Soil Solarization is an organic gardening technique that uses solar energy to disinfest garden soil by getting rid of disease-causing fungal, bacterial, and nematode pests, weeds, and some insect pests. Apart from the expected results, this technique also increases root growth and harvest as a result of changes that happen when the soil is heated.
How to Solarize Soil
Soil solarization is very simple and cheap to implement. At the simplest level, moist soil has to be covered with a plastic sheet for 2-8 weeks.
Use the correct type of plastic sheet: Transparent plastic sheets are best for this purpose because transparent plastic traps heat under the sheet. Black or other colored opaque plastic sheets can also be used but some of the solar radiation that reaches them is transferred to the air outside the sheet. Thinner plastics sheets are more effective for soil solarization than thicker sheets, which is good news because thinner plastics sheets are cheaper and usually easier to get hold of.
Keep the soil moist: Keeping the soil moist during soil solarization makes the soil organisms more sensitive to heat and helps transfer heat in the soil.
Cover the soil long enough: A duration of 4-6 weeks is usually enough but a shorter duration may also work in very warm areas. Likewise, soil solarization becomes more effective the longer the duration. So, if you feel like temperatures aren’t high enough while you solarize your soil, you may want to keep the covers on for longer.
The amount of heat absorbed and the effectiveness of soil solarization varies a lot on soil type. Darker-colored soils absorb more heat than lighter-colored soils, while soil solarization is less effective on sandy soils that drain fast so don’t remain moist enough during the process. This means that soil type also impacts the duration needed for effective soil solarization.
Don’t keep space between the soil and plastic sheet: The plastic sheet should be placed as close to the soil surface as possible to increase the amount that the soil gets heated. This is because an air layer can have an insulation effect and prevent heat from reaching the soil. For this reason, the land has to be prepared and the surface should be smooth before the plastic sheet is placed on it.
Add organic soil amendments to increase effectiveness: Organic soil amendments, such as composts, green manures, and animal manure, can be added to the soil before solarization. Adding these organic amendments to the soil by itself can increase the biological activity of this soil to help reduce pests and pathogens. These amendments are more effective in getting rid of pests and pathogens in combination with soil solarization.
Benefits of Soil Solarization
Soil solarization was developed to be used as a soil disinfection method that was completely chemical-free. Apart from its intended use soil solarization also provides many other benefits.
1. Reduces the number of soil-borne pathogens
Soil solarization kills pathogens in the upper layers of the soil as a direct result of heating. Heat doesn’t travel that well into the lower layers and isn’t enough to get rid of pests and pathogens in the lower layers. Instead, beneficial soil organisms that can control these pests and pathogens become more numerous in the lower layers (see #5 below) due to changes caused by heating.
Soil solarization is most effective against fungi and bacteria that cause soil-borne diseases, such as Verticillium wilt, Fusarium wilt, Phytophthora root rot, Southern blight, damping off, crown gall disease, tomato canker, and potato scab. Nematodes that live in the upper layers of the soil can also be controlled by soil solarization but to a lesser degree, as they can move faster and can recolonize the soil and plant roots faster.
2. Limits weed emergence
In general, winter season and rainy season weeds are more easily controlled using soil solarization than summer season weeds. This may be due to the higher thermal tolerance of summer season weeds.
In addition, seeds with a thick seed coat are also less sensitive to heat and may not be affected much by soil solarization. Seeds that can emerge from deeper in the soil are also less affected by soil solarization since its effects are concentrated at the surface of the soil.
Soil solarization is also effective at reducing seed reserves in the soil and will not only have an impact on the very next crop grown but can reduce weed emergence during successive seasons as well.
3. Increases soil nutrient levels and reduces fertilizer requirements
Decomposition of organic matter or the ‘cooking’ of soil that happens when the soil is solarized can increase levels of soluble mineral nutrients in the soil.
Concentrations of ammonium-nitrogen and nitrate-nitrogen, as well as soluble mineral nutrients, including calcium, magnesium, phosphorus, and potassium, have been found to have increased after soil solarization. This increase in the amount of nutrients will lead to a garden that is healthier and more productive, and will in turn reduce the amount of fertilizer needed.
4. Doesn’t drastically affect beneficial soil organisms
Soil solarization only mildly heats the soil surface to about 45-55°C degrees Celcius and lower layers of the soil to about 35-40°C degrees Celcius, so it doesn’t eliminate all soil organisms, which if did happen would make the soil pretty lifeless and useless. In fact, soil solarization creates room for more beneficial soil organisms to come in (see below).
5. Encourages an increase in beneficial soil organisms
Soil is healthier when it contains a larger number and higher diversity of beneficial soil organisms. Solarization gets rid of many plant parasites and pathogens from the soil. This makes the soil available to be colonized by beneficial soil organisms and for their numbers to increase.
The resulting increase in beneficial soil organisms leads to better root and plant growth and in turn a higher level of productivity.
Disadvantages of soil solarization
Despite being a cheap and easy organic method of controlling pathogens, weeds, and pests, its effectiveness is limited.
- Effectiveness depends on air temperature
Since solar energy is relied upon for soil solarization, cloud cover, lower air temperatures, and rain can reduce its effectiveness. For this reason, soil solarization is most suitable for Mediterranean, desert, and tropical climates that have high summer air temperatures.
- Most effective near the soil surface
Soil solarization is most effective at the soil surface meaning that this technique is most effective at treating small amounts of soil, such as in seedbeds, container mediums, and cold-frames.
- Not consistently effective in controlling heat tolerant pests
Soil solarization is most effective at killing pests and pathogens that have a medium level of heat tolerance and may not be effective on pests and pathogens that have a higher level of heat tolerance.
- Gets rid of some beneficial soil organisms
Heating the soil indiscriminately affects all soil organisms, regardless of whether they are harmful or beneficial. So, soil solarization will affect quite a few beneficial organisms too. Despite this, you can add compost and other organic manures that will encourage the lost beneficial organisms to re-colonize the soil as soon as possible. This minimizes damages done as a result of the loss of these beneficials.
- May interfere with planting schedules
Since soil solarization should be done during the hottest part of the year, in areas with high seasonal variability, the ideal time for soil solarization will coincide with the ideal time for growing many types of warm-season vegetables.
- Requires the disposal of quite a lot of plastic
Home gardeners who are more likely to use organic vegetables are usually those who are most concerned about the environment. This means that gardeners who are most likely to use this method are also most likely to be against creating excessive amounts of plastic waste. This can create quite a dilemma for such gardeners.
You can save and reuse plastic sheets that you get inside large packages, especially those covering large home appliances, which will have to anyway be disposed of.
Soil solarization is an easy and cheap way of disinfecting the soil by getting rid of soil-borne pests and pathogens, as well as weeds. This organic technique also helps the soil become healthier and richer in nutrients, which in turn will make your vegetable garden more productive and less prone to pests and diseases. Despite this, soil solarization needs warm temperatures to work and is most effective on the upper soil layers or small amounts of soil, which limits its use.
