Text by Dr. Ed Brotak
Home gardeners and farmers alike all know how important soil conditions are for a successful crop, be it flowers or corn. But we don’t typically think about soil temperature. Why is soil temperature important? It affects plant growth. If it’s too cold or too hot, plants won’t grow well, if at all. Soil nutrients and useful soil organisms have optimal soil temperatures. Soil moisture and aeration also relate to temperature. But soil temperature is at its most important in the critical early stages of plant life, the germination of the seed and the development of the seedling before it breaks the surface. If the soil is too cold, the seed may not germinate; even if it does, growth may be permanently affected.
What Affects Soil Temperature
We often just assume soil temperatures mirror the air temperature. They don’t. There are different processes involved in how heat is moved. Let’s look at a sunny day. Most of the sun’s rays pass through the atmosphere and hit the earth’s surface, thus heating it. The air is heated from below and with hot air rising (convection), the heat is moved upward quickly. Obviously, that can’t happen below the ground. The heat is only taken slowly downward by the process of conduction. To illustrate how heat works its way down: For a typical soil, the peak temperature at a depth of 2 inches will occur about an hour after the peak surface temperature and it will be 30 percent less. The maximum temperature at 4 inches down occurs 2 or 3 hours later and is 66 percent less. The opposite happens at night. The surface radiates heat out into space and cools rapidly. The air next to the ground is now cooled from below and air temperatures fall. But below the surface, heat travels only slowly upward. Therefore, subsurface soil temperatures remain the same. In winter, the slow loss of heat from the ground helps prevent freezing temperatures from penetrating too deep into the soil. Thus, the root systems of perennials are protected. Interestingly, a snow cover on the surface is actually protective. The layer of snow insulates the ground below from the often extreme air temperatures which can get below 0oF in northern areas.
Soil moisture, the amount of water in the soil, also affects soil temperature. Water has a high heat capacity—it can absorb a lot of heat without changing temperature. Also, when water evaporates, it uses energy or heat. In the spring, wet soils will take longer to warm up. Soil moisture content is strongly related to soil type. Sandy soils drain well. Their surface heats up a lot during the day and cools off rapidly at night. Clay soils hold water; their temperatures change slowly. Especially in the spring, clay soils tend to be too cold. The best soils (for temperature and moisture) are loams, mixtures of soil types.
Why Soil Temperature Matters
Spring planting time is when soil temperatures are most critical. When do you put those seeds into the ground? That depends on what you’re planting. Different plants require various soil temperatures. And there’s more to consider once you’ve determined what you’re planting. For every type of plant, there is a minimum temperature for seeds to germinate and an optimum temperature for seed germination. For example, a cool-season vegetable like lettuce can germinate with soil temperatures just above freezing, but the best temperature for germination is 75oF. If you plant lettuce seeds with soil temperatures in the 30s, they’ll develop slowly, with emergence taking over a month. Wait until the soil warms up some—at least into the 40s—and everything speeds up. But don’t wait until the soil temperature hits 70oF, because even though that’s great for seed germination, the warm summer temperatures would hinder the subsequent plant growth.
Fortunately, all the information you need to know about when to plant various flowers and vegetables is readily available. Check on the back of seed packs or in seed catalogs, or online from a variety of sources including local agricultural agencies and extension services. Do keep in mind that even though soil temperatures may be warm enough for germination and growth, you may have to still worry about air temperatures getting below freezing. This holds especially true in an abnormally warm late winter or early spring.
Measuring soil temperature isn’t that difficult or expensive. Simple soil thermometers are available at many garden or farm supply shops and online. You can get one for under 10 dollars. Stick the thermometer down into the soil as deep as you would plant the seeds. There are also various websites you can check to see local soil temperatures. Check with your state agricultural service or your State Climatologist Office.
How to Increase Soil Temperature
There are various ways to change soil temperature, especially in spring, to help new seeds and plants find success. The simplest is to just use the sun’s heating power. South-facing gardens and fields will get the most sun. Certainly avoid shaded areas.
Covering the ground with a mulch is a typical way of raising temperatures of cold soils. A plastic sheet, either clear or lightly tinted black, will allow the sun’s rays to heat the soil and then hold that heat in. Studies have shown that during the day this can raise soil temperature 5 degrees F at a depth of 2 inches and 3 degrees F at 4 inches. The same covering prevents radiative heat loss at night. The cover will also help prevent weeds and water loss from the soil.
You can do things to the soil itself. Light tilling will break up the soil and help promote excess water drainage. Changing the soil consistency by mixing in other soil types can help. You can use raised beds, which run warmer.
And for some warm-season plants like tomatoes, you can start them inside and just transplant them when the soil is warmer and the threat of frost is past.
Dr. Ed Brotak is a retired meteorologist who lives and gardens in North Carolina.
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