Dear Tom –
This may be a silly question – but, if you use plastic mulch, don’t you run the risk of solarizing the soil? I understand that solarizing the soil can kill diseases – but it also kills off “the good stuff”
Clear plastic is needed to solarize soil and most growers use opaque plastic for that reason. Clear plastic may also threaten your crop with high temperatures and it allows weeds to grow underneath in some cases. My comments in earlier columns assumed agricultural plastic mulch – normally white or black, although more exotic colors and various degrees of reflectivity are available.
While I have never used solarization intentionally for soil treatment, I see it as a tool that may be useful in particular situations. I use solar energy and plastic sheets to dry firewood for home and greenhouse heating. I unintentionally singed grass on several occasions when moving greenhouse plastic from one structure to another and this was with just a few minutes exposure. Finally, I once managed to fuse a roll of plastic into a single melted hunk – an expensive mistake but that experience shows how high temperatures can get inside several layers of clear plastic. I have heard reports of seed flats melting inside greenhouses with vents and fans turned off.
As an organic crop management tool, solarization has pros and cons. In your question you mention the biggest con in my view – killing beneficial soil organisms. The pros may include killing weed seeds and pathogenic organisms that can persist in your soil for years or even decades without solarization.
Dr. Joe Neal with NCSU describes solarization as follows:
Some NC greenhouse growers use solarization to disinfect their greenhouses between crops. They remove electronic equipment and any plastic fixtures that may melt and then turn off the vents and fans. Some growers water the soil first and then cover the soil with a third layer of clear plastic in addition to the two layers of greenhouse glazing. In the Piedmont and coastal plain areas of NC temperatures are high enough that tomatoes do not set fruit in greenhouses in the summer. Solarization must be done when the sun is high in the sky such as near the solstice. In the mountains we continue to set fruit through the summer so greenhouse solarization may be less practical.
Part of an Integrated Approach
Solarization may be useful even in the mountains and even as far north as North Carolina in several different ways. These include:
- Warming the soil early in the season. At low sun angles soil temperatures are unlikely to reach sterilization temperatures but the soil may warm enough to give your crops a head start if you can also protect them from frost damage.
- Land with a history of long-lived pathogens, a heavy load of weed seeds, perennial weeds, or damaging nematodes may benefit from solarization. Beneficial organisms will be killed as well so compost or other inoculants may be needed to repopulate your soil after treatment.
- Brief solarization may be useful in “stale bed” production with slow germinating crops like carrots. Typically beds are sown and then irrigated to prompt germination of weed seeds on the surface. A flame weeder is normally used to kill weedy competitors so that the slow germinating carrots emerge into a bed free of weeds. Timely application of a clear plastic sheet may work as well, although I have not tried this method.
- If growers are concerned about pathogens or weed seeds in nursery container media or soil mix, solarization may be useful (refer to the illustration below.)
- As mentioned above, solarization can be used in greenhouses to compensate somewhat for lack of rotation where the same or similar crops are grown year after year.
Improving Effectiveness of Solarization
Solarization will achieve higher temperatures if:
- It is used at low elevation and at southern latitudes such as Florida and Southern California where day and night temperatures are consistently high in summer.
- The sun angle is high such as late June and early July.
- The soil is damp to help transfer the heat.
- One or more sheets of clear plastic are used.
- The sheets are sealed at the edges to retain heat.
- The plastic is left in place for several weeks.
Here is a large scale solarization operation using mulch laying equipment.
Other references for “further reading” include:
http://edis.ifas.ufl.edu/in824 – a general overview from Florida
http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn74145.html – UC Davis general overview (California)
http://books.google.com/books?printsec=frontcover&vid=ISBN0849368685#v=onepage&q&f=false – an in-depth book on solarization
Thanks for your question.
Ask Tom © 2013 Tom Elmore & Organic Growers School
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Tom Elmore is co-owner and operator of Thatchmore Farm in Leicester NC. He has grown certified organic fruits and vegetables for 25 years and serves on the Boards of the NC Greenhouse Vegetable Growers Association and the Organic Growers School.