This area of gardening needs more input! Why old manure from the farm works so well in your garden may have alot to do with the 100's of works that you are adding to your garden! Should you should gather worms from anywhere you can find them and add them to your raised beds? Yes we should. The $3 box of fishing red worms at wal mart may be a better buy for your garden than a sack of potting soil!
+++
After a rain I go at the edge of the woods and gather up a bucket of leaves and stems from the woods floor. When I go through the mix I find dozens of small babyworms that I add to my seed starting mixes and cold frames! when you dig you garden and see los of worms you will have a better garden!. Why do most farm grounds have lots of worms when plowed?
+++
The reason to me is three fold manure from usually cows or spreaded. The roots of the old corn stocks or even weeds being plowed under that feeds the worms! The third reason is airating the soil by plowing but the plow does not disturb the layering like a tiller does! When you get board go pick some worms and add to your garden or beds they are worth their weight in gold!!