I applied about 4 1/2 yards of horse manure to the north half of my garden in the Fall of 2009.
When you say this I have to wonder if your manure was mixed with bedding, and what kind of bedding? I know lots of people use sawdust or wood shavings in their stalls and the resultant manure pile is mostly wood. Sometimes straw is used for bedding. I think the straw may be better on the garden than the sawdust.
In any case, manure along with other added organic matter is an excellent soil builder. Yes, it has been used in commercial potato production with good results.
There is a point though where too much manure can make a plot too hot and nothing will grow well there. I have seen that on several occasions. One neighbor who thought he was doing his plot a favor really pile on the manure. Nothing would grow. It was too hot. The man lost one years production.
The term "HOT" may be misleading, but is what the farmers here call the situation. What actually happens is the disolved components in the water in the soil gets denser than the solultion in the cells, so instead of the plants taking up water, the opposite is true and the water moves from the plant tissues to the soil. It is a natural thing for the solutions to try to be balanced. In most cases seed cannot even germinate under these conditions. Bagged fertilizers like ammonium nitrate can also easily be overdone, and especially in containers. Fertilizer is good, but be careful and wise in its use. Organic matter and manure are probably the best fertilizers.
I have also observed a corn field that had too much manure put on it. The corn did germinate, but looked sickly all summer. A light green color and many days it wilted for want of water. It only grew about half the usual height for field corn. The yield was way down. In this case a little would have been good, but too much caused problems and actually reduced production. The next year the farmer did not put manure on the field and the crop did well. It took one season to fix the problem.
Manures are good because they contain organic matter and also ammonia which is high in fixed nitrogen.