One of the natural methods of controlling soilborne diseases, pathogens and weeds in gardens is soil solarization. This technique has been used by many farmers around the world because of its adaptability to any location and soil condition. It has been proven effective and it lessens farmers’ and growers’ reliance on synthetic pesticides and herbicides. (See Soil Solarization: Concept and Benefits)
Soil solarization is done by capturing and trapping the sun’s heat with the use of a tarp or covering. After some time, soil temperature can rise as high as 125 degrees Fahrenheit (52 deg. Celsius) at the top 6 inches (15 cm), and even to 100 deg. Fahrenheit at a depth of 18 inches. Over several weeks, that is hot enough to kill a wide range of soil inhabiting pests such as wilt and root rot fungi, root knot nematodes and noxious weed seed. It takes about four to six weeks of sunny weather to sterilize the soil. The method can be done on any size of plot, as long as the plastic covering is enough to wrap the whole plot.
For a more detailed procedure of the process, here are the quick and easy steps:
- Prepare the soil. After tilling the land, thoroughly clean the plot from any unwanted materials and debris. Pull out the weeds or residues from previous cropping. Rake the surface smooth. A fine soil surface will allow the plastic covering to be placed in close contact with the soil Remove stones and large dirt clods which may create air pockets inside the covering and eventually cause uneven heating.
- Water the plot. The soil should be moistened if dry, but not saturated. You can leave the sprinkler on for several hours or overnight to soak the soil. Wet soil conducts heat better that dry soil. Also, there will be 100% humidity inside the covering which acts with heat to eliminate pathogens and weeds.
- Dig a trench all around the bed or plot 6-8 inches deep.
- Lay a clear plastic covering, around 3-6 mm thick deep, over the plot. See to it that the sheet overlaps the trench on all sides. Fill in the trench (you may even place heavy objects on the edges) to weigh down the plastic while pulling it as tight as possible.
Many people always recommend using clear plastic because the idea is that clear plastic produces higher temperatures faster because the sunlight passes through the clear plastic to heat the soil. However, for cooler climate or cooler time of the year, you may use black plastic because it absorbs the heat, making it even hotter.
Polyethylene plastic 1 mil thick is the most efficient and economical for soil heating. However, it is easier to rip or puncture and is less able to withstand high winds than thicker plastic. Users in windy areas may prefer to use plastic 1 1/2 to 2 mils thick. If holes or tears occur in the plastic, they should be patched with clear patching tape or duct tape. Thick transparent plastic (4 to 6 mils) reflects more solar energy than does thinner plastic (1 to 2 mils) and results in slightly lower temperatures.
- Relax, wait, and see. You are done now with setting up and what needs to be done is nothing but wait for the effect. A few weeks of sunshine can effectively improve your soil conditions. In areas where weather conditions are generally cooler or where there are more cloudy days, or when you do not feel like waiting for a long time, you can speed up the process by adding another layer of covering. See to it that the second sheet of plastic cover is raised over ground level so that there is airspace between the two sheets. The airspace serves a buffer zone during cloudy or cooler days. The combination can also raise the soil temperature an additional 6 deg. Fahrenheit.
Read "How to Solarize Soil" at Factoidz.