All crops use up minerals from the soil. A farmer cannot expect his soil to stay rich if he raises crops on it year after year and never puts any minerals back into it.
But how can a farmer put back into the soil the minerals his crops take out? He can use fertilizers. He can also follow a good plan of crop rotation. Crop rotation means changing crops in regular order.
Some crops take more of one material from the soil. Some take more of another. Any crop rotation is probably better than raising such a "heavy-feeding" crop as corn year after year. But to be good a crop rotation must add something to the soil.
Grass is often used in a crop rotation. Plowing it under adds what we call humus to the soil. Humus helps change minerals that plants cannot use into minerals that they can use.
Clover is an even better crop for building up soil. Clover has tiny bumps on its roots. In the bumps there are bacteria of a special kind. They take nitrogen from the air and change it to a mineral that the clover plant needs. If, after the clover seed has been harvested, clover plants are plowed under, this mineral is added to the soil. So is humus.
Clover is one of the plants called legumes. Alfalfa, sweet clover, and soybeans are legumes too. All legumes can be used to help keep soil fertile.
Crop rotation does more than keep soil from wearing out. It also helps in fighting weeds and insects. If wheat were raised in a field several years in a row, weeds that can grow easily in wheat fields and insects that eat wheat would get a better and better foothold. But if crops are changed often, such weeds and insects do not have as good a chance to get established.
