Nematodes are tiny, colorless roundworms that are sometimes used to kill insect pests, such as white grubs. Nematodes live in the soil and attach themselves to a host, killing the host from the inside out. However, some species of nematodes---the root knot nematode, for example---enjoy feasting on the roots of plants. These can include vegetables growing in a vegetable garden. If the roots are destroyed, the plants cannot get the nutrients they need. Nematodes can't be completely eliminated, but they can be controlled.
Follow the practice of fallowing. That means you must go one year without planting anything in your soil. You will need to keep the soil weed-free and the soil moist. This will keep nematodes from finding roots to eat, and nematode eggs will not hatch in the consistently moist soil.
Practice solarization in the months of July and August. To do this you must water your soil and cover it with a plastic tarp. According to the University of California, when the soil temperature reaches 125 degrees F the nematodes will die.
Plant crops that reduce nematode populations. Tomato, bean, barley, and wheat crops are a good choice for this purpose. These work to restore organic matter in the soil, and are less susceptible to nematode damage.
Add peat moss or composted manure to your soil. They contain organic materials that promote the growth of fungi that attack many species of nematodes. Composted manure also adds nutrients to your soil that will help your grass and plants grow healthy.