You can check out the CIMIS station data. Some have soil temp data and some don't.
Either way, soil temperatures can vary greatly depending on microclimates, soil moisture, soil texture, shade, etc...

As part of my job we used to take soil temperatures. It was done twice a day at 8am and 4pm. There were electronic sensors buried at various depths down to I think 100 centimetres. It's all done automatically now. The deeper depths never varied much but the shallow depths had a diurnal variation and of course seasonal variation up here. I suspect California soil never freezes.

There are tons of factors affecting soil temperatures and adding compost would as well, but it would be difficult to say that it would make it warmer in every case.

Lloyd

I'd be inclined to monitor the night time low temp and compare that with soil temp. You can use any probe-type thermometer to measure your soil temp. Don't let the sunlight hit the thermometer directly, though. And stick the probe down to different depths to see if there is a difference. There should be a difference in the top few inches.

If you are out in the desert with clear nights and very low humidity, the night time lows might be lower than the soil temp.

I am in San Francisco and the soil never freezes, but there are parts of Northern CA that could freeze, I guess. Do they sell a soil thermometer, or do you just use a meat thermometer?

Did a search on soil temp in CA ... found some interesting stuff. For instance - San Joaquin soils are designated as official State Soils [..] " The mean annual soil temperature varies from 60 degrees to 64 degrees F and the soil temperature is not below 47 degrees F at any time."
http://www.pssac.org/castatesoil.htm
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This one has recent temps by CM down.. and in celcius
http://landslides.usgs.gov/monitoring/portland/rtd/temperature.php
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This one tells how to take the soil temp:"To measure soil temperature, find an area that is not in direct sunlight. Using a thermometer, measure the air temperature at shoulder height. Hold the thermometer still for about one minute (make sure your fingers are not on the thermometer bulb), read and record the air temperature. Next, measure the temperature at the surface of the ground. Put the thermometer flat on the ground and record the temperature after one minute. To determine the temperature below the ground surface, use a dowel that you have marked at 1 inch, 2 inches, 6 inches and 12 inches. Start by pushing the dowel into the ground till you reach the 1 inch mark. Remove the dowel and insert the thermometer for one minute, then remove the thermometer and quickly record the temperature. Repeat this procedure to obtain temperature readings at 2 inches, 6 inches and 12 inches. Take temperature readings at different times throughout the day at the same location. To compare with soil temperatures for areas in direct sun, just repeat using the same procedure but select an area that gets full sun. You will note that the soil temperatures in these areas are typically much higher than in the shaded areas."
CHART -
Soil Temperature Conditions during growing season
Less than 40� F no growth, bacteria and fungi are not very active
40� F to 65� F some growth
65� F to 70� F fastest growth
70� F to 85� F some growth
above 85� F no growth

http://www.rain.org/global-garden/soil-types-and-testing.htm
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hope this helps .... I'm still processing that we actually have an official "State Soil"....

Soil temps and ambient air temps are not even close to being similar. Soils never reach the same highs or lows of air temps and they are slower to warm up in spring and slower to cool down in fall. And as noted, there are all sorts of variables that can affect soil temps, including the amount of organic matter, light conditions, proximity to structures, raised beds, growing in containers/container color, mulching etc. And optimum seed germination temperatures are often far higher than those occuring in the ground at recommended planting time, which is why many folks start seeds indoors where heat mats can be used and temps controlled. If you are direct sowing, especially veggies or annuals, you need a soil thermometer :-)

Soil temp thermometers are available at any nursery/garden center I've ever been to so they shouldn't be all that tough to find. They are usually right there next to the compost thermometers :-)

The old timers around here told me to seed the wheat once the oak trees flower. As far as gardening, I'd watch the old ladies, somehow they just seem to know when to plant.

There's no way I'm gonna argue with them! :-)

Lloyd

I can see now they are not the same. The Iowa map has soil temps in the 50's, but it's like in the 20's now. I don't understand why the soil temperature would be different from the air temperatures, higher I guess, but why would be different? That map was so cool because it is updated daily and shows the last three days. Too bad we don't have one. I am having good luck with the voila germination in spite of temps in the 50's here. I saw a bunch of little sprouts I think are the voila yesterday.

Water has a specific heat of 4.2. Silica has a specific heat of .7. Seems to me that the predominant factor of soil temperature is water content. A nice warm rain would instantly change the temperature of soil.

Look at that map again :-) Nowhere does it show a current soil temperature in Iowa of 50F - 20's and low 30's in all areas. Heck, even in my zone 8 after an incredibly mild winter and above normal temperatures (mid 60's last week), our soils are only at about 40-45F. We won't see soil temps in the 50's here until midApril at the earliest.

Why is there a difference? Because soil has insulating capacities with respect to both heat and cold and holds heat better than air does. It also has a lower freezing point than does the air, water vapor or just plain water. That's why it takes an extended period of well below freezing temperatures to freeze the soil to any significant depth. And there is biological and chemical activity going on that generates heat as well. The greatest fluctuation in soil temperature will be at or just below the soil surface, responding to both the ambient air temps and solar radiation. In fact, the deeper you go, the more insulation is present and the higher the average temperature, to a point. Depending on where on the planet you are measuring, anywhere from about 30 to 200 feet below the surface, the soil temperature is relatively constant (about 55F).

I couldn't get any useful by taking my ground temp in SoCal. It doesn't rise slowly during the spring so you can't pick a week when it has reached a certain temp for planting. The temps in the upper six inches are inconsistant from week to week. They rise and fall depending on many variables as stated in above posts.

The calendar in conjunction with short term weather forcasts are better for picking start dates.

Zeuspaul

What your soil temperature would be at any time would depend on a number of things starting with how much moisture is in that soil, how much sunlight that soil gets, as well as what the ambient air temperature is. A soil that is exposed to full sun can get hotter then the same soil in the shade, unless that soil in the sun has a lot of moisture in it. Soils with no moisture will not freeze, they may get cold enough to freeze but they will not freeze.