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Compost over concrete?

 
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I have had a compost heap... really almost more of a midden, for about five years now. It is a reasonable solution for biodegradable waste. I've had it in a corner of my garden that is not too visible. But it has grown over time and needs to be turned into a three stage compost system instead of a heap. My hope is to be able to manage it more carefully, get it hot enough to properly kill weed seeds, and make it easier to turn and use.

I've been thinking about new locations for it, and one candidate is a bare concrete slab. Are there any reasons to or not to put compost on top of concrete?

Up until now it has had earth underneath it, and I feel like the native worms had a field day with it because of that, but I suppose I'd be missing out on worm action if I go too hot anyway right?

I'm not much of an expert composter, but the 4 year heap produced some pretty nice soil for topping up my hugel beds last year.

Advice, warnings, words of wisdom welcome.
 
pollinator
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Worms will find it even on concrete it just takes them a bit longer. And yes if you manage to get it hot they won't move in until it cools down anyway.
 
master steward
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Plus even if it gets hot the worms would likely just move to the edges where the temperature makes them happy.  One possible issue would be seepage of compost juices.  Maybe you'd be able to collect them (Yay) or they'd become a problem of some sort (Boo).
 
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Compost juice does stain concrete brown,  which may or may not be a problem for your location.

I have composted successfully on concrete. The bugs and bacteria you need are everywhere  
 
pollinator
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I like the idea.  It would be easier to turn and kerp cleaned up.
 
gardener
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Is your compost pile active, where you turn it every couple of days?  Or is it passive -- pile it up and let it rot down over the long haul?  If it's active, you won't get much leaching of compost juice and the concrete will make it easier to turn the pile.  But if it's like my compost pile (a slow decomposer), you might be losing an important byproduct of your pile: the effluent that leaches from the pile over time.

One of the benefits of a compost pile is that any moisture that seeps through it is chock-full of nutrients and microbes.  If it's directly on the soil, it will feed the ground below.  Because of this, I keep my compost pile directly on the soil and regularly move it throughout the orchard over time.

I've got 4 fence panels that are about 4 feet wide each, so the enclosure made by the 4 panels is 4'x4'.  That's a nice size.  It takes quite a bit to fill it—perhaps 6 months or so—and then once it's full, I can pull the panels off the sides of the pile, move it somewhere else, and start a new one.  Why do I mention this?  Because over time, I've moved that enclosure all over and the compost juice that leaks through the pile gets distributed throughout the garden and orchard --- not a drop is wasted.

The soil is always richest where the compost pile used to be.
 
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The only think I can think to add is drainage. If you have heavy rains, will the compost become waterlogged or will the water drain off?
 
Lew Johnson
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Thanks everyone for the feedback on the idea.

Given a few of the things pointed out - stains, drainage, etc. I might opt for another spot if I can find one.
 
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As for the leaching, Joseph Jenkins author of the Humanure book, puts half a metre of sawdust on the bottom of his compost pile, it sucks up all the liquids before they can get to the ground.
 
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