1. Be choosy. Always look before you buy. If the manure seems to be made up of dry shavings, do not use it in your deep beds but it can be used to quite good effect on the paths as a weed suppresser. This dry stuff comes form stables that are cleaned out very often. Take a look around for more suitable material.
2. Ask the stables owner if any other gardeners use their horse manure; if so try and do a deal with them to get some on a regular bases.
3. Unless the manure is obviously well rotted, -a nice dark colour, with individual shavings not particularly noticeable-do not use it immediately. Best is to heap it up under a plastic sheet and leave it for a good six months.
4. There is a way to increase the nitrogen content and that is to mix it with grass cutting while heaping it up to rot. Add water or diluted urine.
5. If you see, compost worms in the heap that is a sure sign that it is useable.
6. If in doubt, do use with caution. The best is to try it first as a surface mulch on perennial borders where it will not be dug in, adding first a dressing of hoof and horn or some other organic sources of nitrogen, if the soil is poor. It could be also used as mulch on the vegetable path. Before digging it over a large area, test a small patch and then grow mustard on it, with an untreated patch of mustard for comparison. The growth of the mustard should soon indicate any problems.
The best thing to do, in my view with any shaving based manures; is too heap it up for a year to make sure all of the wood bits have rotted down. The problem with wood chipping and shaving is that if they are dug into the soil without composting they will eat up all the nitrogen in the soil. This happens because the shavings are very high in carbon and contain very little nitrogen; if they are added to the soil without sufficient nitrogen, the microbes rob the soil of its available nitrogen so that they can get to work on the shavings.
This weeks Amozon gardening deals: