hi chilly, manure works better if left for a year or so to allow it to break down allowing the nutrients for the plant to feed on, and dont burn roots if planted on really fresh manure.
question:- was the horse manure straw or wood shavings based, as these will work differently, fresh straw will take out nitrogen when breaking down and woodshaving will take about two year to break down,
if your worried about the pH of the soil, do a check, and a good scale for a neutral growing is 5-7 on the meter,
the general rule for adding manure, the more you can add the better the ground will like it this will allow the root to travel through it easier, and if you can double dig your soil, as much as you can, this will help drainage, you'll find after a few years youll have more worm in the soil, add mulches around your plant to help with water saving in summer time, stick to a crop rotation,