Measuring soil temperature

[David]

I'm not sure if I've been recording soil temperature right - I had to consider this when the soil thermometer recorded 25.4C today, and the Grey Lynn station had 21C. I've got the sensor down to a depth of about 12cm. Currently the sun shines on the ground where I have the equipment from mid morning to late afternoon. Am I supposed to have the sensor in shaded ground?

[TonyT]

I'm not sure if I've been recording soil temperature right - I had to consider this when the soil thermometer recorded 25.4C today, and the Grey Lynn station had 21C. I've got the sensor down to a depth of about 12cm. Currently the sun shines on the ground where I have the equipment from mid morning to late afternoon. Am I supposed to have the sensor in shaded ground?

No, the ground area you measure in should be as open as possible. You must be totally consistent with depth. Soil temperature varies diurnally, and of course annually, but the variations change with depth - the nearer the surface you measure the more diurnal variablity you will measure. As you go lower, the diurnal variation reduces, and also gets later. There comes a point where you dont get much if any diurnal variation (about 30cm from memory). The annual variation also delays the lower you go. There is a depth (about 1.5m I think) at which the annual variation is lagged by 6 months - so the temperatures are cooler in the summer and warmer in the winter! But the differences at this sort of depth are very small.

So, make sure you measure at a standard depth (0cm, 5cm, 10cm, 30cm, 50cm, 100cm) which makes comparison with other sites easier, and make sure you measure at the same time of day if you want to make comparisons (eg 9am). Finally, be aware that the soil type and surface treatment will dramatically affect temperatures. Light loams will have a much greater diurnal range than heavy clays (more air in the soil), and a bare, worked surface will have higher variation than a compacted or grassed surface. I think the standard is to measure under short grass (but I'm not 100% sure on that one).

[David]

I think the standard is to measure under short grass

It is under short grass. It does get a small range in Winter (average daily range in July was 0.8C between the soil minimum and the soil maximum, and 1.1C in August). In the last few days the daily range has been about 3C to 6C. I found it hard to believe 25.4C yesterday in the soil, when all of last summer the highest in the soil was 24.6C. It also said 24.5C at 7.40am this morning with a low overnight in the soil of 22.6C. Maybe I try poking a new hole in the ground? - I think maybe the effects of the ground contracting slightly in dry weather could cause a small opening around the entry point of the wire - allowing surface heat in.

About the diurnal lagging, usually the soil low occurs about 10am and the soil max about 8 or 9pm, but recently soil low has been earlier and the soil high about 4pm, once the sun no longer strikes the ground it is in the reading drops. I seriously doubt the accuracy of the 25C readings, so i think I will move it into a new more plugged up hole in the ground.

[David]

Actually my soil thermometer is wasted - when it said 26C just before I knew that was impossible and so I took it out and it said the air temperature was 25C. Maybe it will adjust back if I have it inside for a few days. Its lasted over 4years so I don't see why it should die now

[Manukau heads obs]

also the trick with soil temperature is to take the measurement at the same time every day,for comparisons, e.g 9am
and a grassy surface is needed, as the dark soil colour heats up alot