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Northwest Iowa Dairy Outlooks

A local discussion of current science and issues concerning dairying in northwest iowa

Soil temperature is a factor which few of us consider important enough to check before planting, yet it is probably the most important factor affecting seed germination, stand establishment and seedling growth.

The temperature of a soil is important as it affects how fast plants can grow and how quickly plants take up water and nutrients.

Because clay soils are cold, wet soils germination and seedling growth is usually slow. Wet soil will be cooler because it takes a lot of energy to heat water. Wet soils take longer to heat up in spring than soils that are well drained. Because sandy soils don’t contain much water but lots of air, they warm up quickly and are useful for growing early crops.

Soil temperature affects the speed of chemical reactions with warm temperatures speeding up reactions and colder ones slowing them down. Soil temperature affects the breakdown of parent material and how fast micro-organisms work. Both are important in adding and returning nutrients to the soil. Soil. Temperature is influenced by the climate of the area and the season of the year.

In addition, the slope of the land and the direction that it faces directly affects the temperature of a soil. South-facing slopes warm up faster in the spring than north-facing slopes

Soil covered in plants is somewhat insulated from fast heating or cooling of the soil . In a recently cultivated field, soil will heat up quickly during the day, but it will also lose heat quickly once the sun goes down. Also a dark colored soil with a lot of organic matter in it heats quickly as it absorbs more of the sun’s energy.

Crops such as beans, beets, cabbage, chard, eggplant, pepper, radish, tomato, turnip and corn have an optimum soil temperature for seed germination of 85 degrees F. Yet the minimum soil temperature required for certain of these cold-tolerant crops such as beets, cabbage, hard, radish and turnip is as low as 40 degrees F. The realistic soil temperature (degrees F) recommended for these crops are: beans (72), beets (45), cabbage (54), chard (54), eggplant (75), pepper (64), radish (45), tomato (55), turnip (50), and corn (55).

As might be expected, those vegetables which are really the cold weather champs such as carrots, parsley, lettuce and spinach have lower optimum soil temperatures for seed germination. For instance, the optimum soil temperature for seed germination of carrots is 80 degrees F, for parsley and lettuce is 75 degrees F, and for spinach is 70 degrees F. The minimum temperature required for these crops is 35 degrees F. The realistic soil temperature at which all of these crops should be planted is 45 degrees F.

All of these facts combined for us to add soil temperature measuring capability to the weather station we maintain at the Extension office here in Williamson County. Our weather station is part of the TexasET Network from the Irrigation Technology Program of Texas A&M Agrilife Extension Service.

All the Williamson County data can be found at: and the daily average soil temperature data is available at:

It will be able to be sorted for weekly or monthly statistics as the data is collected. If you have never visited the website, you can find maximum and minimum daily temperatures, average daily relative humidity, dew points, rain fall and wind data plus heat units. The true benefit of this data is that it is accurate and collected here in central Williamson County and not from a station located in another county.

For more information call the Extension office at 512.943.3300.



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