Some people think if they see steam from a compost pile then the compost is too hot and will damage plants, Find out the truth. What is steam? Most people think of steam coming from a pan of boiling water. So when steam is seen coming from a compost pile they think the compost pile must be extremely hot. Well "that ain't necessarily so". Steam appears if the compost pile is warm and moist and cold air collides with this rising warm moist air. Steam can rise from a compost pile even when compost temperatures are simply warm. Warm air rises from a compost pile because it is warm. Compost piles are moist so this warm air carries a lot of water. During the fall and winter months, cold air collides with this warm moist air and cools it. As warm air filled with moisture cools, it can no longer hold the same amount of water. Warm moist air cools and the moisture condenses into a "mist" we identify as "steam". This happens when the compost is only slightly warm at a little over the body temperature of 98F.
Figure 1. Steam coming from a warm compost pile at 105F (40C)
A simple way to see if it is too hot for plants is to stick your hand in it. If it is warm, it is somewhere around 100 to 105F. If it is cool, it is below 98F. If you cannot hold your hand in a compost pile for more than a couple of seconds it is too hot to use for plants. Our compost piles may be warm and moist, but not hot! The Composting Cycle and Temperature All of Viragrow's compost has finished the active phase of the composting cycle. During the composting cycle temperatures will reach 160° F or more during the active phase. It is important for compost to reach these temperatures to kill harmful human pathogens and weed seeds. It is important for compost to reach 160° F during the composting cycle but should be sold only when it has nearly completed the curing stage. The compost is "turned" or mixed as these temperatures are Figure 2. Compost thermometer measuring compost at a reached to regulate temperature and reintroduce good 3-foot depth. microorganisms back into the compost pile. Viragrow compost is not sold until it is safe to use. Viragrow compost is not sold until the temperature of the compost is safe for plants. This is somewhere around 100° F and dropping. We monitor compost temperature on a regular basis to make sure it is safe to use around plants. A professional soil temperature gauge is used and temperatures are monitored at a depth of 3 feet. When the temperature stabilizes after 5 to 10 minutes, it is recorded in our logbook. Compost temperatures are now posted on our website!