Horse Manure needs to be composted before it can be applied to trees or plants. If the manure doesn't have time to break down it will "burn" the tree through dehydration because it is too Nitrogen rich. Cow manure is better but it too needs some time to break down. Depending on the tree you are looking to fertilize, ( chestnuts / oaks / pear / apple ) I'd look at other organic fertilizers.