Author: Kristin Pool

Publish Date: Fall 2008

Composting is a common practice on many farms. While it may seem straight forward, on-farm composting is a complex process with several benefits and some potential pitfalls. The optimal composting system depends on the type of feedstock available (the raw material to be composted), the volume to be produced and the management method. The key to a successful composting system is an understanding of the composting process.

Some Advantages and Disadvantages of Composting

Compost offers numerous benefits to the farm, including:

  • Improved management of on-farm wastes such as manure, livestock bedding, and crop residues
  • Management of plant and human pathogens, weed seeds and odor which can be a concern with some raw organic amendments
  • Valuable amendment for potting soil and field application
  • Improved soil organic matter content and soil tilthAlternative livestock bedding material
  • Reduced need for off-farm inputs
  • Potential income from compost sales or waste disposal services 

Some of the disadvantages of on-farm composting are the investment in equipment and dedication of area for composting. Managing compost is like growing a new crop; it requires a time investment. There is also potential for excessive leachate production, if composting is managed improperly, which can contaminate water.

The Process

Compost is the result of aerobic decomposition which occurs naturally over time under the right conditions. Optimal conditions for aerobic decomposition reduce the time needed for organic matter to decay and improve the quality of the compost produced.

The optimal conditions for aerobic decomposition balance energy, moisture and air. If these components are balanced microorganisms, the agents of decomposition, will thrive and reproduce, speeding up decomposition. In some cases mature compost can be produced in as little as six weeks. Heat is a byproduct of microbial respiration. When the population is high enough and the compost pile is sufficiently insulated, temperatures will climb to 130ºF or more. Therefore increased temperature is an indicator of successful composting. These high temperatures also kill plant and human pathogens and weed seeds. Table 1 lays out the target characteristics that provide ideal energy, moisture, and aeration.

Table 1 - Characteristics of a well prepared compost pile.
Characteristic Ideal Range

 Carbon Nitrogen ratio (C:N)

 25-40:1

 Moisture Content

 50-60& (wet basis)

 Bulk Density

 40lbs/cubic ft

 Particle Size

 0.5-2" in diameter

Temperature

 131°-150° F

Feedstock

The ideal characteristics in Table 1 are created when selecting and mixing feedstock. There are three categories of feedstock: energy materials, bulking agents, and balanced materials. Energy materials provide the nitrogen and high-energy carbon compounds. Bulking agents are dry porous materials that increase aeration. Balanced materials can be composted on their own, because they already contain the above ideal characteristics. They are also good additions to any compost pile to ensure successful composting. Table 2 categorizes common feedstock into these three categories.

Table 2 - Feedstock category
Category of Feedstock Descroption Examples
Energy Materials High nitrogen and moisture, low aeration Grass clippings, crop residue, and fresh dairy, chicken or rabbit manure
Bulking Agents Low nitrogen and moisture, high aeration and porosity Woodclips, sawdust, grass hay, wheat straw, corn stalks
Balanced Materials Low to medium moisture, medium nitrogen, medium aeration Horse manure and bedding, deciduous leaves, legume hay, and ground tree or shrub trimmings
Particle size is also an important factor in selecting a feedstock. Smaller particles accelerate aerobic decomposition because they have more surface area for microorganisms to feed upon, larger particles (i.e. bulking agents) increase aeration. It is advantageous to grind up very large particles.

The On Farm Composting Handbook (NRAES-54 ) and the Field Guide to Composting (NRAES-114) are useful resources with estimates of the C:N ratio, moisture content, and bulk density of common feedstock.

Moisture

Aerobic decomposition will slow as water becomes scarcer. The ideal moisture content is approximately that of a wrung out sponge. The best time to add water is when building a pile. It is surprising to many first time composters how much water is needed to increase the moisture content of organic matter. It is then imperative to maintain the proper moisture content throughout the composting cycle. If a pile is allowed to dry out irrigation may not have the needed effect. Water will take the path of least resistance and simply percolate through a dry pile without wetting the materials. Likewise, a pile should not be over watered or left exposed to excessive precipitation. A pile that is too wet will start decomposing anaerobically, which is an odorous process that produces compounds that are toxic to plants.

Scale

The size of compost pile is also important to the success of a composting system. Smaller compost piles will not heat up to ideal temperatures because they cannot support a sufficient population of microorganisms. The minimum size for a compost pile is about 2-3 yds3. Any smaller and the pile will not heat up; decomposition will be slower and the pile may not satisfy safety guidelines for food production. Composting in smaller piles is possible, but the pile will behave more like a worm bin.

The Finished Product

Mature compost is dark, crumbly in texture, and smells like soil. When turned mature compost will not heat up. This high quality compost holds a wealth of benefits for farms, but creating the optimal compost is tricky work. Understanding the process and how it can be controlled will help you to master your on-farm composting system. With a successful composting system high quality compost is just a few months away.

Composting Research and Education at OSU

This summer OSU researchers, Nick Andrews and Dan Sullivan, began work on a compost program. A Research and Education Facility will be constructed at the North Willamette Research and Education Center (NWREC). This facility will be used to research compost recipes that produce high quality compost for blueberries and other crops. An intensive workshop program for on-farm composters is also planned for 2009/10.

References

  • On Farm Composting Handbook: NRAES-54. Natural Resource, Agriculture, and Engineering Services Cooperative Extension, Cornell University, 1992.
  • Field Guide to On-Farm Composting: NRAES-114. Natural Resource, Agriculture, and Engineering Services Cooperative Extension, Cornell University, 1999.
  • Crogger, Craig and Dan M. Sullivan. Backyard Composting. Washington State University Extension Publication EB 1784.