The pros are that organic matter dug in will break up the soil particles in heavy clay, and aid drainage, and with sandy soil it will help retain moisture around the roots.
The cons are that it is not a natural way of improving soil. Most organic matter falls on to the surface of the soil and is incorporated by worms and other burrowing creatures. As it further breaks down the nitrogen, minerals and other nutrients will be available for the surface feeding roots of the plant which are the fine roots which are barely visible. Large roots beneath the soil are for taking up moisture not feeding.
The con of surface feeding is that you cover the soil making seed sowing difficult.
Personally I always apply my compost and manure on the surface, so either mulch around plants once they have grown or plant through the mulch.
A surface mulch also helps retain moisture in the plants beneath. I also believe it deters things like cabbage root fly (though doesn't prevent it).
The only crop where it's really worth putting lots of rotted manure deep down is for runner beans which really need to have lots of moisture around their roots.

B's right, and onions won't appreciate a very heavily manured soil. Worthy

Many thanks to Bertiefox & Worthy. I need all the help I can get, I'm new to all this so I,ll follow your advice and
just turn it over, plant up then mulch it
Grewsome Bill.

watch out for pests in some horse manure to .

Well answered all

your welcome oldsparky