For the best success for this upcoming garden season, we can’t just blindly follow the planting dates on the back of seed packs to decide when to plant; we need to get down and dirty and get our soils to open up and say “aaahhh” and take it’s temperature so we know when is the best time to sow our seeds based upon the crops we want to grow.
Why should we care about soil temperature?
Soil temperature is critical when it comes to the germination success of the variety of seeds you may be planning to be sowing in your garden.
Soil temperature also affects plants root development and distribution, soil biological activity and nutrient uptake by roots.
Crops like cabbage, kale, onion, peas, lettuce, spinach, turnips and potatoes will germinate when soil temperatures are in the 35 to high 40 degree range and are referred to as “cool season crops”.
However, “warm season crops” like tomatoes, peppers, corn, squash and cucumbers will require temperatures closer to 60 degrees and above before seed will germinate.
For specific seed germination requirements for cool and warm season vegetables please consult the Colorado State University Extension Garden Note #720 for specifics.
What affects the temperature of the soil?
Soil moisture content, soil texture, surface cover, aspect and elevation have the greatest influence on soil temperature.
Soil moisture content is the amount of water in the soil pore spaces, and is influenced by the available water holding capacity (AWC) of the soil. Water has a high heat capacity, meaning that it can absorb a lot of heat without changing temperature. Air, on the other hand, has a much lower heat capacity.
Soils with their pore spaces filled with water will be cooler than drier soils that have their pore spaces filled with air. In the spring, wet soils will take longer to warm than drier soils.
All things being the same, clayey soils will tend to be colder than sandy or loamy soils due to a higher AWC, and a higher soil moisture content.
Potting soils or soils with lots of decomposed organic matter also have a high AWC and tend to be colder than sandy or loamy soils.
Sandy soils with little or no organic matter will be your warmest soils in the garden environment, as they have the lowest AWC of all soil textures.
The greatest variation in soil temperature occurs at the soil surface and becomes more stable at lower soil depths.
Soil within 3 or 4 feet of south facing buildings, fences, garden walls or other garden features that soak up heat will be warmer than soil further away from the heat sink.
South facing slopes in full sun will always be warmer because the angle of the sun hits them more directly.
A good way to determine where the warm spots are in your yard is to watch where the first new green weeds and grass show up in the next few weeks.
How Do You Measure Soil Temperature?
Although you can purchase a soil thermometer at you local garden store, a stainless steel kitchen meat thermometer will also work well, with the instant read digital output models that are at least 6 inches long a good affordable choice.
Glass thermometers should not be used, as they can be easily broken trying to insert them into the soil.
Soil temperatures should be taken between 8:00 AM and 11:00 AM by inserting a thermometer 4 inches deep into the soil surface. After the temperature has stabilized, and after noting the temperature, remove the thermometer from the soil.
Why a depth of 4 inches at 8:00 AM you ask? This is a depth and time in which the soil is in equilibrium between the colder nighttime temperatures and the warmer daytime temperatures.
If you have difficulty inserting the thermometer, you can use a screwdriver to make a small pilot hole, and then insert the thermometer
Different areas of the garden can have different soil temperatures, so be sure to take measurements at several locations.
Soil temperatures should be consistent for several days before seeds are sown to ensure that the seeds are being exposed to optimal temperatures for germination.
Garden Management Techniques to Manipulate Soil Temperature
1. You can apply a thin sheet of black polyethylene sheet on the surface of the soil or on top your garden beds, which will absorb heat from the sun during the day and prevent heat loss during the night.
2. Add a darker mulch or organic matter and compost to the surface of the soil, which will absorb more sunlight and increase the soil temperature.
3. Gently cultivating the soil in the spring will help aerate your soil and allows excess soil moisture to evaporate, warming the soil.
Remember: Even if soil temperatures are warm enough for germination and growth, you should still be concerned about frost damage to tender sprouts, so be prepared after germination to protect your young plants.