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Organic
Composting for Horticultural Use
Prepared by:
James C. Barker, Professor and Extension Specialist
Biological and Agricultural Engineering
North Carolina State University, Raleigh, NC
Composting to biologically
stabilize organics into a humus material similar to soil organic matter is not a
new process. Renewed emphasis on using this process is being fueled by
increasing amounts of agricultural by-products resulting from the production and
processing of livestock, poultry, cotton, peanuts and forestry as well as from
municipal and urban activities resulting in sludge, yard clippings and
biodegradable solid wastes. In some instances, composting may be a less
expensive waste reduction process than alternative treatment methods. In every
case, a well-composted by-product converts a potential liability into a usable
resource such as organic fertilizers, soil amendments or potting media.
Process
The compost process is a
partial breakdown of organics by microorganisms such as bacteria and fungi.
When the process is optimized and goes to completion, the end-product is
biologically stable, has an earthy odor, breeds few flies, has less volume and
weight, and costs less to haul and spread than the original by-product. Heat
created by composting destroys pathogenic organisms and weed seeds. If not
protected, the end-product might reacquire viable weed seeds between the
compost process and point of use.
Before organic by-product
conversion into compost for horticultural or agricultural production, the
non-biodegradable materials must first be removed. Objects such as plastic,
glass, metal or stone must be physically separated. Most agricultural
by-products contain little, if any, foreign objects while municipal solid
wastes could contain substantial amounts. Some municipal wastes might contain
metals such as cadmium, chromium, lead, or mercury restricting their use. Most
agricultural by-products contain very little metals with the possible
exception of copper. Raw materials suspected of containing significant
pesticide residuals should be withheld from compost for horticultural uses.
Factors influencing the
rate at which composting proceeds include raw material particle size,
aeration, moisture content, and the carbon-to- nitrogen (C/N) ratio. The
predominant bacteria in composting are aerobic (needing oxygen). The particle
size of raw materials determines the porosity of the pile which in turn
affects aeration. Material which is too finely ground compacts so densely that
air cannot penetrate the pile resulting in anaerobic (without oxygen) or
undesirable conditions. Materials such as wood shavings-based poultry litter,
leaves, shredded bark, or peanut hulls provide a more porous substrate for
oxygen penetration. Wood chips, twigs and tree branches are dense, do not
readily decompose and should be avoided.
Moisture
Optimum moisture contents
for composting range from 50-60 percent. Above 60% the pile approaches
saturation limiting oxygen penetration and becomes anaerobic. Below 50%
moisture, microbial activity and rate of composting slows down. By-products
and raw materials such as wet leaves, vegetable or fruit refuse, animal
manures, sewage sludge, or food scraps would probably need little, if any,
additional water to optimally compost. Materials such as wood shavings,
sawdust, poultry litter, grass clippings, hay and straw, peanut hulls, chopped
corncobs and corn stover, cotton gin trash, or shredded newsprint would
require additional water.
Carbon/Nitrogen
Ratio
The C/N ratio significantly
influences the rate and degree to which a mixture composts. Microbes use
carbon as energy and nitrogen as a food source to produce proteins. Optimum
C/N ratios range from 20-30 parts carbon to 1 part nitrogen. C/N ratios under
20:1 result in incomplete nitrogen use allowing ammonia to form in the pile
and be released during turning or aeration. Animal manures, poultry litter,
urea, grass clippings, legume residues and some sewage sludges are good
sources of nitrogen. Wood shavings, straw, peanut hulls, and newsprint have
high carbon contents.
Temperature
Temperature is a good
indicator of how the composting process is progressing. Within 2-3 days of
correctly forming the compost pile, temperatures generated by microbial
activity should reach 140 - 160oF. Unless the pile is porous and well aerated,
microbial activity will begin slowing within a few days with a temperature
decrease until the pile is turned or reaerated. When this happens, the
temperature again will peak. The only monitoring tool necessary to determine
if the compost process is proceeding properly is a long-stemmed thermometer.
Methods
Three composting methods
are most commonly employed: windrows, aerated static pile, and bins or aerated
chambers. For high-volume composting of raw materials such as animal manure,
poultry litter, sewage sludge, or municipal solid waste, the windrow or
aerated static pile method is most often used. Windrows 3-5 feet tall and
10-15 feet wide at the base are formed by combining the proper recipe of raw
materials on open earth surfaces. Windrows are turned periodically, with
temperature determining how frequently, by manure collection and loading
equipment such as front-end, skid-steer, or wheel loaders or by special
purpose machinery. The turning machinery has a high initial cost. Minimum
composting time is 1 month followed by 1-2 months of curing before the
material is ready to be used or marketed.
Aerated static pile
composting occurs in a windrow formed over a perforated pipe through which air
is forced. Forced aeration eliminates the turning needs and costs. The
perforated pipe similar to 4" corrugated plastic field drain tubing is
connected to a squirrel-caged blower fan to distribute an updraft of air
underneath and through the compost pile. Aerated static piles may be formed on
earth surfaces or concrete floors and either outside or indoors where
environmental factors can be more closely controlled.
Low-volume or batch
composting can best be accomplished in smaller static or aerated bins where
environmental factors are optimized. Batch composting similar to the backyard
or home garden variety requires more hand labor and is less mechanized. Raw
materials are initially layered in the bin according to the proper recipe.
Material in the bin is either periodically turned by hand, or mechanically
aerated by a forced air blower.
SUMMARY
By-product composting
stabilizes organics, improves handling characteristics, and reduces odors and
pests. Compost enriches topsoil with organic matter and plant nutrients,
improves water infiltration, tilth and aeration of clay soils, and increases
water availability and nutrient retention in sandy soils. Wood ashes added to
compost mixtures sparingly provide some liming effect but in large quantities
promote ammonia loss. Care should be taken at composting sites to protect
ground water and to prevent surface water impairments. Before initiating a
composting operation, the supply of raw materials and demand for the finished
product must be reliably established.
North Carolina Cooperative
Extension Service
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Composting for Horticultural Use
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