I just started composting (again). I added a bunch of greens to a big bin, and then I added some soil.

In the meanwhile, I was uprooting a bunch of grass for a vegetable bed, and without thinking, I threw a bunch of clay/grass into the compost bin. Actually, it's now half full of heavy clay.

Is this a mistake -- will this prohibit or inhibit the decomposition? Should I remove it? It's terribly difficult to mix it as is.

Also, I saw a couple of worms in the compost bin. In the future, should I add them if I see them, or leave them in the real soil? (My bin is covered.)

  • I've found that mixing scraps that could be composted in with our partial clay soil causes them to decompose quickly. If you could mix the clay and the compost together, rather than putting one on top, it might be a good thing. People often add compost to clay (I've never heard of the other way around, but I've thought about it, on account of the observation I mentioned). Commented Sep 8, 2017 at 9:36

4 Answers 4


The clay itself isn't going to break down due to decomposition because it is made up primarily of inorganic material - minerals and such.

If you plan on doing the lazy compost method, where you pile organic material up and forget about it, then it won't much matter if there's clay other than the fact that you'll have a lot of clay in that pile. Eventually, the organic material will break down, though you may find that weed seeds prosper in that environment because it is unlikely to heat up enough to kill them off. That's a big mess to deal with. Not worth the long-term hassle in my opinion.

If you plan on do the actively-involved compost method, where you turn the pile and ensure it is moist and encourage aerobic decomposition, then if there's a lot of clay, you may grow to hate turning the pile and it will likely revert back to the "lazy compost" pile that doesn't heat up much. And there you are with a pile of clay and eventually some rich material veins running through it (and likely weeds).

On our farm, I've thrown clay clods along with weeds and turf into a pile along with horse manure and it has heated up just fine but I'd still hate to turn that by hand. I pick it up with the tractor bucket and toss it that way (it's easy when you can just scoop it up and dump it with machinery) but in my compost bins that are made of pallets I avoid putting clay in it if I can because I actively turn these with my pitchfork and as much as I love a farm workout, clay is (as you said) heavy and I lose my gusto for the turning.

My recommendation is to remove as much of the clay as you can from the pile. It'll break down much faster that way. It will be a pain to deal with it once rather than a pain to deal with multiple times.

With regard to the worms - if it heats up considerably (which is what you want), they will move away from that heat down into the cooler areas. I'm assuming your bin is on the ground and isn't one of those tumbler types. If it is a tumbler, the worms will die if it happens to heat up enough and they have nowhere to go.

  • It's a tumbler, unfortunately. I'll leave them out next time.
    – ashes999
    Commented May 24, 2013 at 18:52
  • 1
    I'd definitely remove the clay from a tumbler. It'll only get harder to tumble later.
    – itsmatt
    Commented May 24, 2013 at 18:59
  • Sorry, my bad. It's not a tumbler, but it's an enclosed space like a giant garbage can. I will definitely remove the clay.
    – ashes999
    Commented May 24, 2013 at 19:24
  • 2
    On the subject of worms, you don't need to add them - you may have seen earthworms which came in with the clay soil, but brandlings are the worms found in compost heaps, and as long as your heap is in contact with the ground, brandlings will appear in there.
    – Bamboo
    Commented May 24, 2013 at 21:43
  • A little bit of soil isn't bad for compost. Helps keep good organisms in the compost pile.
    – Dano0430
    Commented Sep 5, 2017 at 15:18

Clay in a compost can be good. Really good. Never in the form of clods or massed around the roots of a plant though. Clay holds on to the nutrients a plant needs really well. Thats why sandy soils need regular feeding but heavier ones keep pumping longer. If you wet all your compost ingredients with water which has clay suspended in it then the compost may take a little longer to "cook" itself but when it's done you'll be very impressed with the end product. Your compost will be well on the way to becoming humus. Much of the organic carbon and nutrients will have formed complex bonds with the clay [stuck to it] and in that form will last for decades in the soil, keeping that carbon in the soil instead of in the atmosphere, which is where it ends up otherwise. Clay used in this way makes your compost into a long term soil improver. Look for a clay which shrinks a lot when it dries right out, so that cracks appear. Dig some up. break up the lumps as small as you easily can and throw them in a drum with water.. leave soak a couple of days then mix it up with a hoe and a spade or tread it underfoot! Whatever it takes, And wet all your ingredients with it. Easy


I purify my clay by adding water, allowing to stand a little bit then taking the suppernatent portion and allowing it to settle in a seperate container. Then allowing it to dry out thoroughly and screening it through a very fine screen. It can then be sprinkeld as a fine powder and mixed easily.


I've been hot composting on the ground, and have huge numbers of worms around the cool outside of the pile and at the ground contact areas. I started adding clay to my compost piles because I had heaps of it that I needed to turn into rich organic garden soil. As I flip the the compost piles, the worms chew it up on the outside and it gets thoroughly broken up and mixed with the compost. In my case, I was going to have to mix the finished compost with clay anyway; composting it really seems to mix it until it is indistinguishable from the final soil. This is just my personal observation; hope it is helpful.

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