Worms are almost unbelievably good at making the soil perfect for plants. As they eat their way through the soil in search of organic matter, they concentrate plant nutrients in their castings and build a soil structure that is perfect for roots. Worms are continually turning subsoil into topsoil.
Is there anything we can do to help worms? The answer from Ed Berry, an entomologist at the National Soil Tilth Lab in Ames, Iowa, is a definite yes. First, reduce your tillage, he says. Soils that remain untilled have three to four times as many nightcrawlers (surface-feeding worms) per square yard as soils that are tilled in spring and fall. The reason is not so much that tillage actually destroys worms as that it greatly accelerates the breakdown of the organic matter that worms need.
Second, apply mulches like grass clippings or leaves. Berry and his associates found that those mulches produced double the worm population that corn residues did. With a good food source and favorable soil conditions, farm fields in Berry's study had as many as 100 night-crawlers per square yard. On these soils, two inches of water (a very heavy downpour) could be absorbed in just 12 minutes. Soils without worms required 12 hours to absorb that much water.