When nature is left alone, a balance is reached among the animals and plants living in one area. But when man starts his work in nature, the balance is likely to be destroyed. He grows a crop and takes it away to eat; then there are no dead leaves to fall on the ground, holding water while it sinks into the surface, or decaying  and adding humus  to the soil. Unless a farmer acts with knowledge and skill, he is therefore most likely to make the land poorer. To take the place of the useful matter in the crops that he removes, he uses some kind of fertilizer. Chemical fertilizers are of great help, but the waste products of animals and decaying remains of plants should also be put on the land. In some places, it is a habit to burn waste material lying about, but such burning destroys the useful matter in the dead plants. Although the ashes that are left are valuable when put on the land, a better practice is to bury the waste so that it decays and increases the humus in the soil. In the past, when the world population was much lower than it is now, a man had little difficulty in ordinary times in growing the food that was needed. When a field had been used some years and had become tired, the farmer could move to another place. The tired land then slowly recovered. Gradually grasses and other plants would appear on it and its productive power would slowly return to normal through their decay. But nature, left alone, would take a long time to bring back the land to its former state; the length of time required would depend on local conditions, but it might well be ten years.  It is a bad practice to grow the same crop in a field year after year. If the crop is changed, the land will suffer less because it is treated and used in a different way. Different plants have different effects on the soil. Therefore, a change of crop will do less harm than the growing of the same crop year after year and a regular change to grass will do good to the soil. Much will therefore be gained if different crops are grown one after another, a method known as the rotation  of crops. A modern farmer can hardly move to another place as he did before because   _  .
the productive power of a new field isn't higher than that of an old one there are few free fields left for him to do farming it takes a farmer more than ten years to start farming in a new field there will be too many grasses in a new field to grow crops