A criticism of vegan advocacy is that moving away from animal agriculture implies the health of the soil will inevitably deteriorate. I argue there may need to be a transition period needed for crop farmers to learn new and relearn old techniques. Here is a talk on the how farmers may maintain and enhance soil health and thence crop quality without either animal manure or pesticides, and indeed covers the subject or reducing dependency on animal manure etc.
youtube.com/watch?v=qKMf5Z7...
The talk is by David R. Montgomery who is a MacArthur Fellow and professor of geomorphology (the study of the physical features of the surface of the earth and their relation to its geological structures) at the University of Washington.
His recipe for fertile soil is "ditch the plough, cover up & grow diversity."
If you don't like watching videos he has a website Dig2Grow.com.