Soil temperature gauges or thermometers are the common way to take the reading. There are special soil temperature gauges used by farmers and soil sample companies, but you can just use a soil thermometer. In a perfect world, you would check nighttime temperatures to ensure they are not so cold your plant’s health will be impacted. Instead, check in the early morning for a good average. The night’s coolness is still mostly in the soil at this time. Soil readings for seeds are done in 1 to 2 inches (2.5-5 cm.) of soil. Sample at least 4 to 6 inches (10-15 cm.) deep for transplants. Insert the thermometer to the hilt, or maximum depth, and hold it for a minute. Do this for three consecutive days. Determining soil temperatures for a compost bin should also done in the morning. The bin should maintain at least 60 F. (16 C.) bacteria and organisms to do their work. The perfect temperature for planting varies is dependent upon the variety of vegetable or fruit. Planting before it is time can reduce fruit set, stunt plant growth and prevent or reduce seed germination. Plants such as tomatoes, cucumbers and snap peas benefit from soils at least 60 F (16 C.). Sweet corn, lima beans and some greens need 65 degrees F. (18 C.) Warmer temperatures into the 70’s (20’s C.) are required for watermelon, peppers, squash, and at the higher end, okra, cantaloupe and sweet potatoes. If you are in doubt, check your seed packet for ideal soil temperatures for planting. Most will list the month for your USDA zone. Somewhere between the minimum soil temperature for plant growth and the optimum temperature is the realistic soil temperature. For instance, plants with higher temperature needs, such as okra, have an optimum temperature of 90 F. (32 C.). However, healthy growth can be achieved when they are transplanted into soils of 75 F. (24 C). This happy medium is suitable for beginning plant growth with the assumption that optimum temperatures will occur as the season progresses. Plants set out in cool zones will benefit from late transplanting and raised beds, where soil temperatures warm up more quickly than ground level planting.