I suppose if you have a very heavy/clay soil you could but they wont add anything to the actual soil and will compress at some point.

I wouldn't. There's enough plastic polluting the planet already. I can just imagine the mess with all the bits spreading themselves round your garden while you try to work them into the soil. If you manage to get them buried they will get to the top eventually and blow away, getting who knows where.

perhaps you could use them as drainage at the bottom of some outdoor pots?

I definitely wouldn't do that - they'll be out of the bottom of the pots and blowing around PDQ. My MIL used to use broken up polystyrene egg boxes for drainage in pots. They don't break down at all and are lighter than crocks, but they really annoy me when I repot plants that were hers.

Realistically there is nothing you can do with them except put them in your wheelie bin...

secure in a bin bag

Indeed!

Hi there,

I think they are sprayed with a flame retardant, and also an inhalation threat, I sniffed one up by accident once whilst re-using the beans to make draught excluders. I thought my sinuses were going to burst!!!!!!

Happy gardening