You can tell a lot by how your plot looks. It is a learning process, being able to spot, identify disease and nutrient problems. I have a book with good color pictures of typical diseases and nutrient deficiencies.
I usually mix my plots, if one plant doesn't do well I have a backup. I mix by sections, the good sections usually take over eventually. I call it my shotgun approach.
I've never been able to keep a Clover plot going for more than few years. I eventually have to plow and reseed. Either it dies off or something undesirable takes over. Likely the varieties I get here.
Most of my plots need extra iron (and fixer) eventually. Not something you'd normally be able to spot without a little research.