Many seeds and plants have guides that often say something about checking soil temperature, and it's probably the most accurate way to tell if your little plot of land is ready. Now, I know what you're thinking - "Check my soil temperature? Like I'd ever do that!" But in reality it's actually pretty easy! There are soil thermometers sold at garden centers for pretty cheap but I've found that using a simple candy thermometer also works great! You can pick up one of these for a few dollars at the grocery store. Insert the thermometer all the way into your soil & leave it for a few minutes. Check the soil at around midday for the most accurate reading, and continue checking it for a few days in a row and then average the readings. (Remember that the soil in potted plants, like trash-can potatoes, is going to fluctuate in temperature more than the soil on the ground will.)
-Plants that germinate in the coolest soils (down to 40 degrees) include arugula, fava beans, kale, lettuce, pak choi, parsnips, peas, radicchio, radishes and spinach.These temperatures are the lowest that seeds will begin to germinate in but keep in mind that it is only if the temperature continues to rise. To a certain degree, a little warmth is only going to help in your seed success. If your soil temperature is hitting right at 40 degrees it's probably best to let it warm a little more. In those first few warm days of spring it may be wise to spend a little more time planning before planting to ensure a plentiful harvest.
-When soil temperature goes over 50 degrees, plant Chinese cabbage, leeks, onions, Swiss chard and turnips.
-After the temperature gets to 60 degrees, warm-season and many cool-season vegetables can be sown, including beans, beets, broccoli, brussels sprouts, cabbage, carrots, cauliflower, kale, leeks, lettuce, many of the Asian greens, onions, parsnips, peas, radicchio, radishes, spinach, Swiss chard and turnips.
-Wait until soil warms to 70 degrees or more to plant tomatoes, eggplant, peppers, cucumbers, squash, corn and melons.*
Soil 101 - Part 2: Ph Levels - coming soon!
*planting guide taken from "Soil temperature and veggies: How to check before planting" By KYM POKORNY - The Oregonian 3/07