[Mr Fishies]

would it work as a first layer or would it be to rich in organic matter?

This is something I find interesting and puzzling...

Although I have heard this discussion before, I've never seen any conclusive evidence to say yes castings are too rich or that they should not be used. I'm not sure why soil that has passed through a worm and had nutrients extracted from it (which is after all why the worms eat soil unless I am mistaken about their diet) would be more rich than the soil was before.

I suppose it depends on how rich whatever the worms are eating is - does anyone know what a typical commercial castings farm might "feed" their worms? Is it always fresh compost? I'm wishing I paid more attention to the episode of Dirty Jobs where they went to the worm farm!

 

[Mr Fishies]

Cow, horse, and pig manures contain a lot of organics and are dangerous to use in a NPT or anywhere else. I suspect that earthworm manure ("castings") are similar.

Bill

That was my first instinct as well - but farm animal manure, is basically pure waste. When you think about what worms are eating (bits of organics they find mixed in with soil) they are using up organic matter (as they add body mass it has to come from somewhere) but some of the soil is also passing through them, they are essentially further composting the soil they are living in unless I misunderstand things.

For example, this page discusses worm castings and soil and (right at the bottom) says:

"The total mineral balance is not increased by worm activity. Rather the conversion changes the amount of nutrient available to the plant. The minerals have been changed form an insoluble form to a plant - available, soluble form."

This sure sounds a lot like some of what might be happening if you soak and dry your soil a few times like AaronT indicates in the library article on DIY mineralization of soil.