Jim S wrote:
Sorry Rob I have never heard that one.
I used mustard as a green manure mainly to increase my humus levels.
If you put MUSTARD GREEN MANURE in Google you will find several agricultural
sites, here and in the US. It is an old technique which seems to be revived
for environmental reasons.

It's always been used as a green manure but no good at fixing nitrogen
in the soil. Also it grows quite tall. I've never used it as a green
manure - I prefer red clover and facelia - they are prettier and seeds
less freely.