Frost Protection - UTL Repository

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F R O S T P R O T E C T I O N : F U N D A M E N T A L S , P R A C T I C E A N D E C O N O M I C S
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SOIL COVERS
Plastic soil covers
Covering the soil directly with plastic to raise the surface temperature is a
viable method that can provide some protection. This is especially true for
small plantations (e.g. gardens or small orchards), where other protection
methods are unavailable. Because the air temperature above the ground is
related to the surface temperature, any management that raises the minimum
surface temperature will provide additional protection. Often, a simple test
can be used to verify the benefits of a management strategy. For example, a
citrus grower once inquired about whether it was better to keep in place or
remove a clear plastic cover from a newly planted orchard floor before
entering the frost season. If the minimum surface temperature recorded
overnight is consistently warmer for the plastic covered surface than for the
uncovered surface, then it is better to leave the plastic on the soil. If the plastic
covered soil has a colder minimum, then it should be removed. It was
suggested to the grower to remove a small section of plastic and place a few
minimum registering thermometers on the bare ground and a few on the
plastic in the evening after sunset for several clear, cool nights. In fact, the test
does not have to be done during subzero conditions. The grower was
instructed to record the temperatures and note which surface had a colder
minimum temperature. The surface with the warmer temperature is more
desirable for passive protection.
Although the experiments are unpublished, the authors have noted that clear
plastic mulches, which increase heat transfer into the soil, typically improve soil
heat storage and result in higher minimum surface temperature. Since the surface
temperature is closely related to air temperature in a crop canopy, having a higher
surface temperature will provide some protection. Black plastic absorbs
considerable radiation, but the air space between the plastic and the ground
inhibits heat transfer to the soil where the heat capacity is greater. Consequently,
black plastic is less effective for frost protection.
Wetting the soil before covering with plastic further improves heat storage,
which raises the minimum surface temperature and provides more protection.
This is especially true for clear plastic, which allows more radiant energy to reach
the soil surface. Part of the reason for increased surface temperature, when the
soil is wetted before placing the plastic, is that water will evaporate from the soil
and it will condense on the bottom of the plastic as the cover cools to the dewpoint
temperature. This will change latent to sensible heat under the plastic and
it will help to maintain a warmer surface temperature.
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