You can try heating or solarizing the soil by covering it with clear plastic that is 1ml to 3 ml thick and letting the summer sun beat on it for four weeks. This must be done in June through August so that the sun is strong enough to heat and kill the pathogens. To kill nematodes you need to heat the soil to at least 120 degrees. A soil thermometer like the type used for compost piles can measure the temp by pulling off the plastic, inserting into the soil and reading the results after an hour. Be sure to recover the pile and trap the heat so that it stays hot for 4 weeks. After you solarize the soil you should add beneficial pathogens back to the soil with a mulch of composted manure - as long as you are sure the compost is free of nematodes. You may also want to replace the old soil with a soil less potting soil mix if solarizing is not practical. Keep growing, Marianne Binetti