I think an outside heap should be OK without lime but if you are asking about a wormery then you need to make sure it does not get too acid. If you notice lots of thin white worms on inside of the lid then it's getting too acid and you need to chuck in a handfull of lime (crushed limestone is best). It will take a month to get back to a good PH but the worms will appreciate the effort.

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Digger-07

"If you think you can, or think you can't, you're right" Henry Ford.