Soil Temp Vs. Air Greenhouse

Air Temperature

• Circulating air through greenhouses keeps their temperature even at plant-growing heights. Lacking thermal mass, air heats and cools quickly. Three levels of air temperature determine plant production. The minimum level keeps plants warm enough to live but not grow. Optimum level allows plant growth. Maximum level is too hot for plant growth. Heating and cooling units and shade cloth are used to maintain optimum air temperature. Measure air temperature at plant levels.

Soil Temperature

• Soil is easier to warm and cool at a constant temperature than air. Plants that prefer cool soil are planted, watered well and then mulched when the soil is cool. Straw, pine straw and leaves are good mulch choices. Mulch acts as an insulator that protects soil from sunshine and high air temperature. It is also an insulator that will keep out the cold. Transplant warm-crop seedlings when the soil is warm, water well and apply mulch. If you must transplant seedlings when the soil is cool, don't mulch until after the soil warms.

Cooling Air and Soil

• Pollination stops when the air temperature of a greenhouse rises too much. To correct this, use fans in the end walls. One fan pulls cooler air in while the second blows hot air out. Placing the intake fan pulling cool air in close to ground level helps circulate air completely. Place the exhaust fan near the top of the greenhouse to blow out the hottest air. Measure air temperature at plant level in several places to be sure it is even throughout. Use mulch as insulation on cool soil to protect it from sunshine and warm air.

Warming Air and Soil

• Many gardeners use heating units such as a wood stove and propane or No. 2 fuel burning furnaces to maintain optimal air temperature. Many also use a fan attached to or near the unit to circulate the warm air.

  Soil is also warmed by sunshine hitting bare soil, dark mulch collecting and trapping additional warmth and heating cables below soil's surface. Use colored plastic mulches before planting to warm soil. When your soil has reached optimum temperature, cut in the mulch and the seed or seedling planted. You can also bury beneath the soil electric heating cables, which can maintain warmth without burning plant roots. Radiant heat from soil helps maintain a warmer air temperature immediately above the soil.