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Achieve maturity at the end of the manure composting process

Lide Chen for Progressive Dairyman Published on 12 September 2016
manure compost

One of the greatest expenses associated with raw manure is transportation, due to the huge amount of manure (volume and weight) and distances between dairy sites and application lands.

One of the most efficient ways to reduce the volume of manure is composting, which also provides many other benefits. The composting process typically reduces the manure volume by 30 to 50 percent, which allows the material to be significantly more affordable to transport than raw dairy manure.

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What is compost?

Compost is the product resulting from the controlled biological decomposition of organic materials. More specifically, compost is the stable, humus-like product resulting from the biological decomposition of organic matter under controlled conditions.

The controlled biological decomposition process is composting, which is differentiated from the natural decomposition of organic matter because it is under human control.

Organic materials are recycled regardless of whether we compost them or not, but conditions may be regulated and optimized by humans to ensure a smooth process and the generation of a quality end product. To achieve this, people who compost need to focus on creating, examining and optimizing suitable conditions.

How does composting happen?

The degradation of organic wastes is a natural process and begins almost as soon as the wastes are generated. Under natural conditions, earthworms, nematodes and soil insects such as mites, sowbugs, springtails, ants and beetles do most of the initial mechanical breakdown of organic materials into smaller particles, thus increasing their exposure to microbial degradation.

Under controlled conditions, composting operators break down large particles through grinding or chopping. Many of the microbes involved in the decomposition are present in the wastes themselves. Soil microbes (such as bacteria, actinomycetes, fungi and protozoa) are added once the wastes are mixed with soil or inoculated with finished compost.

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Once optimal physical conditions (such as piles of wastes, water, microbes and oxygen) are established, the microbes colonize the organic material and initiate the composting process. The most easily decomposed substances are oxidized first (such as sugars and starch). Compounds resistant to degradation (such as lignin and cellulose) make up the bulk of the finished compost product.

Carbon (C) present in the organic materials is used by microorganisms as an energy source, transformed into carbon dioxide (CO2) and released into the environment. As C is lost from the compost pile, the compost becomes more condensed and air spaces within the pile become smaller. The oxygen (O2) remaining in the pile is quickly consumed by the resident microorganisms and must be replenished to prevent the system from becoming anaerobic.

The mesophilic organisms, which function best at mesophilic temperatures (75-105˚F), initiate the composting process. As microbial activity increases, temperature within piles of sufficient volume and density increases accordingly soon after compost piles are formed. With temperatures in compost piles increasing, thermophiles – microorganisms that function at thermophilic temperatures (above 105˚F) – take over.

The temperature in the compost pile typically increases rapidly to 130-150˚F within 24-72 hours of pile formation, which can last from several days to several weeks depending on feedstocks properties, pile size and environmental conditions. This is called the active phase during which decomposition is the most rapid.

It continues until the bulk of the nutrient and energy- containing materials within the piles have been transformed. Remaining materials continue to decompose, but at a much slower rate. As microbial activity decreases, so does the pile temperature, which represents the curing phase (Figure 1).

Temperature changes in an average compost pile

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In the active “thermophilic” phase, temperatures are high enough to kill most of the pathogens and weed seeds and to break down phytotoxic compounds (organic compounds toxic to plants). During this phase, O2 must be replenished through passive or forced aeration, or turning the compost pile.

Care must be taken that temperatures do not become too elevated, because even thermophilic microbial populations are killed by excessive heat. Excess heat can also be a fire hazard.

As the active composting phase subsides, temperatures gradually decline to around 100˚F. The mesophilic microorganisms
re-colonize the pile, and the compost enters the curing phase. The rate of O2 consumption declines to the point where compost can be stockpiled without turning. During curing, organic materials continue to decompose and are converted to biologically stable humic substances – the mature or finished compost.

The curing phase is important in the composting process since it helps to further decompose and stabilize potentially toxic organic acids and resistant compounds. A long curing phase is needed if the compost is unfinished or immature. This can happen if the pile has received too little O2 or has an inappropriate moisture content (either too little or too much).

Immature composts can contain high levels of organic acids, high C:N (nitrogen) ratios, extreme pH values or high salt contents, all of which can damage or kill plants if the compost is applied to the soil.

There is no clearly defined time for curing. Common practices in commercial composting operations range from one to four months. However, compost piles can cure for as long as six to 12 months.

How to check compost maturity?

Compost maturity is gaining recognition as a significant parameter to use in evaluating compost. Immature and poorly stabilized composts pose problems during storage, marketing and use.

During the early stages of composting, very little if any nitrate-N is formed. As the thermophilic stage ends, the mesophilic microorganisms convert organic N to ammonium-N and nitrate-N begin to flourish.

The appearance of significant quantities of nitrate-N (a couple of hundred ppm to more than 1,000 ppm) is an indicator of mature compost. Therefore, measuring nitrate-N, along with other maturity tests such as the Solvita test, is a useful way to assess degree of maturity.

Compost producers and users must realize that different end uses (organic field, conventional field, bedding, nurseries, landscapes, greenhouses, local and state highway right-of-ways, etc.) have different quality requirements, and the presently accepted methods to evaluate compost quality may not completely or precisely address the most important concern: Is the product appropriate for and does it perform well in the particular end-use? Testing different composts under real situations is the best way to judge their quality and is encouraged.  end mark

PHOTO: Manure compost piles. Photo by Thinkstock.com

Lide Chen
  • Lide Chen

  • Extension Waste Management Engineer
  • University of Idaho
  • Email Lide Chen

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