WHAT IS COMPOSTING?
United States Department of Agriculture [USDA], 2000) defines compost as:
"The product of a managed process through which microorganisms break down plant and animal materials into more available forms suitable for application to the soil. Compost must be produced through a process that combines plant and animal materials with an initial C:N ratio of between 25:1 and 40:1. Producers using an in-vessel or static aerated pile system must maintain the composting materials at a temperature between 131 °F and 170 °F for 3 days. Producers using a windrow system must maintain the composting materials at a temperature between 131 °F and 170 °F for 15 days, during which time, the materials must be turned a minimum of five times." Composting is the controlled management of the normal biological process of aerobic (in the presence of oxygen) decomposition of organic residues by microorganisms such as bacteria, fungi, and actinomycetes. This process is optimized when the various organic residues are mixed to provide certain conditions:
- a balance of energy (carbon, C) and nutrients (primarily nitrogen, N), with an initial C:N ratio of between 25:1 and 40:1
- sufficient—but not excessive—moisture (typically 40–60% by weight)
- sufficient oxygen to support an aerobic environment (typically 5% or more)
- a pH in the range of 6–8
Under these conditions, populations of microorganisms will thrive and organic residues will be decomposed, consuming oxygen and releasing intermediate breakdown products, carbon dioxide, and heat. As the temperature of the pile rises, the community of microorganisms will go through a succession, culminating in thermophilic (heat-loving) organisms at temperatures above 113 °F (45 °C). If the mass of the compost pile is large enough to be self-insulating, temperatures within the pile during this active phase of composting may reach 131–170 °F (~55–70 °C) within 1–3 days. To maintain biological activity and to bring the active phase to completion, temperatures should be monitored and compost moisture and aeration should be maintained. After the most readily decomposable organic matter in the compost is consumed, biological activity will decrease in intensity, and temperatures and oxygen consumption will decline. The compost then enters the curing phase, during which decomposition proceeds more slowly and organic matter is converted to stable humic substances—the finished or mature compost.
TWO MAIN REASONS TO USE
How does composting reduce weed seeds?
Several factors contribute to weed seed mortality during composting. In compost systems assembled and managed in accordance with requirements for organic certification, the most important factors are the interaction between weed species, temperature, time, and moisture (Eggley, 1990; Shiralipour and Mcconnell, 1991; Eghball and Lesoing, 2000; Larney and Blackshaw, 2003; Dahlquist et al., 2007). In general, the higher the temperature to which weed seeds are exposed during the active phase of composting, the higher the weed seed mortality. Similarly, the longer the duration of high-temperature exposure, the higher the weed seed mortality. Thus, Dahlquist et al. (2007) estimated that three of the six weed species they examined under controlled laboratory conditions were unaffected by temperatures of 108 °F, but 90% of the seeds of all six species were killed after less than three hours at 140 °F (Table 1). Furthermore, all six species suffered 100% mortality after less than an hour at 158 °F. Similarly, in Texas, Weise et al. (1998) found that, in composting manure at 35% moisture, barnyardgrass, pigweeds, and kochia seeds were killed after three days at 120 °F; Johnsongrass seed was killed with three or more days of exposure at 160 °F; but field bindweed seeds were killed only after seven days at 180 °F.
Susceptibility of weed seeds to thermal mortality, however, is influenced by the moisture content of the compost; weed seeds in a dry environment are able to survive higher temperatures for longer times than seeds in a moist environment. Some (Egley, 1990; Thompson et al., 1997) have suggested that thermal mortality may be greatest for fully imbibed seeds—seeds that have absorbed water and split their seed coat in the process of germination.
Other factors are thought to contribute to weed seed mortality during composting. Larney and Blackshaw (2003) observed considerable variability in the relationship between temperature exposure in windrows and seed viability for a number of weeds, and concluded that additional factors, such as germination into lethal conditions or pathogen infestation, were contributing to weed seed mortality. Others have implicated plant-toxic compounds that accumulate to sufficiently high concentrations during composting (phenols, ammonium, and acetic acid, for example) in weed seed mortality and suppression of germination.
Reducing Plant Pathogens:
Several factors are known to contribute to the eradication of plant pathogens and nematodes during composting:
- heat generated during the active phase of the composting process
- the production of toxic compounds such as organic acids and ammonia
- lytic activity of enzymes produced in the compost
- microbial antagonism, including the production of antibiotics and parasitism
- competition for nutrients
- natural loss of viability of the pathogen with time
- the production of compounds that stimulate the resting stages of pathogens into premature germination
Of all these factors, heat generated during the active phase of the composting process appears to be the most important in pathogen destruction. Thermal mortality during the active phase of composting was found to be the most important factor affecting pathogen destruction.
In California, Downer found that unturned piles of fresh and aged green waste (note that these piles would not have satisfied organic certification requirements) did not uniformly expose pathogens to lethal temperatures. They recommended that green waste stockpiles should be turned intermittently to mix pile contents and move propagules to a part of the pile where they would be more likely to be killed by heat, microbial attack, or chemical degradation that occurs during active aerobic composting.
What can go wrong?
In general, adherence to a composting process that meets the requirements of organic certification should result in substantial—if not complete—destruction of weed seeds and plant pathogens. Incomplete composting, on the other hand, can result in the survival of weed seeds and/or plant pathogens.
Improperly assembled and maintained piles or windrows may not reach high enough temperatures during the active phase of composting for killing all weed seeds and pathogens. Failure to reach adequate temperatures can have several causes:
- Too high a C:N ratio of initial ingredients, too little water, or too little oxygen can inhibit the rate of decomposition, and thus the production of
- Too much water can starve the pile of oxygen and result in anaerobic decomposition.
- Accumulation of toxic products may inhibit fungal and microbial activity, thus slowing the rate of decomposition.
- Too small a pile or windrow may loose heat too quickly to reach adequate temperatures, whereas too large a pile may have inadequate aeration to
support aerobic decomposition.
To avoid these problems, assemble raw materials carefully to achieve the proper starting conditions of C:N, moisture, pile porosity, and size; monitor temperature and moisture conditions; and turn/aerate as needed to maintain a biologically active, aerobic environment. Temperatures at the edges and surface of compost piles and windrows may not be sufficient to kill weed seeds and pathogens. This is an especially important risk in static piles that are not turned and mixed during the active phase of decomposition, but rely on forced aeration to maintain an aerobic environment. Thorough mixing or turning during the active phase is essential to ensure that all the material achieves elevated temperatures for a long enough period of time to kill weed seeds and pathogens.
Dry heat is less effective than moist heat at killing weed seeds. Ensure that moisture content of the pile or windrow is maintained at 40–60%.
Contamination with soil or uncomposted residues, especially after the active phase of composting has finished, can lead to the reintroduction of weed seeds or plant pathogens. Avoid adding fresh material after the active phase. Finished compost can become recontaminated with weed seeds if weeds are allowed to grow and go to seed on or adjacent to the pile or windrow. Similarly, compost can become contaminated with vegetative reproductive structures from some weeds—Canada thistle and rhizomateous grasses, for example—if they are allowed to grow on or adjacent to the pile. Keep vegetation adjacent to stored compost mowed short, and tarp piles or windrows to prevent contamination by wind-blown weed seeds. When moving or spreading finished compost, avoid picking up soil or other contaminants from under or around the pile or windrow.
To make traditional compost, alternate different types of shredded plant materials in 6- to 8-inch layers. Layering helps compost reach the correct nitrogen balance. Use equal parts by volume of dry and green plant materials in the overall mix. Use caution when you add layers of fine green plant wastes such as grass clippings. Grass
mats easily and prevents water from moving through the mass. Use 2-inch layers of fine materials or process them through a machine shredder. Alternate fine
materials with woody plant prunings to prevent clogging the machine and to create an equal balance of dry and green materials.
Traditional composting includes soil as one of the layers. While soil can serve as a source of microbes to "inoculate" plant wastes, research has found that the microorganisms that break down plants also are present on the surface of the leaves and stems. It's natural for some soil to cling to pulled weeds and uprooted vegetable and flower plants. When you add large amounts of soil, you increase the weight, which makes composting difficult and less efficient. Large amounts of soil also can
suffocate microorganisms. Soilless composting is often practiced.
Add water to the compost after every few layers of material. If the plant materials are dry and no green material is available, add a small quantity of blood meal or a
commercial nitrogen fertilizer free of weed killers. One-half cup of ammonium sulfate per bushel of material is sufficient. Livestock manure also can be added
and supplies some nitrogen. Like soil, manure adds weight and bulk. The space devoted to manure could be used to compost yard wastes.
There is no advantage in adding compost starters or inoculum to the compost. The microbes that cause decomposition multiply just as rapidly from those that are naturally found on the plant cut.
A variety of materials can be composted, but most Gardeners want to recycle collected yard waste. Plants lose between 50 and 75 percent of their volume in composting, so a lot of plant material can be processed effectively.
Composting can be effective on most yard wastes such as leaves, vegetable and flower plant parts, straw, and a limited amount of woody prunings, grass clippings and weeds. Woody twigs and branches that are greater than 1/4 inch in diameter should first be put through a shredder-chipper. Avoid highly resinous wood and leaf prunings
from plants such as junipers, pine, spruce and arborvitae. The resins protect these materials from decomposition and extend the time needed for composting in
comparison with other plant materials. High tannin-containing leaves (oak and cottonwood) have similar problems but can be used in small quantities if chopped
well and mixed with other materials. The easiest way to handle grass clippings is to leave them on the lawn. Research shows that they return valuable nutrients
back to the soil. Some grass clippings can be used for compost if other green plant material isn't available.
Many, but not all, plant disease organisms are killed if the compost reaches 122 degrees F. Temperatures will vary throughout the compost. Outer layers stay cooler than the center and cause uneven kill of disease organisms. If a plant is severely diseased, it is better to dispose of it in the trash.
How you know the compost is done:
Beware of Unfinished Compost
Finished compost is dark and crumbly, does not resemble the original contents, and has an earthy smell. Compost that has not thoroughly processed
could be “hot” with high ammonia content. This could burn plant roots (when applied to the soil) or plant leaves (when applied as a mulch). If the compost
smells like ammonia, it should be processed longer or be worked into the soil at least one month prior to seeding or transplanting in the area. Compost maturity
can be assessed in a lab by measuring the carbon dioxide (CO2) production by the microorganisms living in the material. Lower levels of CO2 indicate more mature compost (i.e. microbial activity is low because they have used the available nitrogen to decompose the carbon in the compost). Conversely, if microbes are producing CO2,
they are consuming oxygen (O2). Unfinished compost can consume all of the O2 from the root zone and greatly inhibit root growth. Finished compost should smell earthy, like healthy soil, not like ammonia.
Applying Compost: Rates and Salt Problems
General application rate for compost is based on the salt content of the compost and soil and on the depth to which it is cultivated into the soil.
Ideally, cultivate the compost into the top six to eight inches of the soil. On compacted/clayey soils, anything less can lead to a shallow rooting system with
reduced plant growth, lower vigor, and lower stress tolerance. Table 1 gives standard application rates for compost. Compost made solely from plant residues
(leaves and other yard wastes) is basically free of salt problems, and higher application rates are safe. Compost needs to be thoroughly mixed into the upper
six to eight inches of the soil profile. Do not leave compost in chunks, as this will interfere with root growth and soil water movement. As the soil organic content builds in a garden soil, the application rate should be reduced to prevent ground water contamination issues. A soil test is suggested every four to six years to establish a base line on soil organic matter content.
Nitrogen Release is Slow
Typical nutrient content includes 1.5% to 3.5% nitrogen, 0.5% to 1% phosphate, and 1% to 2% potash, plus micronutrients. Thus compost is more of a soil conditioner than a fertilizer. In gardens where compost is routinely added, phosphorus and potassium levels are likely to be adequate. Like in other organic soil amendments, the nitrogen release rate from compost will be very slow, over a period of years. When the organic content is below 4-5%, additional supplemental organic may be needed.
ο 4-5% Organic Matter – Soils with 4-5% organic matter from compost will mineralize (release to plants) about 0.2 pound of nitrogen per 100 square feet per year. This should be sufficient for plant nitrogen needs.
ο 2-3% Organic Matter – Soils with 2-3% organic matter from compost will mineralize about 0.1 pound of nitrogen per 100 square feet per year. Additional nitrogen fertilizer will be needed for high nitrogen crops like broccoli, cauliflower, cabbage, potatoes, and corn.
ο 2% Organic Matter– In soils with less than 2% organic matter, the release rate for nitrogen will be too low to adequately provide the nitrogen needed for crop growth. A supplemental organic or manufactured nitrogen fertilizer may be needed.