Good organic matter, whether this is spent mushroom compost, compost from your compost bins, well rotted animal muck, leaf mould... In other words anything you have that will help to enrich the soil.

thanks moon growe would some well rooted horse manure mixed with some compost be ok

Well rotted horse manure is ideal - along with everything else Mg mentioned above.

Yup well rotted horse manure mixed with compost is fine - so long as it isn't bedding or potting compost i.e. your own compost is fine :)

thanks everyone for answers