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OSU Extension

College of Food, Agricultural, and Environmental Sciences

December 1, 2020 - 8:00am --

Composting… it seems self-explanatory. Just toss your yard waste and kitchen scraps into a pile and then it turns to soil, right? Wrong! This synopsis of composting just barely scrapes the surface of the science behind the process. The better that you understand the complexity of composting, the better compost you will be able to make. This goes for anyone; from just having a small compost bin on the back patio, to farms with large manure-based compost piles.

Let’s start with the “why”, of composting. Why is it even worth our time and effort to compost? Composting can be traced back to when groups of people began to transition from “hunters and gatherers” to agriculturally based societies. During that point in history, compost, which was primarily animal manure, was used to enrich soil in order to be able to grow a productive crop. As time went on, composting found its way through many historic populations, including ancient Asia, the Greeks, Romans and Egyptians, the Native Americans, and the American settlers. In the modern world, some scientists found themselves working with composting and people such as J I Rodale and the likes began delving into compost and initiated the work to popularize it with gardeners and farmers. George Washington Carver taught in 1943, that compost was essential to the fertility of the land, and that farmers should "Make your own fertilizer... compost can be done with little labor and practically no cash outlay". Carver could not have said it better. If you have the resources available to you (yard waste, food scraps, manure, organic matter etc.), for no additional expense, use them! Looking at this from strictly a financial standpoint, commercially available fertilizers are an expense. Composting provides fertility and a source of nutrients for your soil and may even diminish or eliminate the expense of buying fertilizers. Obviously, there are a lot of variables that go into the financial value of the compost; however, the principle still applies regardless of the size or scale of the operation.

Just as important as the “why”, is the “how” to compost. Composting is essentially using the natural decomposition of organic matter in such a way that allows for the formation of a uniform, value added product. Composting is accomplished when microorganisms, under the appropriate conditions, break down organic matter. The resulting product is a nutrient rich compost. These microorganisms are most efficient at elevated temperatures, with adequate moisture (40-60%) and in most cases, occasional turning or mixing to provide aeration. The ideal temperature range for compost piles is between 110-160 degrees F. At these temperatures, especially towards the higher end of that range, pathogens, pests and weed seeds can be killed off. Being able to eliminate these things from your compost is a great advantage, however, overheating in the compost pile can occur and cause an overall slowdown in the process due to the die off of the microorganisms that were working on breaking down the organic matter. Likewise, temperatures that are too low will also slow down the process due to microorganism’s inactivity. 

Another important factor is what you are adding to the compost pile. This is where things can get a little more complicated. What we need to look at here is what is known as the carbon to nitrogen ratio (C:N). The Sustainable Agriculture Research and Education (S.A.R.E.) publication “Building Soils for Better Crops” by Fred Magdoff and Harold Van Es has some great information on this topic. Understanding the C:N ratio will help you understand how to build and maintain a more productive compost pile. The C:N ratio is important because this is a major factor when looking at the activity and efficiency of the microorganisms. According to S.A.R.E. “A high C:N will result in low rates of decomposition and can also result in a temporary decrease in nitrogen availability for plants as the microorganisms use much of the available nitrogen.” The acceptable range for C:N is approximately 25:1 – 40:1, although the target should be 30:1. The best way to achieve this target ratio is to mix various organic materials together. When composting with sawdust and woody materials or straw and corn stalks, realize that these materials are high in lignin. Even though they may appear to have extremely high C:N, the lignin prevents a significant part of that carbon from being immediately available for decomposition due to lignin not being easily broken down. Combining these materials with organic matter that is more nitrogen rich, or even adding urea to the compost pile can help to correct the imbalance. Manure and green matter such as grass clippings have lower C:N and can help counter the high C:N from woody materials. More in depth details of different organic materials can be found online and these charts can help you determine how much of a certain material, or what kinds of materials you should add to your compost pile.

Compost piles should be turned every month or so after being set up. This is also the best time to add moisture, if necessary. It should start to look ready for use after 4-5 months. A good way to look at it is that a pile started in the spring, will be ready for use in the fall. Like wise a pile started in the fall will be ready in the spring. When the pile has a crumbly texture, is dark brown and has an earthy smell, you know that it is ready to use. If you so choose, and I recommend it for farmers who are using large amounts of composted material on their fields, get a nutrient analysis test done on your compost. Knowing the nutrient value of your compost, combined with soil test values will aid you in making nutrient management decisions, and prevent you from over applying fertilizers. Over applying fertilizers means that you are losing money, and you may be doing more harm than good to your soils and surrounding environment.

Using the organic matter and materials that we have on hand and treating them as a commodity rather than a nuisance is a great way to improve our soils and return the compounds in these materials to a usable form. Speaking from a perspective of sustainability, composting serves both economic sustainability and environmental sustainability. Saving money on fertilizer expenses, being environmentally friendly, and getting to play in the dirt… what’s not to love? 


Frank Becker is an OSU Extension Agriculture and Natural Resources Program Assistant and IPM Program Coordinator.  He may be reached at 330-264-8722.


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