Soil Solarization

Soil Solarization 101

This method uses solar energy to heat the soil. The process is effective in hotter climates, but the effects on soil microflora are less dramatic. It also requires full-day sun for six to eight weeks. Soil solarization requires plenty of sunlight to build enough heat underneath the plastic. If solarization is unsuccessful, the soil will not warm enough under the plastic. For more information, see this article. For further information about soil solarization, read the entire article.

Less drastic effects on soil microflora

Soil solarization is not without risks, however, and is often accompanied by increased plant mortality. The process can disrupt soil microbial communities. Researchers have identified bacteria, fungi, and pathogens that are susceptible to solarization. Although this process is potentially harmful to plant health, there are still a variety of soil microbes that are not completely eliminated by solarization.

Increasing the soil temperature through the addition of a polyethylene sheet can help reduce pest and disease problems. Soil temperatures increased by up to 65% in soils solarized by just 10 cm. Another study aimed to assess the effects of soil solarization on plant health found that solarization could reduce the incidence of tomato damping-off and reduced disease by as much as 83%. In addition, solarization can also reduce the incidence of diseases, like fusarial wilt.

Time- and temperature-dependent

Soil solarization is most effective during the hottest weeks of the year. In inland California, this means late June through August, with good results starting as early as late May in the southern desert. In coastal regions, consistent, warm, fog-free periods may not appear until August or September. However, in most regions, soil solarization can be done in late June through August. The time and temperature-dependent effects of soil solarization are summarized below.

During the four-week experimental period, soil samples were collected immediately after infestation, after incorporation of green manures, and at four weeks after solarization. The percentage of infected disks decreased significantly in all treatments, but solarization was not more effective than green manure in either 1995 or 1996. Soil samples were used to measure the population densities of pathogens and beneficial microorganisms. Soil cores were collected from each plot and were mixed for analysis. In addition, green manures were applied to the solarized plots at 230 or 930 liters ha-1.

Effectiveness in hotter climates

The effectiveness of soil solarization in hotter climates has been proven, with some research demonstrating a decrease in microbial populations and an increase in rhizobial populations. In recent studies, the top eight centimeters of soil reached 54degC after solarization treatment, compared to 45degC in untreated soil. Dwivedi R. and S.K. (1993) also observed a decrease in inoculum of R. solani at 24 cm soil depth.

Soil solarization is an organic, chemical-free pre-planting treatment that is a viable option for farmers in hotter climates. It has been proven to control weeds and soil-borne pathogens and is compatible with organic crop management systems. Although solarization can be expensive, it is simple to implement, suitable for farmers in developing countries, and requires only a short period of crop-free time. Farmers often use soil solarization in conjunction with other methods to get the most benefit from this method.

Limitations

The adoption of soil solarization is relatively low, but its use is expected to rise over the next several years as ozone depletion bans become more prevalent. This technique is effective in killing many microbes, including weeds and soil-borne pathogens, but can cause negative effects on beneficial organisms, such as fungi and bacterivores, which are necessary for the decomposition of organic matter in the soil.

Soil solarization kills most microorganisms, including beneficial ones. Some of these microbes, such as Bacillus subtilis and Rhizoctonia, are killed by solarization. However, some fungi and airborne bacteria are also killed. It also destroys the seeds of annual weeds. It does not kill root rots. In the case of weeds, soil solarization may be less effective than fumigation.

Plasticulture

Soil solarization is a process in which the soil is heated by the sun. Soil temperatures can reach up to 50degC, and even 40degC in warm climates. This heat can be transmitted as deep as 30cm, killing most soilborne pests and weeds. The temperature of the soil is dependent on the amount of moisture in the soil, and the warmer the soil, the more heat it can conduct.

Soil microorganisms are primarily beneficial to plant health and can help monitor soil quality. Enzymatic activities are also considered biosensors of soil health. Soil solarization can affect soil microorganisms, which may result in increased production of beneficial antagonists to plant pathogens. This process can reduce pressure on the soil microbiota by improving the availability of chemical compounds. Plasticulture for soil solarization also has other advantages.

Raised beds

Raised beds for soil solarization work best when oriented east to west, north to south, or both. This is because solarization is most effective on areas with little slope and a southwest or south-facing exposure. Otherwise, the solarization may not be effective or may be compromised by pest control. In order to maximize solarization, the soil must be wet at least 12 inches deep. You can add moisture after you cover the area with plastic.

Before starting the solarization process, be sure to prepare the soil by removing any existing plants, mulch, and sprouting weeds. Ensure the surface is level and smooth; if it is not, you may have to “crown” it before applying plastic. The top layer should be sufficiently smooth to prevent puddles from developing, which will impede the transmission of light. Lastly, ensure that the plastic is strong enough to prevent it from tearing or ripping.

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