Modern agriculture has led to decline of essential organic matter in cultivated soil
"Organic matter is vital to healthy soils, yet most modern agricultural operations are not managed in ways to retain high levels. Only half the original organic matter remains in most modern cultivated soils. In general, organic matter levels have fallen from 5-6 percent of the soil to less than 3 percent on most cropland soils."
Source: USDA, NRCS, “Healthy Soils Are: High in Organic Matter,” 1.