Organic Nitrogen Sources - What Do You Look For?

The first thing I look for is price. If I know the material is going to contain nitrogen and release it at some point in time, I want to find price. But I also want to find something that is sustainable, again, a terrible word,. regenerative, really, maybe a better word. To me, regenerative agriculture is just following the soil food web practices. The last thing in the world I want to do is buy something.