Science and Engineering

Monitoring Compost Moisture

Composting proceeds best at a moisture content of 40-60% by weight. At lower moisture levels, microbial activity is limited. At higher levels, the process is likely to become anaerobic and foul-smelling.

When you are choosing and mixing your compost ingredients, you may wish to measure the moisture content. After the composting is underway, you probably don't need to repeat this measurement because you can observe whether appropriate moisture levels are being maintained.

If your compost starts to smell bad, chances are it's too wet. Excess water fills the pore spaces, impeding diffusion of oxygen through the compost materials and leading to anaerobic conditions. Mixing in additional bulking agent such as dry wood chips, cardboard pieces, or newspaper strips is likely to alleviate the problem. If you are composting in a bioreactor with drainage holes, you may notice leachate draining out. This liquid is often rich in nutrients and can be diluted for use on plants. You may find it useful to record the amount of leachate produced by each system, for comparison with initial moisture content, temperature curves, or other variables.

If you are blowing air through your compost system, you will need to be careful not to dry it out. If the temperature drops sooner than expected and the compost looks dry, moisture may have become the limiting factor. In this case try mixing in some water and see if the temperature rises again.


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