Amy Brothers
The Edmond Sun
EDMOND
March 23, 2006 11:17 pm
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Soil temperatures are a valuable tool to use for deciding when to plant vegetables and seeded crops. When the soil temperatures are in the optimum range for a particular plant, the seed germinates quickly and has the best chance of growing to be a healthy mature plant. When soil temperatures are ignored and seeds are planted into cool soils, it takes longer for the seed to germinate and emerge.
You may be wondering, “Why is the length of time it takes for a young plant to emerge from the soil so important?” When a plant’s emergence is delayed, these energy reserves can be used up before it has a chance to send out new roots and a shoot.
Once the plant is above ground its cotyledons turn green and along with new leaves, begin to produce sugars. As long as the plant is underground, the food reserves in the seed must meet all of its needs. All of the energy a new seedling has to push itself out of the soil is stored inside the seed.
Rapid seed emergence also reduces the risk of attack by a disease organism or insect. When emergence is delayed, these organisms have more time to damage the tender young plant below ground.
The Oklahoma Mesonet offers soil temperature information on a regular basis at its Web site www.mesonet.ou.edu/public. At this site, click on “Current Maps,” then on “Data Plots,” and next “Soil Temp — 10 cm Sod.” Up will come the latest 4-inch soil temperature for selected Mesonet tower locations across the state of Oklahoma. You also can look at the soil temperatures for the state in a color-filled plot or three-hour color-filled animation by clicking on the appropriate section. The best time to check the Mesonet current soil temperature for planting is after 10 a.m. and before 1 p.m.
Black plastic mulch, clear plastic tunnels or plant protectors like the Wall-O-Water can increase soil temperatures by several degrees. This allows earlier planting and earlier harvesting.
The following are the bottom-end of the optimum soil temperature range for vegetables:
n 40 degree F soil temperature: Plant endive, lettuce and pea seed.
n 45 degree F soil temperature: Plant carrot, radish and spinach seed. Set out cabbage transplants, potato starts, and onion sets.
n 50 degree F soil temperature: Plant beet, parsley, parsnip and Swiss chard seed.
n 60 degree F soil temperature: Plant snap bean, sweet corn, cucumber and turnip seed. Set out tomato seedlings.
n 65 degree F soil temperature: Plant lima bean seed.
n 70 degree F soil temperature: Plant okra, southern peas, squash, pumpkin and watermelon seed. Set out eggplant and pepper transplants.
n 75 degree F soil temperature: Plant cantaloupe seed. Set out sweet potatoes.
If you want to measure your soil’s temperature, use a soil thermometer inserted two to four inches deep in the soil. Leave the thermometer in the soil for three to five minutes sometime between 10 a.m. and 1 p.m.
(Amy Brothers is a horticulture educator for the Oklahoma County OSU Extension Service.)
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