It’s time once again for the annual pitch for soil solarization as a non-chemical cure-all for many common garden problems. Solarization captures the heat from the sun’s rays to heat the top 2 inches of soil to at least 130 to 108 degrees (140 degrees in optimal conditions) and to 90 to 99 degrees at 18 inches. That’s hot enough to bake weed seeds, cook pest insects and their eggs, and kill many fungi and nematodes (microscopic worms that feed on plant roots).
Soil solarization can kill many species of nematodes within the top 12 inches of soil. Although the nematodes in deeper soil can and will move back into the upper levels, nematode control by solarization is very effective for short season crops such as vegetables and flowering annuals. Nematode control by soil solarization may need to be repeated every two to three years. Pull out plants that lack vigor to check their roots; if root systems are smaller and darker than normal and look knobby, nematodes may well be the cause.
If you’ve lost plants to soil-borne fungi including verticillium wilt, some fusarium wilts, damping off (when seedlings rot just after sprouting) and Phytophthora root rot, the heat generated during the process of solarization will kill the fungi and sterilize the soil. Solarization for fungal problems is best done in planting beds that contain short season crops or annuals since it can be difficult to properly prepare the area around mature perennials for solarization.
Solarization can kill weed seeds and weeds if the root systems, rhizomes or bulblets are not too deeply buried; perennial weeds which have stronger, deeper root systems are generally tougher to control than annual weeds. It works fairly well to control for Bermuda grass and Johnson grass, but control for purple and yellow nutsedge is inconsistent. (Solarization over two summers reduced the abundant nutsedge weeds in our vegetable patch by at least two-thirds, making the remaining nutsedge easier to control by hand weeding and by applying much smaller amounts of herbicide.)
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One of the surprises of solarization is that populations of beneficial fungi and microorganisms, especially the mycorrhizai, are either able to survive the heat or are able to recolonize quickly. There is also an increase in soluble nutrients including nitrogen, calcium and potassium because elevated soil temperatures break down organic materials in the soil. After solarization, soils are much healthier and more resistant to disease pathogens. It also seems that earthworms are able to burrow deeply into the soil to escape the heat and their populations also rebound after solarization.
Here in the central San Joaquin Valley, solarization during the hottest months of June, July and August with no rainfall, wind or cloudy days is very effective. Our summer temperatures are so high that that the time necessary for thorough soil sterilization can be reduced to a couple of weeks although four to six weeks of solarization is recommended for complete control.
Next week: the instructions for soil solarization. It’s really simple and cheap.
Elinor Teague: firstname.lastname@example.org