PLSOIL 120
ORGANIC FARMING AND GARDENING

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CHAPTER 4

COMPOSTING

Purpose

The purpose of composting is to convert organic material that is unsuitable for incorporating into the soil into a material that is suitable for mixing with the soil. The main deficiency of uncomposted organic matter is its wide C:N ratio. Incorporation of organic matter with a wide C:N ratio is likely to stimulate immobilization of available nitrogen in the soil. Materials that have wide C:N ratios and that should be composted are sawdust, woodchips, bark, straw, paper, dead leaves and needles of trees, garden residues, and manures with a lot of bedding, .

Another function of composting is to destroy weed seeds and plant diseases. Weed seeds may rot during composting. The high temperatures (130 oF or higher) in the internal region of the pile may kill plant diseases. Destruction of weed seeds and diseases during composting cannot be assumed. Weed seeds on the outside of the pile will not be rotted. Weed seeds may be carried in by wind or birds or by adding of mature plants to the pile after it is almost finished composting. Weeds at the perimeter of the pile may produce seeds that infest the pile. Diseases that are not exposed to the inside heat of the pile will not be affected. The pile may not heat up enough after the first turning to kill diseases of plants that were originally on the outside of the pile. Temperatures of compost piles are insufficiently high to kill viruses. Weeds may be controlled by covering the pile with plastic to prevent the growth and maturation of any plants that start in the pile from the germination of weed seeds. As a safety precaution, diseased material should not be added to the pile.

During composting half or more of the bulk of the pile is lost by decomposition, waste, or unavoidable losses. At least half of the carbon is lost. Microorganisms use the organic material as a source of energy. Respiratory loss of carbon dioxide represents most of the loss of carbon. Nitrogen is lost by volatilization of ammonia and leaching and denitrification of nitrate. Carbon, however, is lost faster than nitrogen, and as a result the C:N ratio is narrowed. Potassium is lost by leaching. Phosphorus is conserved, for it is mostly in insoluble compounds that are neither leached nor lost as gaseous substances. Finished compost is a dilute fertilizer having a composition of about 1-1-1 (%N-%P205-%K2O). The C:N ratio of the finished compost will be about 15:1 to 50:1, varying with the materials entering the compost and with maturity of the composts.

If the bulk of the uncomposted organic materials has a C:N of about 50:1, it need not be composted and can be added directly to the soil. The C:N ratio will soon narrow so that nitrogen will be released. Nitrogen fertilizers can be added with the organic matter to narrow the C:N ratio. If the amounts of materials with narrow or moderately narrow C:N ratios are small, these materials can be used in composting to accelerate the rate of composting of materials with wide C:N ratios.

Procedures for composting

Procedures for composting involve collecting the materials, selecting the site, and selecting the process for composting.

Materials

A substantial amount of material must be collected to build a compost pile. The ideal size of a pile is 5 ft x 5 ft x 5 ft. If enough material cannot be collected to build a pile of these dimensions, efforts should be made to make the pile as high as possible. Flat, pancake-like piles will not compost well and will lose more nutrients that deeper piles. The length and width of the pile can greatly exceed these dimensions, but the height of the pile should not exceed 8 to 10 ft. Piles above this height may become too hot in the interior and kill the composting organisms. Piles that are somewhat deeper than 5 ft will be more active in cold weather than shallower piles due to the insulating effect of the deeper piles.

Materials for composting are divided into two classes, carbonaceous materials and nitrogenous materials (Table 14). Carbonaceous materials have wide C:N ratios, and nitrogenous materials have narrow C:N ratios. A mixture of carbonaceous and nitrogenous materials is used frequently in the construction of a compost pile. Woody and nonwoody materials may be used but should be kept separate if possible. Woody materials will take much longer to rot than the nonwoody materials. Paper can be used but should not exceed 10% of the volume of the pile. Waxed paper and colored paper should be avoided. Waxed paper will be resistant to decay, and colored paper may contain heavy metals.

 

Table 14. Examples of common carbonaceous and nitrogenous materials for compost piles
_______________________________________________________________________________________________

-------Carbonaceous materials---------

Nitrogenous materials

______________________________________________________________________________________________ 

Woody

Nonwoody

Green leaves

Sawdust

Garden residues

Manures

Wood chips

Tree leaves and needle

Sewage

Bark chips

Straw

Soil

Grass hay

Spoiled vegetables

Paper

Legume hay

The carbonaceous materials provide the bulk of the organic matter to be composted, and the nitrogenous materials accelerate the rate of composting. The narrower the ratio of parts of carbonaceous materials to nitrogenous materials, the faster the compost will be finished.

Nitrogen fertilizers may be used instead of or in addition to organic nitrogenous materials. Fertilizers are added usually by sprinkling them over a foot-deep layer of organic matter. Ammonium-based fertilizers are better than nitrate-based fertilizers. The microorganisms that are carrying out the composting prefer ammoniacal nitrogen over nitrate nitrogen. Suitable fertilizers would be urea, ammonium sulfate, ammonium nitrate, seed meals, dried blood, manures, and sewage from nonindustrial sources.

Soil is added often as a nitrogenous material. Soil also adds some weight to the pile and helps to hold it in place. Soil is a source of innoculum of microorganisms that perform the composting. The soil should be the richest available. Soil that is dug from underneath an old compost pile or from a barn or barnyard will be rich in nitrogen. The amount of soil added to the pile should not exceed one-half inch to one inch for every foot of compost in the pile. Too much soil will make the pile heavy and possibly anaerobic in wet weather. The finished product with an abundance of soil will have the characteristics of soil rather than of organic matter.

Phosphorus fertilizers can be added to the pile to fortify the compost. Rock phosphate, bone meal, and superphosphate are suitable. The acids formed during composting will aid in solubilization of rock phosphate, converting some of it to a material that is similar to superphosphate.

Many procedures for composting call for adding of lime or wood ashes to the pile. Adding of these materials should be avoided. The increase in alkalinity from their addition will accelerate the loss of ammonia by volatilization and will lower the value of the final compost. Compost piles undergo changes in pH as composting proceeds. The initial reaction of the pile will be acidic, but development of this acidity is part of a natural process. A particular group or microorganisms adapted to acidic conditions utilize the easily decomposable fraction (sugars, starches, organic acids, proteins) of the organic matter and generate acidity in the process. These organisms die out as the composition of the residues changes. Another group of microorganisms then dominates in the process of composting. These organisms use different substrates and have different effects on pH than the original group. Nothing needs to be done to alter this sequence of events. Finished compost is slightly alkaline, perhaps about pH 7.5. Lime or wood ashes should be added to the soil to which compost is applied, not to the compost directly.

Bones, meat, and grease should not be added to the pile. Bones will not compost unless they are ground. Meat will attract animals to the pile. Grease may be slow to compost and adds little besides carbon.

Site

The site should be handy to the land on which the compost is to be used. A nearby source of water is desirable. Rainfall may not provide adequate moisture to initiate and maintain composting. A partially shaded location helps to keep the pile from drying. Windbreaks will help to prevent drying. If the compost pile is in a residential neighborhood, some screening from view is desirable.

Pits are not necessary for composting unless the climate is very dry or cold. In these cases, composting in pits will help to prevent drying and loss of heat.

Processes

The selection of a process is dictated by the time and materials that are available.

14-day process. This procedure is used when time is short, such as in the spring, and when a source of finished compost is not available. This process is sometimes called the Berkeley process, after Berkeley, California, where it

was developed by engineers who needed a process for rapid composting of municipal organic solid wastes.

High quality organic materials are needed to ensure rapid composting. A mixture with a ratio by volume of 2 parts nonwoody carbonaceous materials to 1 part nitrogenous materials is ideal for rapid composting. A mixture of farm manure and bedding (hay, straw) gives about the proper ratio of materials for the 14-day process. For rapid composting, sawdust and wood chips in bedding should be avoided, for they will be too resistant to decay.

With most materials, the 14-day process is labor-intensive. The materials have to be shredded and moistened as the pile is constructed. Shredding of the materials increases the surface area and accelerates rotting. Shredding is difficult without power-driven equipment. It can be dangerous work with the power-driven equipment. Increasing the fraction of nitrogenous materials and using carbonaceous materials that are already finely divided or soft (needles, leaves, fine garden residues, trimmings or spoiled vegetables) may enable elimination of the step of shredding.

A pile constructed by 14-day method should heat up in about 2 days. If it does not, insufficient water or insufficient nitrogen may be available for rapid composting, and these materials will have to be added to accelerate the rate of composting. The pile must be turned every 3 or 4 days for about 10 days. Turning of the pile aerates it. No additional turnings are needed after the third one. Failure to have sufficient water or nitrogen or failure to turn the pile on time will delay composting. Small piles may not heat sufficiently to be finished in 14 days.

After about 14 days--sometimes longer depending on the success in constructing the pile, the climate, and frequency of turning--the organic material should be decomposed. At this time, some of the original material may be recognizable, but it should fall apart easily on handling. Slick and shiney materials have not composted sufficiently. If some of the compost pile appears to be finished and some not finished, the compost may be screened to separate the finshed product from the unfinished material. Chicken wire may be used to construct a screen. The material that passes through the screen may be ready to use. It should be examined to determine that it is indeed compost and not just finely divided material that still requires further rotting. Finished compost should be black and crumbly and with an earthy odor. The material that does not pass through the screen should be added to another compost pile.

90-day process. Lower quality materials can be used in this method than with the 14-day process. Shredding helps to accelerate composting but is not necessary. A volume ratio of 3 parts nonwoody carbonaceous materials to 1 part nitrogenous materials is a typical mixture of organic matter. Wider ratios may be used, but the time for composting will be lengthened. Times of composting exceeding 120 days are common, even though this process is called the 90-day procedure. Compost made by this process normally is not available for use in the same year in which the construction of the pile was initiated.

Construction of the pile may begin with a foundation of coarse stems or twigs. These materials should not exceed 1 inch in diameter. The organic materials, soil, and optional fertilizers are layered on this foundation. Layering of materials helps to measure material and to keep the proper proportions of carbonaceous materials, nitrogenous materials, and soil. The pile may be watered as it is constructed, but watering is not as essential for this process as for the 14-day process. Most workers rely on rainfall to provide water for the 90-day process.

In a week or so, the pile should heat up to 130 to 150 oF. After 5 or 6 weeks, the pile will cool to ambient temperature (Figure 10). At this time, the pile should be turned to aerate it. Sometimes the piles are not turned, but not turning the pile usually adds to the length of time required for composting. Holes may be poked in the pile with bars to give channels through which air may enter easily. Sometimes poles or boards are placed vertically in the pile, also providing channels permitting air to diffuse into the pile. In some large-scale operations, air is pumped or sucked into the piles through perforated pipes. This practice provides good aeration and generally shortens the amount of time required for composting relative to turning and saves labor.

Figure 10. Cycles of heating and cooling of composts during the 90-day process.

Piles that are not turned or provided with forced aeration likely will take more than one year for completion of composting. Labor savings are considerable if a pile is not turned. Some nitrogen is conserved in the pile if it is not turned. Each time that a pile is turned, some ammonia escapes into the air. Composts from piles that are not turned may have 10 to 20% more nitrogen than turned piles.

After about the third turning, the temperature of the pile may not rise above the ambient temperature. When this situation occurs, it may be a sign that the pile is done. The compost should be examined to ensure that it is finished. The original organic matter will not be recognizable in the finished product. The organic matter should be black and crumbly and have an earthy odor. If it does not fit these criteria, the compost may not be finished. Lack of water is a likely factor that causes the pile not to heat up when it is turned and still remains unfinished.

The finished compost can be incorporated directly into the soil. If the season is not proper, the compost should be stored by covering it with soil, hay, straw, or black plastic. These coverings will help protect the finished compost from leaching by rain and may help to keep weeds from growing in the pile. Small amounts of composts may be stored in closed bins. Large bins for storing compost or even for working of composts can be constructed from snow fencing, lumber, or concrete blocks.

Sheet composting. This process involves composting directly in the soil on the site on which crops are to be grown. Sheet composting often is used in large-scale operations involving large acreages or large amounts of materials. It is a valuable process when labor is short.

Organic materials are worked into the soil 6 or 8 weeks in advance of date of planting of the crop. This lead time is needed in order for the organic matter to decompose sufficiently so that immobilization of nitrogen is not as severe a problem as it would be if the crop were planted soon after the organic matter is incorporated. Nitrogen deficiency in the crop can be a severe problem resulting from sheet composting, and often additional nitrogen fertilizers are added to ensure that nitrogen deficiency does not occur.

Soils in which sheet composting has taken place may be dry. The coarse organic matter gives a lot of open channels through which water may escape from the soil into the atmosphere. A poor seedbed may result from sheet composting if the organic matter has not decomposed sufficienty before planting of crops. The longer the interval between application of organic matter for sheet composting and the planting of a crop, the better the condition of the seedbed will be.

 

Maroon Divider
Description | Syllabus | Notes |Guide | Internet | Lab Manual|Exams and Quizzes|Results|More|
Maroon Divider

Produced and maintained by Your Name Allen V. Barker
University of Massachusetts, Amherst.
last updated - April 25, 1999