Roundworms, or nematodes, are a common pest in the garden soil. They are unsegmented, unlike the common ground worm, and unlike the common ground worm, not all are beneficial. Some nematode round worms feed on the roots of plants, causing the plants to wilt and show signs of drought. Plant nematodes are microscopic, requiring special equipment to see them in the soil. Nematodes need to eliminated from the soil to prevent extensive damage.
Isolate the area that has the nematodes and do not plant new crops there. Nematodes spread with water and along plant roots.
Rotate the crops in your garden each season, or every other season, to reduce the spread of nematodes, says North Carolina State University.
Add 1 to 3 inches of organic matter to the soil and till it in each year to increase the organic matter content. According to North Carolina State University, organic matter decreases nematode populations by promoting good drainage.
Allow an area to lay fallow for a two-year period if there is a bad nematode infestation. Nematodes will not survive in fallow soil.
Add crab meal to the soil at a 10 to 20 percent ration to control nematode population. Add crab meal to the soil before planting.
Cover empty soil in marigold seeds and allow them to grow. According to North Carolina State University, marigolds decrease nematodes by 90 percent.