TECHNICAL CORNER
MONITORING MOISTURE IN COMPOSTING 

FIRST POSTED:  January 2002

By Craig Coker, Vice President
Mountain Organic Materials

Moisture is one of the five key elements of good composting that must be managed (along with C/N ratio, porosity, time and temperature). Too much moisture, and the composting pile can become anaerobic as water fills the pore spaces meant to convey air. Too little moisture, and the microorganisms that make composting work become dehydrated and die off (if there is even enough moisture in the pile to encourage their growth to begin with).  

What is acceptable moisture content? Most references indicate an initial moisture content of 50-60% is preferred, and the final product should have a moisture content of 35-40%. Moisture content is traditionally determined by the “squeeze test”, a non-quantitative method of estimating moisture. In this test, a handful of the material is squeezed together in the fist. If water actively drips out while the compost (and/or raw mix) is squeezed, the material is too wet. If the material does not release water, and crumbles apart when the fist is opened, it is too dry. By this test, only if the material does not release water and stays compacted when the fist is opened, is the moisture content “just right”. 

More accurate means of measuring moisture content exist. The most frequently used method is a “gravimetric” procedure of weighing samples before and after the water is removed. The basic concept of this procedure is: A sample of the material is weighed to determine its wet weight (minus the weight of the container). Then the sample is dried to the point where all or nearly all of the water is evaporated and the sample is weighed again. This weight (minus the weight of the container) is the dry weight. The difference between the wet and dry weights is the weight of the water that the sample originally contained. Dividing the water weight by the wet weight gives the moisture content as a fraction. This type of calculation is known as the “wet basis”, or: 

Percent Moisture = [100% x water weight (grams)] / wet weight (grams) 

The primary pitfall in this gravimetric procedure is the loss of volatile compounds in the compost during drying. This can be minimized by the drying procedure used. Drying methods include air drying, oven drying at various temperatures, and microwaves. Microwaves are the quickest and most convenient but also the least accurate. Air drying is slow but most accurate if the material is not actively decomposing. Oven drying is the middle ground and tends to be the standard. 

The EPA standard method for determining moisture content is to dry the sample in a vented oven at 103 to 105^o C (217 to 221^oF) until the sample stops losing weight. The method adopted by the U.S. Composting Council’s Test Methods for the Examination of Composting and Compost requires oven drying at 65 to 75^o C (149 to 167^o F) until no change in weight is observed. As the temperature range in the EPA method is above the boiling point of water, drying is relatively quick (1-2 hours). At the TMECC-recommended procedure, drying may take one or two days. The advantage of the lower drying temperature is to minimize the loss of volatile compounds.  

Determining moisture content for the purpose of managing the composting process (as opposed to regulatory reporting of metals concentrations, for example) does not require great accuracy and precision. Therefore, drying samples in a microwave oven is usually good enough, in addition to being fast and convenient. Drying time depends on the power of the microwave, the size of the sample, and the initial moisture content. As an example, starting with a 600-watt microwave (or a more powerful one set at a lower “cooking” level) and a sample of 100 grams (3.5 ounces), heat wet samples (50-80% moisture) for six to eight minutes and dry samples (20-40% moisture) for four to six minutes. Remove the sample, weigh it and place it back in the microwave oven, rotating its position 90 degrees (unless the microwave has a rotating platform). Heat the sample again for two minutes, weigh it, and return it to the oven, rotating it another 90 degrees, in the same direction. Repeat the procedure at one-minute heating intervals until the weight change is less than one gram. Scales used for weighing samples should be accurate to 0.1 gram.  

Moisture sensors are now available for purchase that measure some quickly-measurable property of the material, such as electrical conductivity, electrical resistance, or the dielectric constant. One drawback to these tools is the influence of soluble salt levels on electrical characteristics of the material (with the exception of Time Domain Reflectometry, which measures the dielectric constant). Another drawback is that precision and repeatability are not as good as they are with standard gravimetric procedures. The key advantage of these instruments is that they allow quick determination of moisture variation within and across a compost pile or stored finished compost product. 

Any of these procedures will help the composter understand and manage one of the most important elements of composting.