Re: CULT: Black plastic vs. Clear

Kitty, here's why clear plastic works and black plastic doesn't.  Clear
plastic lets the sun's energy through, where it is absorbed by the soil and
converted into heat.  However, the plastic is opaque to heat radiation, so
just as in a greenhouse, the heat is trapped in the space between the
plastic and soil, and gets hotter and hotter.  It's the greenhouse effect
in miniature.  The temperatures just under the plastic on a bright day can
reach 160F.  Also, because of the clear plastic, there is less cooling at
night and the next day's heating gets a head start.  After several weeks of
this, high temperature can reach several inches into the soil.

Black plastic does heat up, but does not transmit much of the heat to the
soil below.  In fact, black plastic is used by commercial growers of
vegetables to keep the soil cool whilst radiating warmth to the plants
above.  Stuff dies under black plastic because as you say, there is no
light, but it has minimal effect on soil pathogens, weed seeds (which just
stay dormant), etc.

Hope all is well in Ukiah.  I'm still enjoying happy flashbacks of my trip
there this spring!  What a great meeting you guys organized.

Bill Shear
Department of Biology
Hampden-Sydney College
Hampden-Sydney VA 23943
(804)223-6172
FAX (804)223-6374
email<bills@hsc.edu>

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