Smell Testing Compost

The vermiculture and composting stations were the first two learning stations to come online at our flagship Edible Learning Lab in Wyoming.  I plowed through the lessons in those two curricula quickly as the hot composting and vermicomposting processes unfolded before our very eyes.

The students were eager to step outside their comfort zones, to try new things and to be daring.  Afterall, everything about these processes was new to them.

But there was an ah-ha moment one afternoon.  I described the composting process to the students and guided them through the science that would produce various smells as a result.  I informed them that their sense of smell could be useful in determining whether or not the compost was finished and described the characteristics to look for when smelling compost.  We gathered around the worm bins, removed the lids, and each carefully cupped a handful of worm castings.  Each student slowly raised the sample to their noses, hesitantly took in that first breath, and before I could comment, one of the students screamed, “Wait!  This is worm poop!”

It’s hard to recover from that!

How to Perform the Test

This test is very simple.  Take a sample of the compost in your hand and give it a good sniff.  Your nose will tell you what you need to know.

What Completed Compost Smells Like

Earthy. Woodsy. Rich. These are the words that we use to describe what finished compost should smell like.  Compost that has completed the natural process will drop in temperature and have a smell similar to what you might expect from rich organic soil that you would find on the forest floor.

What Unbalanced Compost Smells Like

Sour. Sharp. Ammonia. When compost is breaking down and the bacteria is hard at work there will be a smell.  However, that smell should not be all that unpleasant. It’s the “bad” smells that indicate that the balance between browns and greens is off.

Why Smells are Created

If airflow is restricted or the compost pile has not been aerated or turned enough the process may switch from aerobic to anaerobic, producing a sour smell.  This indicates that the bacterial process has shifted from good to bad.  It’s the presence of the bad bacteria which thrive in environments where oxygen is absent that produce the smell.  When large amounts of nitrogen-rich inputs are used, the smell of ammonia can often be detected as well.

How to Remedy It

It will happen.  You will open the door on your Earth Cube or catch a whiff from the compost pile as you approach.  Your compost is not balanced or perhaps needs more oxygen. The good news is that the aerobic bacteria, once reestablished, will eat the bad compounds responsible for the smell and bring your compost back into a proper balance.  Often, it’s just as easy as simply adding some additional carbon material like leaves or wood chips to reduce the moisture a bit and balance out the C:N ratio.