The power of the sun is a force man has long tried to harness and put to use. Remember focusing the rays of the sun with a magnifying glass to burn holes in paper? Plants are unique in that they can actually harvest the sun's energy in the process of making food. We gardeners can also harness the sun's powerful energy to help us in our gardening efforts. There is at least one good thing about our sometimes oppressive summer heat - you can put it to work for you by solarizing your garden soil. Hence, soil solarization must be done during the middle of summer.
Solarization is a simple method to reduce harmful soil organisms, like weeds, nematodes, insects and soil-borne diseases, which will help your vegetables and flowers grow and produce better. Research has shown that increased vegetable yields gained by solarization are greater than what would be expected from just destroying insect and disease-causing pests. No one is quite sure exactly why this is true, but results consistently show increased yields. There is evidence that nitrogen is more readily available to plants. And beneficial soil organisms can be favored by solarization.
The process works by simply trapping the heat of the sun under clear plastic to pasteurize the upper layer of the soil where most of your plant's roots are located.
To solarize your garden or flower bed, first prepare the soil. Eliminate all weeds and old garden plants. Next, rototill the soil as deeply as possible to produce a uniform soil texture. If your soil is too dry to easily work, irrigate, wait a few days and then rototill.
For solarization to work, the soil needs to be moist and damp to allow sun's heat to penetrate the ground as deeply as possible. So, give the garden a good soaking before covering it with plastic. An ideal garden setup incorporates drip irrigation under the plastic to maintain soil moisture during solarization and later for watering your garden crops. The plastic used to cover the area should be clear, not black. Clear plastic lets light energy pass through and then traps it, much like a greenhouse. Black plastic absorbs most of the sun's heat without letting it pass through to the soil below. 1-to-6 mil plastic will work fine - the thicker the better. Pull the plastic tight and cover the edges with soil to help keep the soil moist and to prevent strong gusts of wind from blowing it away.
Leave the plastic in place for at least a month and the longer you leave it on the better the results. Two to three months would be ideal, but good short term weed control can be gained in a month. The soil in the top several inches should heat almost to 150 degrees F. which is hot enough to pasteurize the soil and kill many of the harmful organisms. Apparently beneficial soil organisms bounce back quickly and are not greatly harmed by the treatment.
Mentioned earlier as one of the pests solarization can reduce, nematodes are one of the most common, yet frequently overlooked, garden pests. The reason that nematodes are overlooked is because they are very small - so small, in fact, that you need a microscope to see them. Nematodes are tiny round worms that live in the soil and damage plants by feeding on the roots. There are also beneficial nematodes that control other pests, but we'll focus on the types that damage plants.
The root-knot nematode is the most easily identified and common of the types that hurt plants. Infested plants usually are stunted, yellow, and often die prematurely. At the end of the growing season, gardeners digging up plants should inspect the roots of all plants while cleaning up the garden. Look for signs of root knot nematode - swollen, thickened sections, with galls or knots on the roots. Often roots are decayed in severely infected plants. Some parts of the garden may be infested while other sections of the garden may be free of nematodes.
In gardens and flower beds, there are various steps that can be taken to reduce the numbers of nematodes so the gardener can raise a successful crop. Be advised that you will never be able to eliminate them from the soil, so an ongoing management strategy will be needed.
During the summer you can try one of several options. Solarization is one very good option. Another is summer fallowing. Fallowing involves keeping the garden area free of all plants and weeds and as dry as possible. Repeated tilling will further dry out the soil. Adult nematodes require soil moisture to survive.
A third option would be to plant a trap crop of marigolds. However, in order to be effective, marigold seed needs to be planted solidly and thickly so their roots penetrate the entire soil mass. A single row of marigolds planted around your tomatoes will do nothing for the nematodes, but will attract spider mites which can quickly spread to your tomatoes and other susceptible plants. The type of marigold is also important. The small flowering, French type marigold, Tagetes patula, is more effective in controlling root-knot nematode than the larger flowered, American type marigold, Tagetes erecta, also referred to as African or Aztec type marigold.
Keep vegetable gardens and flower beds free of weeds, even when they are not growing a crop. Not only do weeds compete for water and nutrients, but also act as hosts for nematodes. Use nematode-resistant varieties of tomatoes. Look for the capital letter 'N', which stands for nematode resistance, in the series of letters (such as VFN) that follow the varietal name on the tomato tag.
Whether nematodes are present in the soil or not, one of the very best things every gardener can and should do is to maintain high levels of organic matter in the soil. The higher the organic matter, the more likely that organisms antagonistic to nematodes will develop. Some soil fungi actually trap nematodes with lasso-like strands of hyphae and "eat" them. Turning under a green manure crop such as small grains or legumes several weeks before planting is one of the best ways of increasing organic matter. Cereal or Elbon rye is one of the most highly recommended trap crops for nematode control and should be seeded during mid-October to mid-November.
There are not many good things that can be said about our sometimes blistering summer heat and sunshine, but such conditions are ideal for soil solarization.
This web site is maintained by Master Gardener Laura Bellmore, under the direction of William M. Johnson, Ph.D., County Extension Agent-Horticulture & Master Gardener Program Coordinator.
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