I've thought about pre-warming the soil, and plan to do that this year. My neighbor is a market gardener. He puts down the drip tape and black plastic a week or two before transplanting tomatoes, squash, peppers and eggplant. I helped him last year, and the soil was nicely warmed under the plastic. My knees were cold as I was kneeling on the ground beside the plastic. The soil under the plastic felt warm. I'd guess it made at least 10 degrees difference. This year I will remember to take my soil thermometer to test it. We had a light frost a couple of nights after planting. A few leaves were nipped, but it didn't faze the plants. I planted some of his plants in my ground which had not been pre-warmed, then put used plastic around them. Mine sat, his grew. His tomatoes ripened at least 2 weeks ahead of mine. Our growing season is about 120 days with cool nights. Last frost is usually about June 1, and first frost sometime in early October.
