Warming the soil - clear or black plastic?

[Colin_M]

Many of us lay down some sort of covering over winter (eg. plastic sheeting, cardboard, carpets etc). One reason for using black plastic that I have heard many times to is to help warm the soil before spring sowings.

I know that JB also makes use of clear plastic later into Spring (as part of clearing weed seeds before he does his main sowing etc).

My question is this - which is more effective at helping the soil to warm up:
- Black?
- Clear?

If we ignore the weed sprouting requirement, I'd have thought that clear plastic might allow a weak spring sun to warm the (dark) soil directly. By comparison, black plastic may become warm by having the sun's rays on it, but will much of that warmth get into the soil below?

The physicists amongst you will probably have a view. How about those of you who have tried both methods - can you advise your findings?


Thanks, Colin

[Chantal]

I'm no physicist but I do know that I get much hotter from wearing black clothes (as opposed to white) in the sunshine. The black seems to absorb the heat, the white to reflect it. Surely the same would apply to the soil?

[PLUMPUDDING]

That is an interesting thought Colin, so I'm going to try it out - when the sun has thawed the frost this morning. I've got some sheets of both clear and black plastic so I'll leave them both on for a few days and then test the soil with my soil thermometer and report back.

As you say, the black soil under clear plastic may warm as well or better than soil with the light totally excluded.

[Colin_M]

Thanks Plum Pudding. There's nothing to beat actually trying it! I'd be interested in your findings.

Last year, I did experiment with using clear plastic on a bed I'd sowed carrots in, partly to try & help keep the soil from drying out. However this wasn't a controlled experiment like yours.

Colin

[PLUMPUDDING]

I've not put the plastic down yet as the ground didn't thaw out much at all, but I checked the temperature under some black plastic I had put out a couple of weeks ago before the cold weather, and at lunch time the soil under the black plastic was still frozen whereas the open soil next to it was warming up in the sun.

I'll wait until the ground has thawed out properly and lay fresh black and clear side by side.

[Colin_M]

I checked the temperature under some black plastic, and at lunch time the soil under the black plastic was still frozen whereas the open soil next to it was warming up in the sun.

Hi PP, in a way this sort of makes sense to me. I also feel sure that when you are able to lay some clear plastic, you'll find the soil under it even warmer than the exposed soil.

Maybe later into the spring when the air temperature warms up, the results are less influcend by direct radiant energy from the sun?? Of course, the main reason for using black plastic is to suppress weed growth during this period.

Colin