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Colored
Mulches Allow Growers More Control of Light, Heat
By Kimberly Warren Associate Editor Adding color to your field can do more than brighten it up according to studies done at Michigan State University (MSU) and other universities across the country. Using colored plastic mulches will affect the light radiation at the surface and either reflect certain amounts of it or absorb certain amounts of it – allowing growers a little more control in soil temperatures or plant exposure to light. “Basically there are three things that happen to solar radiation: it can be absorbed by the plastic or it can go directly through the plastic or it can be reflected back to the atmosphere,” said Mathieu Ngouajio, assistant professor and Extension specialist at MSU. “All those three things depend on the color of the plastic.” For instance, Ngouajio said, black plastic will absorb all radiation and transfer it into the soil in the form of heat. The opposite of that is aluminized plastic that will reflect most everything. Somewhere in the middle lies white plastic, he said. “Then you get the new technologies of infrared transmitting (IRT),” Ngouajio said. “For the IRT, it will allow the infrared radiation to go through the plastic but will block the radiations that are for photosynthesis.” Using these plastics,
he said, will combine the advantages of increasing soil temperature
while reducing weed pressure because there is no light that can be
used for photosynthesis. “If it reflects light up, it can be used by the plant for photosynthesis. Those plastics that ultimately affect soil temperature will affect plant growth,” he said. “That’s why most people do it: they use plastics to warm up the soil in the spring to be able to grow the crop a couple of days earlier. “It is what you call modifying the microclimate around the crop, which is in the form of light and heat,” he said. Ngouajio’s research is specific for Michigan’s vegetable crops. Using the colored mulches in California, for instance, would lead to different results. A grower in California would not want to use plastics to warm the soil, but to cool the soil. In the lab, Ngouajio and other researchers use a spectroradiometer to measure how much light the plastic reflects and how much light is going through the plastic. With this, they use a data logger to monitor soil temperatures on each type of plastic. Most of the plastics
are made of a low-density polyethylene. For example, Ngouajio said, silver plastic on cucumbers can help divert aphids by disorienting them so they can’t recognize the plant. However, this silver plastic will not have the same results on tomatoes. “Even if you have two plastics that reflect the same amount of light, the quality of the light is important also,” Ngouajio said. “The red and yellow plastics will reflect the same amount of light, but the light will be very, very different if you look at the spectrum.” Before growers decide to use colored plastic mulch, Ngouajio said it is important for them to talk with researchers to see which color of plastic will work best for their needs. In discussing plastic mulches of any kind, Ngouajio said it is important to consider disposal. The trend in the industry, he said, is to develop plastics that will biodegrade in the field. Research is being done for those right now on the white and black plastics. “(Plastics disposal) is the biggest problem of the industry – the only limitation to the widespread use of plasticulture,” he said. “The limitation is not science, but it is finding the right technology to make those (biodegradable) plastics inexpensive.” Ngouajio will be presenting information on his colored mulch research on tomatoes on Tuesday, Dec. 9 at the Great Lakes Fruit, Vegetable and Farm Market Expo. For more information on the Expo, visit www.glexpo.com.
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