Soil temperature regulates seed germination, root growth and the availability of nutrients. Soil temperatures range from permafrost at a few inches below the surface to 38°C (100°F) in Hawaii on a warm day. The colour of the ground cover and its insulating ability have a strong influence on soil temperature. Snow cover will reflect light and heavy mulching will slow the warming of the soil, but at the same time they will reduce the fluctuations in the surface temperature.
Below 50 cm (20 in), soil temperature seldom changes and can be approximated by adding 1.8°C (2°F) to the mean annual air temperature.
Most often, soil temperatures must be accepted and agricultural activities adapted to them to:
- maximize germination and growth by timing of planting
- optimise use of anhydrous ammonia by applying to soil below 10°C (50°F)
- prevent heaving and thawing due to frosts from damaging shallow-rooted crops
- prevent damage to desirable soil structure by freezing of saturated soils
- improve uptake of phosphorus by plants
Otherwise soil temperatures can be raised by drying soils or the use of clear plastic mulches. Organic mulches slow the warming of the soil.