Soil temperature is just as critical to plant growth as sunshine and fertilizer. To test the soil temperature, push the probe of a soil thermometer 4" to 6" into the soil. Note the temperature that displays on the gauge. Do this in a number of locations in the garden to get a good range of results.
Low-lying spots and those subject to flooding make poor garden sites. To determine how well a potential site will drain water, it is wise to perform a percolation test. Using a spade, dig several 12"-deep holes around the site of the proposed garden (Image 1). Using a garden hose, fill all holes with water (Image 2). Allow the water to drain out completely. Refill the holes with water, this time measuring with a watch how quickly they drain (Image 3).
Minimum soil temperatures for seed germination is around 45 degrees Fahrenheit. The optimum range for cold weather crop seed germination is between 45 and 85 degrees. The optimum range for warm weather crop seed germination is between 65 and 85 degrees. Garden soil should drain at a rate of 1" to 2" per hour. Any slower or faster and it may be necessary to amend the soil or move to a more suitable spot.