I am a farmer and about half my watershed for my main pond comes off my own crop land and nearly all the watershed for my old pond is crop land.
Here is what I did for my main pond. It was not something I had to do, but I do feel it was very beneficial.
Building a sediment pond And here is a thread where there are a lot of sediment and specialty ponds that might give you some ideas.
Sediment and other specialty ponds I've not had any problems with chemicals from my ag land (Corn and soybeans mostly) but at times there is some nutrient load that comes into the ponds. Nothing that I have found to be any serious problem.
