There are various things you can use. A pumice stone is really good for shifting stubborn bits and lemon juice in water will do the same as your pal's bleach. It will also help to whiten and condition your nails.
The main problem though, is that you gets cracks and suchlike in your hands and nails, especially when gardening. This is where the dirt harbours. So having got rid of it you need to help your hands mend. Rub them in olive oil every night and you could wear a pair of cotton gloves to protect your bedding (sexy huh?). Make sure your nails are trimmed and cut off (don't rip) any snags. Should be lovely in a week or two.
AND WEAR THOSE GARDENING GLOVES - sorry for shouting
