Plants grow best in a certain range of temperatures. Most plants stop growing at soil temperatures under 50 degrees. Temperature also affects the speed of chemical reactions. Higher temperatures provoke more rapid soil reactions, up to a point. For example, some organic fertilizers release nutrients only at relatively warm temperatures. On the other hand, the availability of chemical fertilizers is only slightly reduced at low soil temperatures. Also, the beneficial effects of soil organisms occur only in warm soil.
Because soil temperature depends on sunlight, it fluctuates according to season and climate. Other factors that influence soil temperature are described below, with those easiest for the gardener to manipulate being listed first:
Mulch acts as an insulation. It keeps heat from being absorbed during the day and released at night. It keeps the soil cool in the summer and warm in the winter. The surface of the soil can reach 120 degrees under hot summer sun, much too hot for plant roots to grow. A mulch cools the soil, allowing roots to grow right to the surface.
In the winter, mulch delays the soil’s freezing and thawing. This is important in preventing heaving, the loosening of soil that results from alternate freezing and thawing. To allow soil to warm early in spring, remove the mulch when the weather warms.
Loose soil contains air spaces that insulate the soil, acting a little like a much. Compact soil heats faster in the sun and cools faster at night than cultivated soil.
Water in the soil conducts heat rapidly. Dry surface soil insulates the soil below. Water the soil in the spring to help the day’s heat be conducted to the depths.
Since the sun is the source of most soil heat, shading the soil keeps it cooler during the day. You can shade soil with ground covers or other plants, or with structures.
The direction the soil faces influences the soil temperature. North-facing slopes are coolest. East, west, and south-facing slopes follow in order of coolness.
The soil can also be used as a source of heat to keep plants from freezing in light frosts. Damp, compacted soil with a clean surface radiates the most heat to plants above it. When protecting plants from early frosts, cover them with fabric or cardboard and clean the soil under them of mulch and plants. This allows heat to be stored in the soil during the day and radiated to the plants and cover at night.