Plastic mulch has been a feasible and useful technology for specialty crops including fresh vegetables and small fruit crops for over six decades but like any technology, proper selection and installation is critical to get the most out of it. Growers use plastic mulch because of the advantages it provides. High value vegetable crops well suited to production with plastic mulch are tomatoes, peppers, melons, squash, cucumbers. Strawberry is also a well-known crop that benefits from plastic mulch. Some benefits of plastic mulch are soil moisture retention and improved water use efficiency, weed and pest control, reduction of soil compaction, erosion and nutrient leaching, ability to absorb or reflect sunlight (solar radiation) to warm or cool the soil depending on mulch color, season extension, and improved yield and quality. In contrast, disadvantages of using plastic mulch are increases in inputs and costs from the use of specialized equipment, labor/time for installation, and removal and disposal of used plastic unless adopting biodegradable mulches. This article discusses how plastic mulch color and proper installation influence energy flow and soil temperature.
Plastic mulches are available in many colors. Black, clear, silver reflective, and white are commonly used but infra-red transmittance (IRT-green) is gaining popularity. Black plastic mulch is widely used in spring and late fall to warm up the soil and is usually the least expensive. The black plastic absorbs the energy from the sunlight and heats up (figure 1). Then the heat is transferred by conduction from the black plastic mulch to the soil when there is tight contact with the soil. Heat conduction is Inefficient and soil warming is suboptimal when black mulch is not tightly in contact with the soil. Improper bed preparation and/or plastic installation may result in clumpy soil and/or loose plastic mulch. At night, the air temperature decreases, especially with clear sky and dry air, and soil heat loss occurs by conduction through the plastic mulch back to the atmosphere. However, water condensation beneath the plastic acts as a barrier to heat conduction and soil cooling is slower than in bare ground (table 1).
There is a controversy as to whether black or clear plastic mulch heats the soil better and is due to the different mechanism of heating the soil. In contrast to black mulch, sunlight passes through the clear plastic mulch reaching and heating the soil directly. Tight contact with the soil is not as important but recommended to avoid flapping and transplant damage. However, herbicides may be needed to control weeds growing under clear plastic mulch. Also, red plastic mulch, which is sometimes used in tomatoes. does not have good weed control and weeds push the plastic up and loosen it.
White and silver reflective mulches reflect most of the sunlight back to the atmosphere, so there is minimal energy absorption and heat transfer to the soil (figure 2). Therefore, the soil usually stays cooler than bare ground or clear, black, or any colored plastic mulch (table 1). At night, soil heat loss occurs by conduction through the plastic mulch back to the atmosphere. Silver reflective or aluminized plastic mulch, however, has been shown to have some insulating capacity since it reduces heat/energy loss at night keeping the soil warmer than other plastic mulches or bare ground (table 1).
Wavelength selective plastic mulches (IRT-green, PST-green, SLT-red, SRM-red, Al-Or brown) can absorb, reflect, and let through specific wavelengths of the solar radiation. These mulches absorb most of the visible light transferring the heat to the soil by conduction when in tight contact (figure 3). Absorption and exclusion of the photosynthetically active sunlight allows excellent weed control. In addition, these mulches allow part of the sunlight spectrum (e.g., IRT=infra-red transmitting) to pass through and heat the soil directly (figure 3). Therefore, both mechanisms (heat transfer from plastic and direct soil heating) play a role in the more efficient soil warming characteristic of wavelength selective plastic mulches (table 1), which is useful for early plantings.
Good heat transfer starts with proper soil preparation. All organic matter must be turned over and there should be no clumps on or near the surface. These clumps interfere with a smooth, sharp-edged bed and tight contact of the plastic mulch with soil. Some growers use rotary tiller as the final step since they mix soil horizontally, not vertically. Another helpful practice is to pre-shape the bed. This is as simple as pre-shaping the row/bed with coulters/hipper to throw soil into the center of the ridged row/bed. Not enough soil in the center leaves pockets and plastic stays suspended above the soil. The air in that space will be heated, but heat transfer to the soil is inefficient. In addition, proper adjustment to the bed shaper and plastic layer itself is also important to getting enough soil into the bed.
There are two basic types of plastic mulch to choose: smooth or embossed plastic. Embossed plastics can stretch so they are less prone cracking due to expansion and contraction under fluctuating temperatures as smooth plastic are. They also maintain better contact with the soil for efficient energy transfer. Warming the plastic before laying helps with tight contact with soil. Plastic mulch is often laid cold after being in storage. Once it is laid in the field, sunlight warms it and it becomes loose on the bed due to expansion. Loose plastic not only is inefficient to transfer heat to the soil but can be caught by the wind and eventually pulled off. If possible, store the plastic in a heated area before use or let it sit exposed to sunlight before laying.