Plants grow by a very complex process. The leaves, which are above the ground, take in some of the nourishment that the plant needs by absorbing sunlight and turning it into chemicals. The roots, which are below the ground, soak up minerals dissolved in water, these minerals coming from the soil in which the roots grow. A plant never runs out of sunlight, but the soil does run out of chemicals. When this happens, healthy plants will no longer grow in it. There must be a waiting period in which the soil can enrich itself with more minerals of the kind the plant needs. So long ago that history does not go back so far, men learned that they must let the ground rest between crops.
The ancient Egyptians, nearly ten thousand years ago, let a field "lie fallow" every few years, which mean that they planted nothing in it or else planted a crop (such as alfalfa or another hay) that could grow without taking any important minerals out of the soil, and in fact would cause the soil to acquire new minerals to replenish the ones it had lost. Not long after that, farmers learned that various fertilizers (which were then generally called manure) would supply minerals to the soil and make it grow better plants. Finally, scientists began to apply their knowledge of chemistry to the problem and to analyze the soil to see what chemicals and minerals it needed and did not have. These missing chemicals and minerals, especially nitrates, carbon, iron, and others, they could feed to the soil so that the soil in turn could feed them into the roots of the plants.
One of the ways of preventing the soil from wearing out, and one that has been known for thousands of years but is still practiced, is called crop rotation. First a certain field is used to raise a crop that takes from the soil some of the minerals but not all. Then a crop is raised that does not take those same minerals, but does use the ones that were left from the year before, and that restores to the soil the minerals that were taken out by the previous crop. These missing minerals are restored because they are in the roots and stems of the plants that die, and rot, and become part of the soil. Rotation means "turning around," and the rotation of crops is a changing from one crop to another, then going back to the first one, so that the whole process goes around as though in a circle.
The farmer learned all these things, and he bought the best seed and the best fertilizer, and he worked very hard, and he still found his farm becoming less and less valuable. The reason, very often, was "erosion," which means "wearing away"; in spite of everything he did, the topsoil, which is the most important soil, was wearing or washing away from his farm. The topsoil is the layer of soft, black earth that lies on top of the harder, poor level of clay or packed sand that is called subsoil. The topsoil is rich in the stuff that life is made of. It is the level of soil that is soaked when it rains. Through this topsoil worms and insects burrow, both loosing it while they live and enriching it when they die. Just because it is on top, the topsoil is most likely to wash away in a heavy rain or blow away when it is dry and the winds are high, leaving for the farmer only the barren subsoil in which he can not grow anything of value. The problem of soil erosion became so severe that even in the rich United States, not more than thirty years ago, many farmers were unable to make a decent living because their soil was no good. It was one of the great problems of the country. The problem has now been solved to a very large extent. One way has been to cut level fields or territories into hilly country, so that when the water pours down from the hills