What Makes a Good Compost?

Composting involves the combining of manures with organic materials in such a method that the organic materials are “broken down” through the work of macro and microorganisms resulting in a rich humus that serves at least as a soil conditioner and, if done carefully, a source of nutrients, natural chelates and a source of plant protection for plants.
Authors

Bob Morris and Dr. Dale Devitt*

Originally appeared in Southwest Trees and Turf
Used with permission from the authors

            Compost results when organic matter decomposes through a series of chemical and biological processes. Composting in the natural form has been occurring since time memorial while the actual manipulation of organic matter by humans has been recorded for two millennia. The word compost comes from the Latin “componere” which means to combine or put together. It generally involves the combining of manures with organic materials in such a method that the organic materials are “broken down” through the work of macro and microorganisms resulting in a rich humus that serves at least as a soil conditioner and, if done carefully, a source of nutrients, natural chelates and a source of plant

protection for plants.

            The Indore Process of composting, recorded by the British in India 75 years ago, has been used worldwide and still found in composting manuals. The Indore method of composting relied on the stacking of separate layers of manure and degradable organic materials in a bin or pit to a depth of about five feet. This method required no turning or mixing of the layers principally due to the porous nature of the organic material which allowed air to circulate through the pile. The effluent draining from the pile or pit was generally reapplied to the pile to keep it moist. In later variations of the process, piles were turned to speed up the process.

            Researchers Waksman and later Gotaas in the early 20^th century described the composting process we know today by identifying and elaborating on the different variables needed for successful decomposition such as the environmental, microbial and characteristics of the necessary ingredients. Their investigations revealed the importance of temperature, moisture, the ratio of carbon to nitrogen in the compost, inocula or types of microoganisms, and particle size of the organic material. Gotaas’s studies in particular revealed the relationship between the types of organisms present during composting and the condition of the compost during decomposition.
            Mechanization of composting, the use of structures for speeding up the process and making it more cost effective at a larger scale, were explored primarily in Europe for cities faced with waste disposal problems. Although not as widely appreciated in the West, Asia made significant contributions in refining the composting process and were later added to our present body of knowledge.

Finished compost has feestocks that are basically unrecognizable any more, dark brown and an "earthy" smell

            When composting is monitored and done correctly the end product is stable, has an earthy smell and consistent brown color, smaller in volume than the combined, uncomposted ingredients, with a decrease in temperature compared to that during composting. Factors that influence the rate of speed of the composting process include the organic matter particle size, regulation of the air and moisture content, activity and populations of the microorganisms, and the amount of nitrogen present in relation to the carbon content. 

            The amount of nitrogen present significantly influences the rate and degree to which a mixture composts. Microbes use nitrogen as a food source so that they can “feed” off of the organic material to be composted; the source of carbon. Optimum carbon to nitrogen ratios range from 20-30 parts carbon to 1 part nitrogen. Carbon to nitrogen ratios under 20:1 allows ammonia to form in the pile and released during turning or aeration.

            The microorganisms which dominate during composting need oxygen. The particle size of raw materials determines the porosity of the pile which in turn affects aeration. Material which is too finely ground will compact so densely that air cannot penetrate the pile, resulting in anaerobic (without oxygen, fermentation) conditions. Composts made during anaerobic decomposition can be toxic to plants.

Static piles are turned periodically and kept moist. Turning helps aeration and keeping the pile from overheating.

            Optimum moisture content for composting ranges from 50-60 percent; moist to the touch but not saturated. Above 60% moisture, the compost is approaching saturation which limits the availability of oxygen to the microorganisms and leads to anaerobic decomposition. Below 50% moisture, composting slows down due to a lack of available moisture and the process is slowed.

            There are three methods of composting that are now commonly used: composting in windrows which are turned, static piles which are not turned but aerated passively or actively and the use of bins or aerated chambers. Aerated static piles used for composting, on the other hand, are unturned windrows that have perforated pipe, similar to drain tiles but used for aerating, are laid within the pile or the pile is built in such a way that passive air movement ventilates the pile. Perforated pipe may be connected to a low volume blower fan to push air through the compost pile. For commercial, high-volume composting the windrow or aerated static pile methods are most often used.

            Bins or aerated chambers are most typical for small volume or home composting.              Low-volume or batch composting are static or aerated bins which require more hand labor and is typically less mechanized. Raw materials are layered into a bin and either periodically turned by hand, or mechanically aerated by a forced air blower.

Windrow turner

            Windrows consist of long piles, 3-5 feet tall and 10-15 feet wide at the base, formed by combining raw materials on earthen surfaces. Windrows should be continuously monitored to ensure that the necessary temperature and moisture levels are attained for thorough composting. Heat generated by good composting techniques destroys most pathogenic organisms and weed seeds.

            The temperature produced by the compost pile can be used as an indicator of how the composting process is progressing. Within 48 to 72 hours after composting has been initiated temperatures within the pile should reach 140 – 160F. A drop in temperature below this range, provided the soil moisture is monitored, indicates that the pile should be turned so that new food sources from the cool, dry surfaces of the pile can be brought into contact with microorganisms in the interior of the pile.

Compost windrow

            Windrows are turned by equipment such as front-end, skid-steer, or wheel loaders. This allows oxygen into the center of the pile because microorganisms, which break down the organic matter, require oxygen to carry out the composting process. The pile then again peaks at temperature between 140 – 160 F when it is turned again.

            Temperatures staying below this range after turning, along with a change in color, texture and smell, indicate the pile has nearly finished the active phase of the composting process. The only monitoring tools needed for determining when a pile has finished this phase are the physical appearance of the ingredients and a tool for monitoring the interior temperature of the pile. Minimum active composting time is 1 month followed by 2-3 months of curing in stockpiles before the material is ready for use.  

Windrow turner and shaper

            As the active phase is completed another phase, the curing phase, begins. The curing phase is a critical and frequently a neglected phase of the composting process and can take from two to nine months.  A long curing time can be required if the compost pile was poorly managed such as too little oxygen or improper moisture content. Immature composts can contain high levels of organic acids, high carbon to nitrogen ratios, extreme pH values or high salt contents which can damage plants.

Taking the temperature of a compost pile

            Compost is considered finished or stable after temperatures within the pile core approach average air outside air temperatures and oxygen concentrations in the middle of the unturned but adequately moist pile of at least one cubic yard remains greater than 5% for several days. The curing phase begins when the monitored temperatures inside the pile decline to about 100F, even after turning. At this phase, organisms tolerant to lower temperatures begin to recolonize the compost pile.

            The demand for oxygen during curing declines to where the pile can be stockpiled without turning. During this phase is when humic and fulvic acids are being produced which are stable, natural organic acids which can aid plants in mineral uptake. The presence of stable, humic acids and a drop in temperature are some common indicators used to declare that compost has been cured adequately.  

            The Minnesota Department of Transportation has listed specifications for what they call a Grade 2 compost that they consider acceptable along with any chemical nutrient analysis, pesticide and heavy metal analysis and stability tests provided by the producer.

Carbon/Nitrogen Ratio	6 (minimum) - 25 (maximum)
Ammonia Levels	10%  of Total (maximum)
Percent Moisture	20 % (minimum) - 40 % (maximum)
pH	5.5 - 8.5

*Used with permission from the authors