Here are my findings comparing the soil temperature in open ground, under black plastic, clear plastic and under a fleece tunnel:-

The average temperature over the nine days I tested the soil was:

Black plastic 9C
Clear plastic 10.5C
Fleece cloche 10C
Open ground 9.5C
Air temperature 10.5C

On sunny days the open ground warmed faster than ground under black plastic, and as expected was colder than all the covered ground on frosty nights.

The clear plastic closely followed the air temperature, being 1C warmer in dull weather and 1C less in the sun.
On sunny days the soil under clear plastic was up to 3C warmer than under the black plastic.

On frosty days the soil under clear plastic was .5 to 1 degree higher than the black plastic.

The soil under the fleece tunnel was .5 to 1 degree higher than the black plastic over the period of the test, but not as warm as the clear plastic. This was probably because the fleece was a tunnel and not in contact with the soil.

The black plastic was a little warmer than open ground in cold weather but was slow to warm up when it was sunny, and over the period of the test was .5C less than the open ground, 1C less than fleece cloche and 1.5C less than clear plastic.

The weather during the test covered sunshine, 1 day frost and snow, five frosty nights, cool cloudy and rain, very cold winds and intermittent sun and showers. On the whole quite cold, but warm when the sun came out.

The coldest soil temperature recorded was 6.5 under the black plastic when clear was 7 and the highest was 14 under clear plastic when black only reached 11C.

My conclusions are that clear plastic performs much better than black plastic, warming faster and remaining warmer. Black plastic prevents the soil from warming in the sunshine but keeps it slightly warmer than open ground on cold nights.

Open ground warmed much faster than the ground under black plastic and even though it cooled a little more at night, achieved .5C warmer over the test period.

There were only a few degrees difference in temperature between them all, but I think the black plastic would be better for weed suppression and drying the soil ready for seed sowing in wet weather than for warming the soil.

Brilliant bit of proper science, beats opinions every time! I had thought of buying a soil thermometer to do something similar so you've saved me a few quid. Thanks for taking the trouble to share your results with us.

I would just like to second Geoffs comments.
Thanks for that Plumpudding.

That's fascinating, PlumPudding, and not at all what I would have predicted with the black plastic!
Don't suppose you fancy getting out there at night and seeing if you get similar results at the coldest part of the day, do you?
Was that a 'no'? Or something ruder?
I wouldn't blame you - as much as I'd like to know, I don't fancy doing that myself, either.
Is there anyone out there who suffers from insomnia, who fancies a challenge?

Hi Plumpudding,
I applaud your scientific approach but must inform you that black polythene is not used as a warming agent. Black polythene is used to kill off weeds and keep the soil in a state ready for use. I use both clear and black. Black for most of the time but clear for a period long enough to allow the first crop of annual weeds to emerge then cover with black again to kill them off. As far as I am concerned temperature doesn't come into the equation.
JB.

Thanks Plumpudding,

this confirms something JB and I have talked about in previous postings.

Also, it ties in with what you'd expect from a scientific point of view. As JB says, the regime is probably to use black plastic in the earlier periods, then switch to clear when you want to warm the soil (or get the weeds to sprout so you can lop 'em off).

In previous years, I've found the cheap black plastic I bought never lasted more than one winter (sometime it had sharded or torn before Spring). I've replaced that with woven black sheeting, which is more substantial & flexible and seems to resist tearing in the cold. However I guess the stronger option isn't available in a clear version...

Hi Johnboy, I did the test as I had seen several articles mentioning using black plastic to warm the soil and had my doubts about this advice. It was interesting to see the results from the different materials and confirmed what I suspected.

My conclusion was that the black plastic was better for weed supression and drying out the soil for sowing or planting, also the results would support the idea of using clear plastic to encourage weed germination.

I got some quite heavy gauge clear plastic sheeting from an agricultural merchant a couple of years ago and it is still in good condition, but I agree that the black plastic doesn't last very long. I did use similar thicknesses for the experiment though, not the thick stuff.

Hi Colin,
The only trouble with woven black covering is that it does allow light onto the soil which in turn allows weed seeds to germinate.
I have had Heavy Duty Black Polythene 1250 gauge for simply years. I think that it has been in regular use since 1994 and is now getting towards the end of it's useful life. I buy from builders merchants and it is so long since I have actually bought any my pricing would be well out of date but I enquired for somebody last year but I cannot remember who. It was a female and that is all I can remember.
The HDBP I use is what builders use as Damp Proof Membrane. I think it comes in 25Mx 25M lots.
JB.

Hi Plumpudding,
I think your last paragraph sums it up nicely.
Of course just to throw some confusion into the matter is the use of Polythene sheeting black one side and white the other!! I take it that you would lay that black showing to allow the heat in and white on the underside to prevent the loss of the heat gained. What do you think? Or have I got it arse about face?
JB.