Question: What is “compost,” exactly? I’ve heard the term my entire life, but I’ve never really understood what it means.
Answer: Most gardeners and farmers know compost to be a rich, organic soil amendment, which can be used to improve soil tilth and supply plant nutrients. The term compost is also a verb for a rapid way to decompose and stabilize different types of organic materials, such as leaves, grass clippings, food scraps, manures, woody debris, and municipal biosolids.
During the composting process, different types of fresh, organic waste materials are mixed. Ideally, they’re mixed to have a total carbon: nitrogen ratio of about 30 or 40: 1. The moisture content of the mixture is also important, because rapid decomposition and stabilization depend on having a well-aerated pile.
Not too wet, not too dry
Too much moisture—i.e., too little air—and the process will become anaerobic. An anaerobic compost pile will undergo very slow decomposition and stabilization. More importantly, it will start to smell really bad! It’s also important to make sure the pile doesn’t get too dry. A dry pile will also stop the composting process and may even catch on fire.
If oxygen conditions are right, though (i.e., there’s about 60-70% moisture in the pile), microbes will start to decompose the organics. The pile will heat up to about 55 Celsius and should stay hot for several days or even a few weeks.
Heating, then curing
During this process, microbes in the pile will decompose a portion of the organic material all the way to carbon dioxide, but will also transform much of it to complex compounds typical of well-aged organic matter in soils. A stable compost will have gone through the thermophillic, or high-temperature, decomposition phase (this will also kill all pathogens and weed seeds) and then a slow stabilization or curing phase.
The resulting product will smell like a rich soil, and will provide a slow release source of nutrients for plants and a conditioner for soils. The critical parts of the composting process are the high temperature phase and the curing phase.
It’s also possible to get a product that’s very similar to compost by simply curing the same organic materials (i.e., without going through the thermophilic phase first) that are normally used to make compost in a controlled and monitored facility. In this case, curing will take many, many months, as opposed to weeks. But that is not true composting.
All compost is not the same
It’s important to know that not all composts are created equal. The nutrient value of compost will depend on the materials used to produce it. So, for example, yard waste composts made from a lot of branches in winter months will be very low in nutrient value.
In the summer, adding fresh leaves and grasses to the mix will produce a much more nutrient-rich compost. The nitrogen and phosphorus content of the compost should be available from the compost producer or listed on the bag for a bagged compost. In a compost that will be used to supply nutrients to plants, nitrogen should be between 1-3%, and phosphorus between 1-2%.
If you have a low-nutrient compost, it can be used as a mulch. Some composts, such as those made with food scraps and yard waste or municipal biosolids, will typically have higher nutrient values than a purely yard waste compost.
–Answered by Sally Brown, University of Washington