Soil-borne pests tend to accumulate in intensively managed gardens. Traditionally, pesticides applied before planting have been used to control a variety of insect pests, weeds and nematodes. Although the treatments are effective, most people object to them because of their toxicity to animals and humans. Also, many soil-applied pesticides are restricted to use on certain vegetable crops.
If you are looking for a non-chemical method of soil-borne pest control, consider solarization, achieved by placing clear plastic film on moist soil during the summer. Plants often grow faster in solarized soil and produce higher yields, both of which are attributable to improved pest control and increased nutrient release from organic matter decomposition.
The plastic film allows the sun's radiant energy to be trapped in the soil. Soil temperature at a 2-inch depth can rise to 130°F, hot enough to kill many disease-causing organisms, nematodes, insects and weed seeds. Although soil solarization controls many weed species, it will not affect deeply rooted perennial weeds such as bermudagrass, johnsongrass and nutsedge (nutgrass). Use a herbicide before solarization to control perennial weeds.
Plant-available nutrient levels increase due to accelerated organic matter decomposition that occurs during solarization. Consequently, you will want to wait until after solarization to soil test. You may not need to apply fertilizer prior to planting your next crop. Also, pay close attention to soil salinity. Soluble salt levels increase with solarization. Consider leaching beds with water if reported soluble salt levels exceed 1000 ppm.
Before laying plastic film, prepare beds following the procedure outlined in the 'Seed Bed Preparation' section. Solarization is most effective if the seedbed is smooth and the plastic film rests snug against the soil.
Wet soil conducts heat better than dry soil, making soil organisms more vulnerable. Before laying plastic film, thoroughly soak the beds by using the drip system in combination with a surface watering. Most beds will not need to be irrigated again during solarization. However, if the soil is sandy or if the beds have a high profile, it may be necessary to re-irrigate using the drip system.
Plastic solarization film can be applied to beds separately or in combination. When applied to single beds, the film is laid like mulch film. See Plastic Mulch Application section for details. In complete coverage, film is laid down to form a continuous surface over a few beds or the entire garden, depending on the size of the sheet. The edges of the film should be held in place by burying them in the soil.
For small gardens, UV-inhibiting clear plastic film, sometimes called drop cloths, can be purchased from hardware or home improvement stores. The plastic film can be cut to size or spliced with clear patching tape. The heavier, thicker film is more tear- and puncture-resistant and can be reused.
For market gardens, clear film can be purchased in rolls 6 feet wide by 2,000 feet long or longer. The 6-foot width is ideal for 40-inch-wide beds. For a list of companies that carry large rolls of clear film, see the appendix.
In southern states, film should be left in place for four to six weeks to allow the soil to heat to the greatest depth possible, while in cooler climates, film should remain in place all summer. Results are best with high solar radiation and minimal wind, regardless of geographical location.
Higher soil temperatures and deeper heating may be achieved with a double layer of plastic. The top layer of film can be applied loosely to form an air pocket capable of reducing heat loss from the soil. Even higher soil temperatures can be obtained when beds are solarized within a closed greenhouse or hoop house. At the Noble Foundation, hoop house bed temperatures at a depth of 6 inches commonly reach 120°F during August.
Once plastic has been removed, the cooled, treated soil can be used. Plastic may remain in place through the winter; however, it may start to degrade, making cleanup difficult in the spring.
There is no reason to cultivate the soil before planting if the beds have been prepared properly. If required, fertilizer may be incorporated prior to planting. Try not to incorporate fertilizer deeper than 3 inches, or you risk bringing weed seeds and pathogens to the surface.
Because soil solarization is laborious and will reduce production for a portion of the summer, limit its use by employing other pest control strategies instead, such as cover crops, organic amendments and crop rotation.