The crop is the receipt. The soil is the bank account. Most of farming reads the receipt every harvest and never looks at the balance.

The number that moves

Soil organic carbon is the balance. It builds in fractions of a percent a year, but it's the only number that tells you whether the farm is getting richer or poorer under your feet.

Why the receipt lies

A good season hides a dying field. Rain falls, prices hold, the yield looks fine — and the account quietly drains. You only notice when the bad year comes and there's nothing left to borrow from.