How does soil temp correlate to air temperature?

[JerryHaskins]

I am not sure if this has been discussed already, if so, please point me to it.

I found this great table showing the effect of soil temperature on seed germination:

http://tomclothier.hort.net/page11.html

And I have one of those little soil temperature measuring instruments.

But does anybody know how soil temperature relates to daily maximum and nightly low temperatures.

Obviously, it matters how long it's warm or cool during the 24-hour period, whether it is cloudy or sunny, how much it rains, the hours of daylight, amount of mulch, etc.

But daily weather forecasts on TV and the internet give the minimum and maximum air temperatures. That's easier to find than soil temperature.

I just wonder if anybody has tried to correlate air temperature to soil temperature.

[jmsieglaff]

Temperature of the soil (assuming you're talking outdoors) depends significantly on depth. The top 2" of soil will swing dramatically over 24 hr periods, especially on sunny days compared to cloudy days. Moist soils will have more dampened swings compared to dry soils. As depth increases, diurnal swings quickly diminish, but since you're referencing seed germination, you're likely only interested in the top 1" anyway. Effectively you can just use the air temperatures--the top 1" of soil will not get quite as cold as the air, but it will be within a couple degrees. Moist soils will also warm more slowly than dry soils--a lot of energy required to warm water compared to soil and air pockets in the soil.

[jmsieglaff]

Oh and a cold spring rain will crash what was a warm soil temperature.

[JerryHaskins]

I agree. And with the top 1 inch of soil varying so much throughout a single day and from day to day, I am not sure what use the optimum soil temperature table serves.

That's probably why they recommend planting dates based upon the normal last frost date for an area.

As you mentioned, the optimum soil temperature table would be more relevant to starting seeds indoors.

[Worth1]

Lest we not forget the reason for planting on hills.
The mound will be warmer than a flat surface.

[Cole_Robbie]

Originally Posted by Worth1

Lest we not forget the reason for planting on hills.
The mound will be warmer than a flat surface.

Good point, same for raised beds. Black plastic mulch does wonders, although getting a seed to sprout through a hole in it is a little tricky. The plastic can't be loose and flap in the wind.

Now that I think about it, anything black would absorb heat from the sun. I would think that seeds covered in black sand would sprout faster than those covered in any other color of media.