You can till the moss and lawn right into the soil in early spring, add lots of compost or aged manure - at least 6 inches on top of the soil then worked into the soil and then plant. Are you sure you have round worms or just assuming? If you must grow carrots or potatoes that are eaten by these worms I suggest you make raised beds, bring in good compost and topsoil and plant in these beds rather than try and kill the worms in your existing soil. You can also try growing crops in pots to avoid the worms. In cool climates growing crops is raised beds is more productive then tilling up the lawn and planting in existing soil. Keep growing, Marianne Binetti