The Importance of Checking Soil Temperature

eggplant harvest off of three plants for one day

eggplant harvest off of three plants for one day

A lot of people understand that when there is a hard frost, many of the less hardy plants in the garden are likely to die. What fewer people understand is that soil temperature is just as important as air temperature in keeping the plants alive. This can be confusing because a frost is caused by a combination of air temperature and humidity. With the emphasis that even weather stations put on frost reports and warnings, it would seem that soil temperature is unimportant. This would be an incorrect assumption.

For most garden plants to flourish, they require a healthy root system. If the soil temperature is too cold, the roots die or struggle to develop. This is the reason that plants may continue to grow in the fall even after they’ve been ‘singed’; that is, a light frost has occurred and perhaps killed a number of the leaves. If the soil is still warm and the roots are still in good shape, the plant continues to grow.

Taken a step further, have you noticed how some plant seeds will lay on or in the ground, not germinating even when the air temperature becomes quite warm? Many seeds in this situation will actually rot, rather than germinating. This is because of the ground or soil temperature. If it isn’t warm enough, the delicate roots of seedlings won’t survive, so the seeds don’t germinate. Some seeds won’t germinate until the soil temperatures get substantially above what we might consider to be warm.

There is a great example here in the Rocky Mountains. In the winter, it isn’t uncommon for the ground to freeze solid to a depth of a foot or more. When increased sunlight and warm air comes in the early spring, the surface begins to warm up and thaw out, however the ground can still be frozen a few inches below the surface. Mother Nature knows this isn’t the time for some plants to start to grow. It takes longer for the soil that is a few inches below the surface to get warm than it does for the top layer of dirt to warm up. Thus, if I planted green beans right now, in the middle of March, it would be extraordinary for any of them to germinate. They will simply rot. Soil temperatures should be at least 50 F, and preferably closer to 60 F, before I plant green beans.

Last year, many area farmers planted eggplant seedlings in early May and they never did get a good harvest. The reason is again soil temperatures. By early May, we’d had several days that had air temperatures above 80 F. However, the ground hadn’t warmed up. Eggplants require even greater heat for the roots than green beans. In fact, though they will bloom at lower temperatures, eggplant fruits won’t set until the overnight low temperatures and the soil temperature stabilize at above 65 F. The fact is that in early May last year, the soil temperatures had barely reached 55 F.

In contrast, I planted eggplants in mid June, and despite how this abbreviated the growing season, I had a bumper crop. By that time, the soil temperature was above 65 F and the eggplants thrived. The picture is a single day’s harvest off of three plants. We had nine plants that could be harvested every three days, so this is how many eggplant fruits we were getting daily, for over a month.

It isn’t hard to check the temperature of the soil, either. There are thermometers that are specifically designed to do this. For instance, this one is available at Mountain Man’s Affiliate Store:

Notice that this looks a lot like a meat thermometer. In a pinch, a meat thermometer will work, but it is better to use one that is specifically designed for soil temperature readings. Using it is straightforward, though. Just stick it at least three or four inches deep, let it set for about five minutes, then check the temperature. Since different areas of the garden and flowerbeds usually vary in temperature, it is a good idea to take readings in more than one location.

If you plan on growing plants year after year, it is also worthwhile to invest in a garden journal. There is one that can be downloaded from Arbico Organics for free and which can then be printed out, or you can make your own. The worth of a garden journal will be covered in another article, but you can keep track of soil temperature in such a journal, which eliminates a lot of the guess-work about when to start checking the soil temperatures, year after year.

Soil temperature can make the difference between a bumper crop, a crop that struggles or seeds that don’t even germinate if planted outdoors. It is worthwhile to pay attention to the soil temperature.




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