Soil solarization offers gardeners and agricultural operations a non-chemical approach to sterilize soil and manage soil-borne pests. This method utilizes solar energy to heat the soil, effectively reducing populations of weeds, pathogens, and harmful nematodes. As a sterilization process, it raises concerns regarding its effect on beneficial soil organisms, particularly earthworms. Understanding the thermal dynamics of solarization and the natural behavior of earthworms is necessary to determine the impact on these important soil dwellers.
The Process of Soil Solarization
Soil solarization begins by preparing the treatment area, tilling the soil six to twelve inches deep and removing debris. The soil must be thoroughly moistened, as water is a more efficient conductor of heat than dry soil. This moisture maximizes the thermal effect on stationary pests and weed seeds.
The prepared soil is covered with a clear, thin sheet of plastic, typically one to two mils thick, and the edges are securely buried. The clear plastic acts like a greenhouse, trapping solar radiation and converting light energy into heat. This process is most effective during the hottest months, usually lasting four to six weeks in warm climates.
The resulting heat depends on the sun’s intensity and treatment duration. Solarization works by maintaining high temperatures over an extended period. The goal is to raise soil temperatures sufficiently to disrupt the life cycles of target organisms and cause thermal death.
Thermal Tolerance of Earthworms and Beneficial Soil Life
Earthworms are ecosystem engineers, performing crucial functions such as aerating the soil, improving water infiltration, and accelerating nutrient cycling through the breakdown of organic matter. Their presence is associated with healthy, fertile soil. Like all organisms, earthworms have specific temperature tolerances that govern their activity and survival.
The activity of most earthworm species decreases significantly when soil temperatures drop below 50°F (10°C) or climb above 104°F (40°C). The upper lethal temperature for many common earthworm species ranges between 77°F and 95°F (25°C to 35°C) if exposure is prolonged. Unlike stationary organisms, earthworms are highly mobile and have a natural defense mechanism against unfavorable conditions.
When faced with excessive heat or drought, earthworms instinctively burrow deeper into the soil profile, a survival behavior known as aestivation. This movement allows them to seek cooler, more moisture-stable soil layers. This natural mobility is the primary factor that differentiates their survival rate during solarization from that of the non-mobile pests the treatment is designed to eliminate.
Analyzing the Impact: Temperature Zones and Mortality
Soil solarization creates a significant thermal gradient, with the highest temperatures concentrated near the surface. In the top two to six inches of soil, temperatures commonly reach between 108°F and 140°F (42°C to 60°C) in hot, sunny conditions. These temperatures are lethal to many shallow-dwelling pathogens, nematodes, and annual weed seeds.
The effectiveness of heat penetration decreases rapidly with depth, creating distinct temperature zones. While the surface layer reaches temperatures exceeding the earthworm’s lethal threshold, the subsoil remains significantly cooler. At a depth of 18 inches, the maximum temperature achieved typically drops to a lower range, often between 90°F and 99°F (32°C to 37°C).
Because lethal temperatures are confined primarily to the upper six to eight inches of the soil, earthworms possess a functional escape route. They retreat to the cooler subsoil below the lethal zone to avoid the heat. While any earthworms trapped near the surface may perish, the overall population of mobile earthworms is generally not eradicated by solarization. The treatment is considered selective, targeting non-mobile pests while allowing mobile beneficial organisms to survive by migration.
Minimizing Harm to Beneficial Organisms
Gardeners can take several actions to maximize the survival of earthworms and other mobile beneficial soil life during solarization.
Soil Preparation
The most important step is ensuring the soil is consistently and thoroughly moistened before laying the plastic sheeting. Moist soil facilitates the burrowing movement of earthworms, making it easier for them to quickly migrate to deeper, cooler soil layers.
Timing and Duration
The timing of the solarization period influences the outcome for earthworms. Performing the treatment during the peak of summer heat achieves the highest temperatures and deepest penetration, but presents the greatest risk to soil life. Using the minimum effective duration—often four weeks—rather than the maximum, reduces the overall heat stress on the soil ecosystem.
If pest pressure is moderate, a slightly shorter treatment duration or planning the process for early spring or late summer may be considered. These slightly cooler periods still raise the soil temperature sufficiently to control many pests. They also create a less severe thermal gradient, increasing the chances of earthworm survival. Monitoring the soil moisture throughout the process is advisable, as persistent dryness in the deeper layers can be detrimental to earthworm survival.