Sunday, 6 October 2019

Compost Heaps: How (not) to Water Them

The other day, I wrote about retrieving a compost pen that was not rotting, and regular commenter Mal *waves excitedly* commented as follows:

"As for watering in of each layer, I would just have run a hose over the finished pile and let gravity and capillary action take it from there."

Now, this is a really interesting point, and a very common misconception about how to manage a compost heap.

(Brace yourselves for relentless self-publicity) I do know a fair amount about composting, in fact I have actually written a book about it:

 (Two books for the price of one! Not just compost, folks, but leaf mold as well!! End of relentless self-publicity.)

... and in this book, one of my earliest points is that more compost heaps fail through being too dry, than ever fail through being too wet.

And this is a point to be made, because it is actually very difficult to re-wet a compost heap which is too dry.

There's a fairly simple reason why: generally speaking, compost heaps are "too dry" because they have too much grass in them - grass clippings from the lawn, and long grass that's been pulled up, or scythed.

Why should grass be such a problem? Cast your minds, for a moment, dear readers, towards thatching, which has been the standard roofing material for hundreds of years, and which is still perfectly serviceable today.

A thatched roof keeps out the rain. We all know this to be true (as long as the thatch is in good condition, of course).

What is "thatch" made of? Answer: grass.

OK, technically, it's straw, or reed, but basically they are all types of grass. So what conclusion can we draw from this? Grass is Waterproof.

I would now draw your attention to Exhibit A, m'lud: the photo on the earlier article about the thick layer of compressed grass clippings at the bottom of that failed compost heap. Above this layer was three planks-worth of long grass scythed material, all loose and airy.

The material in this layer of the compost pen is bone dry and pale grey in colour.

You can see there are two stripes of much darker colour: that's where I earlier emptied about five buckets of water onto the heap. That water went straight through the loose long grass, not even dampening it, then stopped dead when it hit this layer.

It has not spread out evenly through the heap.

It has not been forced by gravity to soak the entire heap: it came to an abrupt halt at this point, and most  of it went out the sides of the pen.

It has not been forced by capillary action to work its way around all the material: no, it's made two damp splodges but the rest of the heap is bone dry, and I can assure you that the "damp" layer was minimal in depth - underneath, it was all bone dry.

Here's another example, from another garden:

This one was a non-rotting heap which I'd been asked to sort out, and when I delved into it, there was a thick layer of grass clippings from a year or more earlier. They were bone dry and compressed almost into chipboard.

You can see where I've levered them apart with my daisy grubber: they de-laminated into hard "plates" of compressed grass.

(Honestly, there was a time about 15 years ago when I seriously considered creating a business to manufacture eco plant pots from compressed lawn clippings...) 

So the point is that merely bunging some water on the top of the heap does not work, if the lower levels of the heap contain thick undisturbed layers of grass.

Nor does it work at all, if the heap is indeed a "heap", and is more or less conical in shape: all my Clients and all my trainees will tell you that I am always banging on about spreading out the material within the compost pen, never leaving corners unfilled, and ensuring there is a central depression so that all water/rain which falls upon the pen, stays within the pen, and doesn't run uselessly off the top and out the sides.

So there you have it: the importance of dampening down your compost pens as you fill them, to ensure that there are no pockets of dry material at a lower level, which would prevent water getting through, and as we all know, worms don't have teeth, so they are completely uninterested in bone-dry garden waste.

Rats, on the other hand...... 


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  1. I stand corrected. In mitigation I use the high fibre method relying on brandling worms to do my mixing.

  2. I use the "three-pen, no mess, no fuss, no faff" method, and I certainly rely on brandlings to do all the hard work for me!

  3. "I seriously considered creating a business to manufacture eco plant pots from compressed lawn clippings.."

    The egg box I bought today is "50% rye grass" so someone is making money from grass!

    1. Yay! I thought it was a good idea! I just didn't have time to develop the idea further - I did a few trial runs, and decided that it needed either mechanised presses (not very eco) or an awful lot of moulds!


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