Tracie Meskell, Landscape Architect and Compost Operations Manager

Not all compost is made the same and the differences have important impacts on how it will support your garden. When researching compost, you often read about the following qualities of what to look for– an earthy smell, dark and moist, and no garbage. The difference between compost and a great compost however, is that a great compost is stable, mature, and full of rich organic matter. To help narrow your search and save you time, there are certain compost qualities you can look for based on its lab analysis and ingredients. The following are some important qualities to look for: 

Organic Matter- What is the percentage of organic matter in the compost? 

The number one reason to use compost is for the wonderful benefits the organic matter will provide to the soil’s microorganisms, in turn benefiting your plants. An optimum range for compost is between 40% and 60% organic matter. If the levels are below 35%, the compost likely has sand or dirt mixed in from the turning process, so you’re not getting as much direct impact on your soil and plants. Composts higher than 65% may not be fully composted yet. 

Compost Maturity- Has the compost aged enough to be safe for plants? 

If a compost isn’t mature, organic acids and other byproducts can be toxic to plants. A germination test checks for the maturity of a compost; cucumber seeds are often used for a germination test because they are sensitive to phytotoxins. An 80% emergence rate or higher from a cucumber seed test is considered acceptable. A test using standards set by the US Composting Council will provide this information. Another indicator of a mature compost is that most of the inorganic nitrogen is present in the NO3 (nitrate) form rather than as NH4 (ammonium). 

Compost Stability- Is the compost still actively decaying? 

A compost that is not stable, or insufficiently cured, will continue to decay rapidly, generate phytotoxins, emit odors, and possibly tie up its nitrogen. A stable compost will have converted all of its easily decomposable materials, leaving the more stable forms of carbon to decay slowly. It is at this stage that a compost has an earthy smell. Very stable composts will produce less than 2 mg of CO2 per gram of organic matter per day. A compost test using the US Composting Council standards will provide a stability measurement based on the CO2 output. Another stability indicator is the pH of the compost. The pH should fall between 6-8.5 if it is stable, with most composts around 7.5. In many cases, a compost with a low pH is not completely stable and is still releasing organic acids, a sign that rapid decomposition is still taking place. 

Feedstock- what is the compost made from? 

Many of the landscape supply yards offer composts made from yard waste (sometimes referred to as green waste). This can be a great option if your main goal is to add organic matter to your soil with small amounts of slowly released nutrients. However, these composts can have small bits of metal, plastic, and glass because it is difficult for the composting facilities to remove all waste matter from the initial inputs. For a higher quality compost, look for a lab result with less than 0.5% inerts. 

A compost made from manure can be a great option if you want to add organic matter to your soil with a larger amount of slowly released nutrients. However, you’ll want to check that the manure is free from pathogens, so request a copy of the compost analysis if one isn’t available on the website. If you want to use an organic compost, find one certified by CDFA or OMRI which are appropriate for use in organic production and are verified to comply with the USDA National Organic Program Standards. 

Carbon to Nitrogen Ratio- what does the C:N ratio say about the compost? 

It doesn’t necessarily indicate if a compost is mature or not. For example, a mature compost with a high C:N ratio likely has more wood based materials in it- causing a higher ratio even when mature. Some composts with large amounts of manure or green plant material, like grass clippings, can have a low C:N ratio but not be fully composted. It does, however, act as a useful predictor of plant available nitrogen for release by the compost following its application to the soil. If the C:N ratio is above 20, it may tie up the nitrogen the first year and supplemental nitrogen is recommended if your soil needs it. A C:N ratio between 10-20 means the compost will provide a small amount (about 5%) of its nitrogen. If the C:N ratio is below 10, then more nitrogen is available (10-20%) right away. For several years after its initial application, compost will continue to provide a slow release of nitrogen to your plants.

Source:

Interpreting Compost AnalysesBy Dan M. Sullivan, Andy I. Bary, Robert O. Miller, and Linda J. Brewer
https://catalog.extension.oregonstate.edu/sites/catalog/files/project/pdf/em9217.pdf


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