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Soil Temperature Rules, So Early Planting is Not Always Best

 Daniel E. Mullins
IFAS Extension Agent IV
Santa Rosa County Extension
6263 Dogwood Drive
Milton, FL   32570

Profitable vegetable production begins with the prompt germination and early, uninterrupted growth from seeds.  If a seedling slows down even once during germination and emergence, it is in trouble.  The results of this condition are long term, resulting in poor stands and reduced yield potential.
Stem disease on a vegetable seedling. This condition is often the result of planting when the soil temperature is too low. Photo Credits: Daniel E. Mullins

Since the rate of seed germination, emergence and seedling growth is largely controlled by soil temperature, the producer should choose planting times very carefully.  Though vegetables vary somewhat in the soil temperature range for best germination and growth, the optimum range for most warm season crops is 70 to 90 degrees F., with the minimum being approximately 60 degrees F.  Obviously, the cool season or winter vegetable species have lower optimum temperature requirements. 

The late spring growing season during 2010 serves to show the impact of soil temperatures.  Using our IFAS FAWN system, a quick check of archived weather data shows that the soil temperature did not reach 60 degrees F. in central Santa Rosa County until March 28.  The optimum temperature of 70 degrees F. and above, was not reached until April 25.

The result of the cold spring last year obviously had its effects.  Many growers who planted at the usual time last year (early to mid-March) experienced poor stands or found it necessary to replant.

Each spring is of course different, with some years allowing for earlier planting success than others.  In 2009 the minimum acceptable soil temperature for germination occurred on March 9, while it was again April 25 before the soil warmed to the optimum temperature range.  Records further show that 2008 was similar, but 2007 allowed for an early start.  The minimum temperature that year was reached on March 3, while optimum soil temperatures began stabilizing in early April.

The best way to monitor current soil temperatures, as well as long term trends is through the use of the FAWN weather system.  Become familiar with it and you can also keep current on rainfall amounts, air temperature, monitor chill units  and use many other features.  It’s fast and easy.

For example, following are the steps that I took to learn about local soil temperatures over the past few years.  First, to get on the system, go to:  http://fawn.ifas.ufl.edu/  A map is displayed with several locations around the state of Florida.  Click on the one nearest your location or farm.  You will then see a page showing current conditions which are recorded every 15 minutes.  These include soil temperature, air temperature, wind speed and direction, relative humidity and solar radiation.  There is a menu that also allows you to mine archived weather data and view or print it and see past trends.

In obtaining archived soil temperature information, I clicked on “database” in the menu, then “report generator”, next found my nearest location, which was Jay, Florida.  The “range of dates” was included.  In my case, I wanted to see soil temperatures from early March through April for the past several years, so a separate table was viewed and printed for each year.

Monitoring, studying and applying soil temperature information should be tempered with the knowledge that soil temperatures bounce around – fluctuating even with time of day.  It is not unusual to see early spring soil temperatures rise into the 90s F. during mid-day and then drop to the 40s or 50s at night.  Watch for the times that the soil temperature stabilizes at or above the desired germination threshold before planting seed.

Vegetable producers can also even more closely monitor local soil temperatures by using a soil thermometer.  This device has a probe that is placed in the ground to read the temperature at the desired depth.  It is waterproof, allowing it to be left outdoors for constant reading. 

Most soil temperatures for agricultural applications are taken at 10 centimeters or at approximately a 4 inch depth.  Since most vegetables that are direct seeded in the field are planted one to two inches deep, it would be interesting to monitor the temperature at this depth too.

One season I placed a soil thermometer in our yard, so that I passed it on my way to and from the house every morning and afternoon.  The large dial allowed me to read it by simply slowing down on the way to and from the truck.  I found it amazing how the soil temperature changed in only a few hours.

Late winter and early spring temperatures would be in the forties just after sunrise, but might jump to the nineties by mid-day.  By seeing these temperatures show great extremes early in the season, it was easy to determine when they began to stabilize at warm temperatures.

Panhandle Agriculture

Permanent link to this article: http://okaloosa.ifas.ufl.edu/newsletters/2012/02/28/soil-temperature-rules-so-early-planting-is-not-always-best/