Soil temperatures should remain within the same range as optimum air temps. For vigorous, healthy growth, moderate temps between 70 and 80 degrees are best. Absolute minimums and maximums are difficult to pinpoint, as all cannabis grows differently under different circumstances. Some strains can withstand a mild frost or two before dying, some can withstand the heat of the tropics, but constant, extreme temperatures will slow growth significantly. Soil temperature becomes a problem when it gets too hot, as is sometimes the case when compost that has not been properly decomposed is added to the soil. When compost rots, heat is given off during the process. This can cause problems to the roots of a plant. Make sure that any compost you use has been fully decomposed before you add it to your soil.