During the last 50 years, farmers have widely used chemicals to killnematodes, microscopic soil worms that feed on plant roots and cause tremendous yield losses. However, because of environmental concerns, many nematicides have been banned. The most common nematicide in the world, methyl bromide, faces almost certain U.S. prohibition in the year 2000 because it has been implicated in ozone depletion. "Losing those pesticides is giving us the opportunity to look more critically at the beneficial effects of some nematodes," says UC Davis nematologist Howard Ferris. By combatting harmful nematodes with crop rotation and resistant plant varieties, farmers should benefit from nematodes that feed on bacteria and fungi, according to Ferris' research. Since the beneficial nematodes take in more nitrogen than they need when feeding, they excrete nitrogen, making the nutrient available to the plant. "In the past few decades we've treated soil as a medium for holding up plants. We add water and nutrients and the plants grow very well," Ferris says. "Now we're beginning to see that the more abundant the bacterial and fungal nematodes are in the soil, very often the more fertile the soil is."