A handful of healthy soil contains more microorganisms than people on Earth. Amish don't know this (no microscopes). But they feel it.
Signs
Good soil: dark, crumbly, smells like forest, full of worms. Bad: gray, clumped, no smell, no worms. Amish farmer takes a handful, smells, squeezes, counts worms. Result: more accurate than any lab test.
Secret
Manure (10 tons/acre/year). Cover crops (winter clover — keeps soil covered). Minimum tillage (don't turn deeper than 6 inches). Rest (one field in four always 'sleeps').