Solarization most commonly refers to a method of using the sun’s heat to sterilize soil, killing weeds, fungi, and other pests without chemicals. The term also applies to two other processes: a tonal reversal effect in photography, and the gradual discoloration of glass exposed to ultraviolet light. All three share the same root idea of sunlight transforming a material, but soil solarization is by far the most widely searched and practically useful meaning.
How Soil Solarization Works
Soil solarization is straightforward: you cover moist soil with a sheet of clear plastic during the hottest weeks of summer and let the sun cook it. The plastic traps radiant energy the same way a greenhouse does, raising soil temperatures high enough to kill weeds, plant diseases, root-eating nematodes, and insect larvae in the top 12 to 18 inches of earth.
At a depth of 2 inches, soil under plastic typically reaches 108°F to 140°F. Deeper down, at 18 inches, temperatures settle between 90°F and 99°F. The sweet spot for pest control is the top 6 inches, where daily highs need to stay at or above 110°F to 125°F. At 158°F or higher, virtually all soil pests die within 30 minutes. In warmer climates like inland California, soil inside black plastic bags can exceed 160°F.
Why Clear Plastic Beats Black
This surprises most people: clear plastic heats the soil more than black plastic does. Black plastic absorbs sunlight at its surface, while clear plastic lets light and heat pass straight through to the soil beneath. That means the ground itself becomes the heat collector rather than the tarp. Black plastic does block light and can smother weeds through a related technique called occultation, but it takes longer and doesn’t reach the same temperatures.
What It Kills and What Survives
Solarization is especially effective against fungal pathogens that live in soil. Research on lettuce production found that solarization outperformed both chemical fumigants and biological control agents at reducing populations of several common root-rot fungi. It was the single most effective treatment tested across two years of trials.
The effect on beneficial soil life is more nuanced than you might expect. Rather than sterilizing the soil completely, solarization reshapes the microbial community in ways that can actually help plants. Heat-tolerant bacteria in the class Bacilli, which includes many species known to suppress plant diseases, nearly doubled in relative abundance after solarization in one study, jumping from about 15.5% to 24.6% of the bacterial community. Meanwhile, many harmful fungi were dramatically reduced. Populations of beneficial Pseudomonas bacteria, another group linked to disease suppression, showed no significant change. The net result is soil that’s not sterile but shifted toward a healthier microbial balance, which helps explain why plants often grow more vigorously in solarized soil than you’d expect from pest reduction alone.
How to Solarize Soil
The process works best during the four to six hottest weeks of summer, when daytime highs consistently exceed 80°F to 90°F. Start by clearing the area of any plant debris and large clods. Water the soil deeply, because moisture conducts heat far better than dry soil does. Then lay clear polyethylene plastic (standard painter’s plastic from a hardware store works) directly on the soil surface, pressing it down so there’s as little air gap as possible. Bury or weigh down the edges to seal in heat.
Leave the plastic in place for four to six weeks. In cooler climates or at higher elevations, you may need the full six weeks or longer. In hot inland valleys, four weeks is often sufficient. After you remove the plastic, try to avoid deep tilling, which can bring untreated soil and weed seeds up from below the zone that was heated.
Solarization in Photography
In photography, solarization refers to a partial reversal of tones in a print or negative. The classic technique, more precisely called the Sabattier effect, involves briefly exposing a partially developed print to light, then continuing development. The result is an image where some mid-tones appear reversed, creating an ethereal, almost metallic look with distinct outlines between light and dark areas.
The chemistry behind it involves what happens at the surface of light-sensitive silver halide crystals. During normal development, a chemical reaction converts exposed crystals to metallic silver, creating the image. When the developing print gets a second flash of light, the already-developing areas release ions that migrate inward, essentially locking the crystals in a state that surface-acting developers can no longer reach. Those mid-tone areas stop developing and appear lighter than they normally would, creating the characteristic tonal flip. It’s not a true reversal of the image so much as a selective shutdown of development in certain zones.
Solarization of Glass
If you’ve ever noticed old window glass with a faint purple or amber tint, you’ve seen glass solarization. Prolonged exposure to ultraviolet light causes certain mineral impurities in glass to change their chemical state, shifting how the glass absorbs visible light. Manganese dioxide, commonly used as a clarifying agent in glass made before the 1920s, is the classic culprit behind the purple tint in antique windows. The UV energy triggers electron transfers between impurity atoms, permanently altering the glass’s color. Modern glass formulations use different clarifying agents to avoid this effect, which is why it’s mostly seen in older buildings and collectible bottles.