How to determine moisture content of compost

  • By: composthq
  • Date: October 1, 2022
  • Time to read: 5 min.

How vital is moisture to compost?

All life needs water, even microbes. Microbes are everywhere: inside our bodies, in our houses, and your compost.

They are what break down your greens (grass clippings and yard waste), food waste, and browns (cardboard, paper, and sawdust).

So, the question is there: what are the effects of moisture in the decomposition of compost? Water has a lot to do with the breakdown of organic waste in compost.

Too much, and it stinks and slows. Too little moisture, and it stops.

Should you have moisture in your compost heap?

It’s a good idea to water your compost pile as you build it. If the moisture content becomes too low, lower than thirty percent, the microbes stop being active.

They continue to slow down until they start to die. Drying out can happen in areas with arid, hot summers.

On the flip side of that compost coin, too much water is a big problem for a compost heap.

Not only does it cause a foul smell and slow down decomposition in a pile, but it can also leach nutrients right out of the compost, which is supposed to nourish your plants.

How much moisture do you want in your compost pile?

The perfect moisture content for a compost pile is around 40% to 60%. The idea is to have a thin film of water on all the compost particles but not fill the air spaces around them.

The air spaces, or pores, allow water to circulate through the pile.

The lower range of moisture in a compost pile is 40% to 45%. The compost absorbs moisture through food, which is very wet, and green waste, which is wet as well.

It’s ok if there’s some rain at this point; the pile is ready for it.

You want to increase the compost pile’s moisture content to 55% to 60% at the beginning of the hot and dry season.

It prepares the mound for the evaporation that is coming. The optimum moisture level during the main composting period is 50% to 55%.

You want there to be less moisture in the compost during the maturation period. It needs to be lighter to make quality potting soil and seed starter for your garden.

Let it get dry, but not too dry. Excellent moist soil for your plants is what you want.

What happens when there’s too much moisture in compost?

When there’s too much moisture in the compost, there’s no airflow. Our microbes need oxygen to thrive.

Without our air-loving (aerobic) microbes, the anaerobic microbes take over. The compost pile gets slimy and starts to stink.

This usually means there’s over 60% moisture in the compost heap, too much for our aerobic microbes to work.

Water displaces too much air, and nutrients leach out of the compost that could be going into your plants. Anaerobic microbes dominate the matter.

What to mix with your compost to manage moisture

When you add food scraps, you need to know that they are wet, and you should mix them with something that’s considered bulk, like sawdust, woodchips, or wood pellets.

Adding bulk to food scraps will make a better mix with the moisture content you want.

Food is high in nitrogen. You want to get the right carbon to nitrogen ratio in your compost pile.

When you add food, mix it with something that has a lot of carbon in it to get that ratio just right in your compost.

Turning your compost frequently is essential to keep those aerobic microbes breaking down the scraps, brown, and green waste at an optimum level.

Mix some good browns, such as cardboard, paper, chopped straw, or woodchips with grass clippings to keep your compost from becoming too wet.

What to do with a dry compost heap

A dry compost pile tends to heat and cool rather quickly, especially with the summer days’ fluctuating temperatures. Unfortunately, this can give it the possible ability to combust spontaneously.

When the heap is dry, the pile is cool because the bacteria can’t do their job and heat the compost.

The activity of the microbes stops at less than 15% moisture in the compost pile. There are a few solutions to this problem you can try.

One thing you can try to do is to add green waste to the compost pile. Greens add moisture. Or you could add water to it and turn the compost to get it started again.

You may have too many browns in your pile and need to establish balance again. Browns take up a lot of moisture.

When you live in a dry, hot area, it can be a problem because it can regularly dry out your compost pile. You could try keeping your compost in a pit rather than a bin that’s in the sun.

Or you can keep your compost covered to stop the constant evaporation of precious moisture.

Testing for moisture in the compost

You can do an easy test for moisture in your compost pile.

  • Reach 18″ to 24″ into the pile with a gloved hand and grab a handful.
  • Squeeze it in your fist.
  • If it stays compressed and acts like a wet sponge, putting off very little water, the moisture should be between 40% to 60%

You can’t do this test with worms, as you may squeeze and crush them.

  • If the compost makes a ball but doesn’t stay in that shape when you open your hand – there’s around 40% to 45% moisture.
  • If the compost makes a ball when you squeeze it, but it falls apart when you tap it with a knuckle from the other hand – 45% to 50%.
  • If the ball stays together and you can’t see water on your hands – 50% to 55%.
  • If the ball stays together, and you can see moisture but not droplets on your skin – 55% to 60%.
  • Some few drops of water come out when you squeeze the ball, and you can see water between your fingers – 60%.
  • Compost is wet, and water runs between your fingers when you squeeze it – 65%.

A fantastic article from the “For Dummies” series on moisture and composting is right here.

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