I think I agree that clay is better than sand, Neal - plants here in France need constant feeding and adding compost and other organic matter seems to make little difference. On the other hand (!) the soil in parts of the English garden that have not been heavily cultivated are so bad you could make pots out of it...
The Strulch (mineralised straw) website says 'added iron minerals', nothing more specific. Later it says, 'Strulch is biodegradable and will break down over time, adding valuable organic matter and nutrients to the soil. N – 0.6%, P2O5 – 0.2%, K2O – 1.6% percentage dry matter, plus calcium, magnesium and iron (0.75%) and organic acid preservative. Strulch encourages earthworm activity by acting as a long term food source for surface feeding species.'
We find it lasts for a couple of years, although we top it up annually in places, for a bank of hostas we have on a wooded slope (love hostas, hate hostas with lots of slug damage!) Here's a link to their website:
http://www.strulch.co.uk/about...
Since we started using it we haven't had to use any other slug/snail products.
Perhaps, since you already know about straw bale gardening (very interesting, that, I might give it a try!) you could chuck in some 'iron minerals' and make your own strulch?
