Composting: Great promise that comes with challenges, is it a fit for your farm?

Author: Nick Andrews, Small Farms Program, Oregon State University

Publish Date: Winter 2015

Compost is an excellent soil amendment that can quickly improve degraded soil. The composting process is also an excellent pest prevention strategy that can reduce weed seeds, plant and human pathogens, and other pests in raw materials. By many, compost is considered a fundamental component of organic and ecological agriculture. But composting requires quite a lot of effort, and a significant financial investment if done on a larger scale. It is well worth practicing on a small scale before undertaking larger scale composting. If you are considering composting on your small farm, these are some questions you should consider.

Can I use this material raw, without composting?

 It is usually cheaper and more efficient to apply a raw material to your field, rather than go to the trouble of composting. If you could apply your material raw, ask yourself if the following attributes of composting will improve the raw amendment you have in mind:

• Composting can reduce the particle size of chunky materials, making it easier to apply evenly with manure and fertilizer spreaders.
• Composting can prevent high carbon materials like straw or wood shavings from immobilizing nitrogen when applied to the soil.
• Composting can reduce pests in raw material like weeds, plant pathogens, human pathogens, and insect pests.
• Composting reduces the volume of most raw materials by about half, making it easier to transport.
• Composting can reduce the amount of mineral nitrogen (i.e. ammonia and nitrate-nitrogen) in materials like poultry manure, and convert it to organic nitrogen (i.e. proteins and amino acids) that are much less available to plants. This can be a positive or negative attribute.

Can I just let it decompose slowly rather than intensively managing a compost pile?

 Slow, passive decomposition is sometimes adequate, but it doesn’t usually provide all of the benefits of well managed composting. Slower decomposition can reduce particle size and produce a material that is relatively easy to apply. However, human and plant pathogens, weed seeds and propagules (i.e. rhizomes) and other pests may survive in a pile of decomposing organic material that doesn’t heat up enough (see below). Neglected piles of decomposing organic waste are also often left uncovered, which can leave them exposed to weed seeds and nutrient leaching from rain. Composting is essentially a natural process that can be quite simple to manage. Relatively simple changes to your packhouse waste or manure pile could help you produce a product you can apply to your fields with confidence. If you want to add organic matter to your soil, composting can prevent you from spreading a pest problem around your farm, or introducing a new pest problem to your farm.

I have a lot of one material, how do I compost it?

When you build a good compost pile, you are creating habitat for the microbes that will digest the raw material and convert it into a useful soil amendment. The most important observable factors are C/N ratio, moisture content and particle size. Large piles of one material may or may not create a good compost pile. Consider which materials you should bring on to your farm to improve the C/N ratio and moisture content of the materials you have readily available. Remember that even if an imported material is free, trucking costs can add up quickly.

Most materials (i.e. most vegetable, meat or crop waste, and many types of yard debris) will break down into smaller particles during the composting process. Larger woody materials (i.e. tree branches) usually require grinding before composting. A flail mower or manure spreader can help to chop up and thoroughly mix materials with larger particles and solid clumps.

Nitrogen feeds the microbes that do the work in a compost pile. Ideal C/N ratios for compost are in the range of 20-40:1. You can test your product for C and N content or look up typical “book values” (see WSU Compost Mix Calculator below) for individual materials. If you are combining materials with different C/N ratios, use the WSU calculator to estimate the C/N ratio of your compost pile.

Compost microbes live in the moisture film on the surface of the organic material being digested. Their ideal moisture content is 40-60%. Typically the heat of a compost pile dries the pile out as the process continues. Unless you are building a pile just before heavy rain is forecast, try to mix feedstock or add moisture so that initial moisture content is around 50- 60%. When you squeeze most materials tightly in your hand you can estimate moisture content. For example, if no drops come out, but it leaves a moist sheen on your hand, the material is probably about 58-63%.

Andy Bary from WSU developed a Compost Mix Calculator that can help you calculate C/N ratio and moisture content for combinations of a wide variety of materials: CompostMixCalc.html. It is an easy to use Excel spreadsheet.

What about food safety and pests like weed seeds and plant pathogens?

 If your main concern is managing pests such as plant or human pathogens or weed seeds, make sure that your compost pile reaches good thermophilic temperatures (i.e. 131-170°F). Make sure that all material reaches this temperature for at least three days, perhaps a bit longer for some weed seeds. When the core of your pile is hot, material closer to the surface is much cooler. To ensure that all of your material undergoes the composting process for pest reduction, make sure your pile is turned often enough during the initial thermophilic stage (i.e. first 3-6 weeks) so that all the material reaches 131°F. Aerated static piles require should be insulated with pathogen free material (i.e. finished compost) or mixed often enough to ensure that all the raw material reaches these time and temperature requirements.

To learn more join us at ACRES: Agricultural Composting Resources and Education Series in April. Our instructors include Dan Sullivan (OSU Soil Scientist), Andy Bary (WSU Soil Scientist), Bob Barrows (Oregon DEQ, Western Region), and Wali Via (Wintergreen Farm).

I described Wali’s composting practices in the summer 2013 issue of Oregon Small Farm News: http://