The Importance Of Soil Temperature To Growing Plants

Gardeners can't always rely solely on the calendar to tell them when it's time to plant, especially when it comes to measuring the temperature of the soil. Plants that are started in cold soils may grow slowly, develop poorly, or succumb to disease-IF they ever get off the ground at all! Here's what you need to know about soil temperature and the adverse affects that too-cool soils can have on your plants.

How Soil Temperature Affects Germination and Growth

Life is fragile, so seeds are fussy. They are happy to put forth the extraordinary effort needed to produce a plant, providing of course, that the conditions are just right. Soil temperature, light, and moisture all work together to trigger seed germination. When the soil is cold, the seed may not germinate in the ground. If it does, it may be weak and lack the strength and vigor needed to develop properly, and may be at greater risk of succumbing to pests and disease.

Heat is the catalyst for many biological processes. When soil temperatures are low (and biological processes slow), certain nutrients are made unavailable or less available to plants. This is particularly true in the case of phosphorus, which is largely responsible for promoting the development of roots and fruit in plants. So, no heat = less nutrients = poor growth.

How Warm is Warm Enough?

The answer to this depends on what type of seeds you are trying to germinate and/or what type of plants you are trying to grow. Most cool season vegetable crops will germinate when soil temperatures reach the mid-40's to mid 50's. They grow best when in daytime air temperatures of between 60-80 degrees F. Warm season vegetables crops prefer soil temperatures in the mid 50's to mid 60's for germination and need summer-like air temperatures (70-95 degrees F) to achieve optimal growth.

Many extension agencies websites and gardening books include charts listing both the minimum and optimum temperature ranges for vegetable seed germination and growth. Here is an example from the Alabama Cooperative Extension System:www.aces.edu

Flower seeds have soil temperature preferences, too. Check the back of seed packets for specific instructions.

Using a Soil Thermometer

An easy way to take the guesswork out of calculating soil temperatures is to use a soil thermometer. From the low-tech dial probes to thermometers with high-tech digital read-outs and automatic shut-off features, most hardware stores and garden centers carry them. Probe thermometers are designed to fit easily into your shirt pocket and are inexpensive ($7-$10).

These will work just fine for most gardeners. Keep in mind that some thermometers do not give readings past 140 degrees F, so if you want a thermometer that will also read the temperature of your compost pile, look for one with a range that exceeds 160 degrees F.

To take a reading, simply insert the probe into the soil to the appropriate depth--2 inches for small seeds and early season crops, and 4 inches for warm season vegetables like tomatoes. Leave it there for at least 5 minutes before checking the results. Readings should be taken over a period of days, in different locations throughout the garden. Use a journal to record how the temperatures fluctuate daily and which part of your garden warms up first. This will help you when its time to plan next year's garden. Once the temperatures stay at the desired level for 3 days in a row, it is safe to plant.

Working With Mother Nature

Mother Natures takes her own sweet time to warm up soil temperatures in the spring. To speed up the process and avoid problems, gardeners can do several things:

• If your winter has been unusually long and cold, frost pockets may be deep and the soil slow to warm up. You can speed up the process by raising the temperature artificially. This can be done by applying plastic sheeting or commercially made row covers to the soil.

• Start your seedlings indoors. Many (although not all) vegetable seeds can be started easily indoors. This allows you to control light, moisture, air and soil temperature until conditions in the garden are right for transplanting.

• Amend your soil. Soils that drain well warm up earlier in the spring. Heavy soils like clay can be amended over time by adding organic matter on regular basis.
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• If all else fails, simply wait. Eventually, Mother Nature will work her magic. She may dip into your growing season a bit before your soil reaches the optimum temperature, but if you wait long enough, it will happen.