soil temperature

[Mike Vogel]

I wanted to test the temp of my soil after the cold spell, so I pushed a soil thermometer about 6" deep into soil which had not been covered. I meant to do this in a number of places, including where I have had a fleece and again where I have had black polythene on the soil.

But of course, doing other things, I forgot completely about it until I had to go home. Pulling it out, I found to my surprise that it was showing 50 degrees, which is about 10 degrees celsius.

Should I be surprised? I know it had been a sunny day, but the thing was 6" down and we have had some snow and/or cold rain over the past few days.

mike

[Geoff]

"Never throw anything away."
Not even a dodgy thermometer?
See what temperature it thinks your fridge is.

[Primrose]

Don't know what type of soil you have Mike, but I suspect that heavy clay soil will take much longer to heat up but lighter soil will warm quite quickly. Also if the soil has been well dug with lots of air spaces between clumps, the warm air could perhaps dissipate further than once would think.

[PAULW]

MIKE We had 10 down here the other day hopefully we have turned the corner now and things will start to grow.

[The Mouse]

You've given me an idea.I've been wondering myself just how effective it is to cover soil, either before or after planting. In another thread, the question of whether potatoes should be planted yet or not has been discussed, and quite a lot of people are covering them after planting. Is it necessary if they are are some six inches underground?
If I get a chance this weekend, I will get some min/max thermometers and do a bit of testing. That is, unless anyone else out there has already done it!!!

[Urban Fox]

"I found to my surprise that it was showing 50 degrees, which is about 10 degrees celsius. Should I be surprised? "

I wouldn't be surprised that the soil was warm. Soil stays warm for quite some time and of course the deaper you go the warmer it gets. In fact there are ways of heating buildings using underground heat.

There is an interesting web page on a heat exchange project here

http://www.livingrainforest.org/ig/WhatIsaVSHE.php


Christina

[Mike Vogel]

That's an interesting one, Urban Fox. I didn't realise the soil would be so resistant to getting cooled by freezing water. To be truthful, it hasn't been such a cold winter, has it? All the same, i think I'll put my thermometer in the fridge just in case, Geoff.

mike

[PLUMPUDDING]

We've actually had the coldest winter for quite a few years here. There were two weeks of continuous frost when night time temperatures went down to minus 10C on at least three occasions. Even after that I was going to do an experiment with the black plastic, clear plastic and open ground and I couldn't get the thermometer through the frozen soil, so haven't tried again. I'm hoping to have a gardening day tomorrow - potato planting - so will test the soil temperature and put the plastic on an empty space somewhere and compare the results over a couple of weeks. It was frosty again last night, so should be a reasonable test.

[Mike Vogel]

Unusualy for Bedford, those very cold nights seemded to miss us. We are supposed to be close to a frost hollow, so it is surprising that my allotment soil is as warm as it is. interestingly, when I put the thermometer into a patch which had been covered in black polythene,it showed a lower temperature than in the exposed area. This suggests to me that it takes time and a few sunny [rather than warm] days for the black polythene to really start having an effect.

mike

[The Mouse]

That's interesting, Mike.
You would expect black to absorb the heat of the sun -when there is any- and the polythene should help prevent heat loss through windchill. On the other hand, although I'm no scientist, I think black also speeds up temperature loss from the soil when the air temperature falls, by acting like a radiator.
If we use white fleece instead, we probably reflect away the heat of the sun, but lose less heat when the air turns cold (I think white radiates heat less effectively than black).
So for maximum soil temperature, maybe we should be covering the soil with something black while the sun is out, and white the rest of the time. Or maybe my theory is total nonsense! If anyone out there has any more temperature readings that might help us work out exactly what's happening, I'd be interested to see them.

[GIULIA]

If we're testing whether the soil is ready to receive its first spring sowings, here's a tip my grand-dad was taught by the head gardener who trained him as a boy in the 1920's - the oldies are often the best.
Here goes...
when you think it's nearing time to make some early sowings, sow a test patch of spinach seed, just a 6" circle will do, and label it with the date. Do the same thing again 5 days later, and so on. Spinach is very sensetive, especially to fluctuations between night and day temperatures, so if the soil is too cold and wet it will take two or even three weeks to come up, keep watching, and as soon as a spinach patch breaks the surface within seven days of sowing, you know germination conditions are spot on and can sally forth with your line and hoe confidently. (You can eat the spinach or hoe it under if you don't want it).

[Primrose]

Gulia - sounds a pretty good test to me. I tend to watch for weeds beginning to shoot through the soil but everybody knows that weeds can thrive where nothing else seems to !

[PLUMPUDDING]

I thought you were going to suggest the old gardeners' trick of sitting on the soil with a bare bottom - and if it was comfortable to the skin then it was OK for the seedlings. Not something I fancy doing!!!!