Demystifying Compost: A closer look into the pile

Av Singh, Ph.D.

For many livestock producers the first signs of spring coincide with clearing drylots and bedded manure-packs. Traditionally, these farm wastes were combined with crop residues and set aside to decompose naturally over long periods of time. In recent years, many farmers have realized the economic benefits of compost and are beginning to intensify their compost management to generate a uniform and stable product in a relatively short time. This article will briefly highlight the mechanics of composting and comment on the economic viability and environmental sustainability of intensive composting.

Composting of manure and other organic residues help to stabilize nutrients and organic matter, reduce the volume and density of manure, and destroy weed seeds and pathogens found in manure. Composts, in general, can improve soil nutritional availability and soil tilth because of their complex microbial population. Composts bring with them a wide array of bacteria, fungi, protozoa, nematodes, and arthropods (including earthworms) along with the food resources needed to feed these organisms. The optimal environment for these organisms to survive is integral to successful composting and is affected by the following: 1) moisture; 2) carbon:nitrogen (C:N) ratio; 3) oxygen; and 4) temperature.

Moisture
Microbial activity is sustained if the moisture content of the pile is in the range of 40-60% (about as damp as a wrung-out sponge). Microorganisms require water as a medium for chemical reactions, to transport nutrients, and to move about. A pile with too much water may prevent proper airflow hindering the activity of aerobic microorganisms and lead to odours (e.g., hydrogen sulphide). Compost with too little moisture (less than 35%) will permit ammonia to be lost as a gas and if the pile becomes too dry and dusty it is more likely to be populated by molds rather than beneficial microorganisms. Water is often added when the compost pile is being created.

C:N ratio
Animal manures are high in N and often require large amounts of C (in the form of a bulking agent such as straw, wood shavings, hay) to provide the right environment and food for the composting microorganisms. In general, these organisms require about 25 times more C than N. Bulking agents that persist longer in the pile and have larger particle sizes are more effective at maintaining aerobic conditions because they provide pore space for air. In contrast, readily available C sources such as newspaper or sawdust may permit the pile to compress resulting in anaerobic conditions overtime. In such cases, compost pile management should include regular turning to maintain aerobic conditions.

Oxygen
Since aerobic microorganisms need oxygen to work, oxygen must be able to move into the pile and carbon dioxide and water vapour must be able to escape. Initially a compost pile will contain about 20% oxygen, however as oxygen is consumed, levels may drop as low as 5%, dramatically slowing down the composting process. Active turning of the compost pile is one way of restoring pore spaces and thereby replenishing the needed oxygen. Passive composting (minimal turning) of manure, most common for on-farm composting, may include increased use of bulking agents such as straw to ensure air pockets are formed to sustain oxygen levels. Often in composting literature terms such as bulk density and pile porosity are used to convey the pile structure and its influence on oxygen levels. Quite simply, bulk density guidelines (weight/volume) are useful to optimize aeration and porosity (pore space) in your pile and serve as a useful tool in applying a finished compost product.

Temperature
Temperature is both a necessity and a result of microorganisms' work. If the temperature is too low microorganisms are not very active, which means heat will not be generated and decomposition will occur at a slow rate. Maintaining temperatures in the thermophillic range (between 113oF to 155oF; 45oC to 68oC) will kill most fly larvae, weed seeds, and plant pathogens. Some believe that these temperatures must be sustained in the pile for several days or weeks to destroy pathogens. Researchers at USDA Woods End Laboratory suggest that even in unturned compost piles pathogens can be destroyed in 50 days at ambient temperature because pathogens become part of the microbial food chain. The pile size can also affect temperature. Pile sizes 3 to 5 feet in height and 6 to 10 feet at the base in long windrows may be best for on-farm composting, while assuring that heat is retained in the pile and that airflow is optimized.

So, why has this rather simple recipe using only four ingredients and natural processes become so complex? As the benefits of compost became more accepted within the farming community there has been an interest in intensifying these natural processes. This intensification has come with a variety of composting products and technologies, including: compost pile turning machines; aeration systems; pile covers; in-vessel compost-reactors, etc. Questioning the economic viability and environmental sustainability of intensive composting, the Woods End Laboratory and the Centre de developpement d'agrobiologie du Quebec conducted several studies to examine the cost and quality of end-products with intensive composting. In one trial, addressing the frequency of pile turning they revealed that oxygen levels were increased only for a short time but were not sustained and suggested that self-aeration (using bulking agents such as straw) can adequately furnish oxygen needs for composting. In terms of time-efficiency for creating mature or stable compost (as determined by no increase in pile temperature) frequent turning (twice weekly) required 106 days versus 123 days for no-turned piles for dairy manure and 130 days vs. 145 days, respectively for poultry manure.

Organic matter is lost during the composting process as evidenced by the reduction of the pile, but is the loss greater in intensive composting? Many argue that nitrogen is lost as ammonia with increased turning. In the Woods End trial, organic matter and nitrogen losses, 88% and 86%, respectively, were greatest in poultry manure that was turned twice weekly versus 75% (organic matter) and 72% (nitrogen) in the no-turned pile.

All of this mechanization/intensification came with a cost. Economic costs associated with compost intensification resulted in cost ($) per wet ton to be $3.05 for the no-turned pile versus $41.23 for the compost turned twice weekly.

From the above information composting should be viewed as an excellent nutrient management tool that requires little management. Organic producers should take advantage of the natural decomposition process and compost on-farm, whether it's wasted hay, animal manures, or crop residues. At present, certifying bodies (CBs) in Canada have different standards on the use of off-farm compost. Most CBs require a minimum of six months before land application and many CBs prohibit compost importation of manure from genetically-modified fed livestock or industrial livestock operations. Such decisions may have scientific merit. The OACC, in collaboration with Nova Scotia Agricultural College researcher Nancy McLean are determining if genetically modified DNA (from corn and soybean) can still be detected in dairy and poultry manure after composting. Their findings may have significant impact on organic producers reliant on off-farm compost sources.

For those more interested in learning more about composting, the OACC offers a web-based course on Basic Composting Skills.


For more information please call the Organic Agriculture Centre of Canada at 902-893-7256 or email oacc@nsac.ca.


en français