
bacterial activity; too much moisture (>65%) results in slow decomposition, odors and leaching.
The moisture contents of some common materials are shown below.
| Material |
Moistur |
Carbo |
| Lettuce |
87% |
45% |
| Peaches |
80% |
42% |
| Food scraps |
80% |
42% |
| Grass |
77% |
45% |
| Leaves |
35% |
50% |
| Dry dog food |
10% |
|
| Newspaper |
5% |
|
| Dry leaves |
3% |
50% |
To calculate the percent moisture of the materials you plan to compost:
- Weigh a small container
- Weigh 10 grams of the material and place in the container
- Dry the sample for 24 hours in a 105-110C oven
- Reweigh the sample, subtract the weight of the container, and determine the moisture
content using the following formula:
Moisture Content =((Weight of Sample - Dry Weight)/Weight of Sample) X 100
To calculate the quantities of various material to combine the reach your moisture goal of 50-60%
the general formula is:
Total Moisture Content=(quantity material1 X moisture content1)+(Q2 X M2)+(Q3 X M3)+ . . .
Q1 + Q2 + Q3+ . . .
Carbon/Nitrogen Ratios
Carbon is an energy source and the basic building block making up about 50% of the mass of
microbial cells. Nitrogen is a crucial component of proteins, and bacteria need plenty of nitrogen
for rapid growth. For most materials, a C/N ratio of about 30 to 1 (by weight) will keep these
elements in approximate balance.
If composting high carbon material such as leaves, additional nitrogen may be required. This can
be achieved by adding a small amount of nitrogen fertilizer. An ammonia odor will indicate that
too much nitrogen was added.
Monitoring the Composting Process
Moisture
If your compost starts to smell bad, chances are it's too wet. Adding material such as wood
chips, cardboard pieces, or newspaper strips is likely to alleviate the problem. If the temperature
drops sooner than expected and the compost looks dry, try mixing in some water.
Temperature
A well-constructed compost system will heat up to 40 or 50C withing two or three days. You
can check the temperature by using a probe that extends to the center of the compost pile. The
temperature will drop at a rate that varies by the type of material being composted, size of the
pile, moisture, etc.
Odors
A well-constructed system will not produce offensive odors, although it will not be odor-free.
Your nose is the only equipment you need to monitor odors. An ammonia odor will indicate that
your system is too rich in nitrogen, and you need to add a carbon source such as leaves or wood
shavings. A musty odor indicates the compost is too wet. Mix in a bulking agent. A foul
sulfurous odor shows that your mix has gone anaerobic: aerate and add an additional absorbent
material such as wood chips or sawdust.
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