Organic Composting for Horticultural Use
Prepared by:
James C. Barker, Professor and Extension Specialist
Biological and
Agricultural Engineering
North Carolina State University, Raleigh, NC
Published by: North Carolina Cooperative Extension
Service
Publication Number: EBAE 171-93
Last Electronic Revision: March 1996 (JWM)
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.
Distributed in furtherance of the Acts of Congress of May 8 and June 30,
1914. Employment and program opportunities are offered to all people
regardless of race, color, national origin, sex, age, or disability. North
Carolina State University, North Carolina A&T State University, U.S.
Department of Agriculture, and local governments cooperating.
EBAE 171-93
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