Have you thought of increasing the soil temperature to give a head start to your seeds before spring comes, to protect young seedlings on cold days, or just to protect your garden from frost?
You can increase the soil temperature by covering the ground using plastic sheets or mulch, a low tunnel, a cold frame, a “wall of water” around the seedlings, a propagator, an electric blanket, or simply planting in south-facing locations for direct sunlight.
In this article, we look at the factors that affect soil temperature and the practical ways to increase it.
Before we look into the ways of increasing soil temperature, it is important to first understand the factors affecting it.
1.1 Air temperature
Naturally, you would think that the soil is warm when the air is warm, and vice versa. But the relationship is not so straightforward.
In spring and summer, the soil typically has a lower temperature than the air. This is because the soil takes longer to absorb heat, even though the soil temperature will rise as air temperature.
In winter, the reverse happens: Soil temperature is higher than the air temperature. That’s because the soils have a larger heat-retaining capacity, allowing them to store up the heat from sunny summer days (Longstroth, 2012). Additionally, the ground is insulated by the snow, vegetation, and topsoil, making the deeper sections warmer than the top layers.
You can think of the relationship between air and soil temperature as a highway, a “heat highway.” In the summer heat moves down through the soil, but in the winter the heat rises through the ground instead.
1.2 Soil texture, composition, and structure
The soil texture, or the amount of sand, silt, clay in the soil, can also affect soil temperature.
Generally, sandy soils both heat up and cool faster than clay soils. Sandy soils heat up and cool faster because they don’t hold water, and water takes longer to heat, and it also takes time to cool (U.S Geological Survey, 2019).
In terms of soil structure or how soil particles are arranged in the soil profile, granular (or crumbly) soils warm faster than other soil structures because they aren’t waterlogged.
1.3 Soil color
Soils come in a wide range of colors like black, gray, yellow, brown, red, white, and even green.
The darker soils like black, brown, and gray absorb more heat from the sun, becoming hotter than the lighter yellow and white soils.
1.4 Soil moisture
The moisture content in soil can affect how it warms up and how long it stays warm.
Wet soil warms up more slowly compared to dry soil because latent heat is needed to dry wet soil. It takes more energy to raise the temperature of 1 kilo of wet soil (0.35 cal/gram) by 1 degree than 1 kilo of dry soil (0.19 cal/gram). One reason for this is
1.5 Slope of the land
Soil temperature decreases as the slope increases.
The amount of sunlight that reaches a sloping land is scattered over a wider area compared to the same amount of energy that hits a level ground. As a result, the sloping soil will have a lower temperature than the flat land.
1.6 Vegetative cover
Crops and vegetation act as insulators. They limit the amount of heat that gets in and out of the soil. Soils with vegetation don’t get too hot in the summer or too cold in the winter as bare soil would.
In addition to the vegetation (and snow in winter), each layer of soil is insulated by the one above it. So the temperature increases as you go deeper.
However, the temperature increase isn’t continuous. Between 30 to 200 feet (9-61 meters) below the surface, the soil temperature is relatively constant, around 55 Fahrenheit (13 Celcius).
2.1 Cover with plastic or mulch
Cover the soil and the irrigation lines with a piece of plastic or organic mulch like straw or bark chips can help increase soil temperature. The plastic and mulch act like vegetation covering, keeping heat from escaping the soil and preventing cold rain and late snow from getting to the soil.
The most common type of plastic mulching is black plastic. It can also be “rows of covers” which is a light, semi-transparent and porous fabric that allows sunlight and water to pass through it while keeping the soil moderately insulated from the cold. They can increase the soil temperature by 3 to 5 °F (1.7 to 2.8 °C).
Plastic sheets, especially black ones, are more effective in raising soil temperatures than organic mulches, as they can absorb more heat.
Plastic mulch is easy to use. You simply spread the plastic over the land and hold it down at the sides with a heavy object like a brick. Also, push out any air trapped underneath the plastic.
2.2 Build a cold frame or low tunnel
Low tunnels and cold frames are insulating structures. Like plastic mulching, they protect the soil from cold rains, cold breezes, and snow. At the same time, they prevent heat from escaping, thus creating a mini-greenhouse over the crops.
Low tunnels are typically 4 feet tall and 3 – 4 feet wide. They can be easily constructed by staking a series of metal or PVC hoops in the ground and covered with a waterproof membrane. They’re suited for ridges because of their long and tunnel-like structure.
Similarly, cold frames are low square or rectangular structures typically made from wood, with a transparent top of glass or plastic sheeting to allow sunlight but keep heat from escaping. The top can also be opened to provide ventilation during the daytime or when it is warm.
Cold frames can easily be constructed by adding a transparent lid to an existing raised bed.
2.3 Use Wall O’ Water
A Wall o’ Water or season extender is a tubular structure made of thick plastic sectioned into smaller cells filled with water.
The water absorbs the heat from the sun during the day and releases it to the seedlings at night.
You place it over the plants and fill the individual cells with water. This allows it to stand upright and creates a barrier around the plant, shielding it from the cold and preventing heat from escaping. The water will also absorb some heat in the daytime and release it to the plant at night.
Wall o’ Water
2.4 Construct a Propagator
A propagator can be a polystyrene box, a raised bed, soil heating cables, a shower curtain, or other enclosure.
It is a heated enclosed environment that warms the growing media so that you can germinate seeds even when outside temperatures aren’t ideal.
It provides a head start, giving you up to a month of growth before spring comes so that there’s a longer growing season and enough time for seed vegetables like cucumbers, tomatoes, and pumpkins to produce.
Here’s how to put them together:
- Rake the soil in the raised bed to make sure it is level.
- Cover the soil with polystyrene sheets. These will serve as insulators to keep the heat from leaching into the ground.
- Place a waterproof membrane like a shower curtain over the polystyrene sheet to serve as a water barrier
- Cover the membrane in about 2 inches (5 cm) of sand
- Level the sand
- Starting at the edge, lay the cable over the sand making several lines 3 inches (8 cm) apart
- Use fencing staples to hold down each line of cable
- Cover the cables with another 2-inch layer of sand
- Add little water to the sand, just enough to make it moist. Water conducts heat well
- Level the sand, and arrange your seedling trays and pots over it
- Connect the thermostat, and turn it on. The thermostat allows you to control the heat so you can give your seeds just the right amount of heat they need to germinate and grow.
2.5 Use an electric blanket
You can also make a propagator using electric blankets. This method is a cheaper alternative to the heating cable method, as it doesn’t use a thermostat. Instead, you can use an old electric blanket to provide the heat.
You will need a raised bed, waterproof membrane, an old electric blanket, and polystyrene sheets.
- Level the ground in the raised bed
- Cover the ground with the polystyrene foam
- Place the electric blanket over the foam
- Cover the blanket with waterproof material. The material will prevent water from the plants from getting to the blanket and causing damage.
- Set the blanket to medium heat, and place your plant trays over the propagator.
2.6 “Window sill” technique
The window sill technique is an effective way indoor to take advantage of the sunlight from windows and the warmth in the house to grow seeds and seedlings 2 or 4 weeks before spring.
This method works with a window sill propagator, which is a small rectangular container that sits on the window sill. It has several cells to grow different seeds and a transparent lid to let in sunlight, warmth, and air, too, if need be.
Window sill propagator
2.7 Grow in south-facing locations
South-facing window sills or gardens receive the most sunlight than the other orientations in the northern hemisphere.
This is because in the northern hemisphere, the sun always travels along the eastern horizon, through the southern sky at noon, and sets in the west.
Naturally, more sunlight hitting the soil means it will be warmer. And, you won’t need to do much to raise the soil temperature to ideal levels for warm-season vegetables like tomatoes and pepper.
The most effective way to increase soil temperature is by covering it with plastic mulch when the air temperature falls below 45 Fahrenheit (7 Celcius). This method is cost-effective and easy to install.
A “wall of water” is equally inexpensive, easy to install, and lasts for long. You can use it in combination with the plastic mulch. The plastic covers the soil while the wall of water protects the plants from cold air or snow.
Weber State University (n.d.). Understanding Astronomy: The Sun and the Seasons.
Michigan State University Extension. (2015). Warm-season vegetables to grow during summer. MSU Extension.
Onwuka, B., & Mang, B. (2018, January 12). Effects of soil temperature on some soil properties and plant growth. Advances in Plants & Agriculture Research.
Snyder, K., Murray, C., Grant, A., & Wolff, B. (2015). The Effects of Plastic Mulch Systems on Soil Temperature and Moisture in Central Ontario. Research Gate.
Longstroth, M. (2012). Moist, weed-free soil retains more heat. Michigan State University Extension.
Irmark, S. (2016, June). Impacts of Extreme Heat Stress and Increased Soil Temperature on. CropWatch- University of Nebraska.
Journey North. (n.d.). Journey North Tulip Garden and Climate Change Study.