I take it that you are spreading manure then?
I used to see a lot of people spreading manure as a kid, and my family certainly did too, but that was when many people still had both livestock (beef cattle, dairy cattle, and hogs, sometimes chickens) and crops. The people back in the day were just as much getting rid of a waste product as they were applying fertilizer to their fields when they spread manure. But then many farmers went to either just row crops and no livestock, or just beef cattle and no row crops or other livestock. There are still a few hanging-on dairy operations around these parts, so there are still people spreading manure, but it's not many. That's pretty sad considering this state was the #1 dairy state in the whole nation 120 years ago.
I am pretty much stuck with "outside" fertilizer as I hay fields that livestock never go in to supplement and winter feed livestock in a pasture. It is a whole lot harder to collect scattered manure out of a pasture to spread it on a different field than it is to muck out a dairy barn, hog barn, or poultry house and spread it somewhere else. That and I sell hay too, and those nutrients just disappear.