Soil scientists have shown that the soil teems with millions of living things, many of them useful, others harmful. The living things which are useful include earthworms and various kinds of bacteria. Earthworms loosen the soil and so enable air and water to enter it. Bacteria, which are microscopic living things break down dead plants and animals and make humus, or take nitrogen from the air and change it into substances that plants use. The living things that do harm include other bacteria and fungi which cause diseases. Other harmful things are pests such as wire worms which feed on the roots of grass and other plants. While the farmer can usually keep weeds in check by careful cultivation, this alone may not protect his crops from insects, pests and diseases. Nowadays, however, he is much better able to control these enemies. He may plant specially resistant types of seeds or he may keep the pests and diseases in check with chemicals. With better seeds farmers have been able to increase their crop yields. They can grow crops that ripen more quickly and have a stronger resistance to disease, frost or drought. The living things that do harm