A great advantage of crop rotation comes from the interrelationship of nitrogen fixing - crops with nitrogen demanding crops. Legumes, like alfalfa and clover, collect available nitrogen from the soil in nodules on their root structure. When the plant is harvested, the biomass of uncollected roots breaks down, making the stored nitrogen available to future crops. Legumes are also a valued green manure: a crop that collects nutrients and fixes them at soil depths accessible to future crops.
what crops were discovered that could replenish the nutrients of the soil
Legumes, like alfalfa and clover