Apr 22, 2016
Tight plastic important for good heat transfer

Plasticulture is a well-accepted cultural practice for many fresh vegetable crops. Plastic mulch helps control or reduce many diseases and weeds. It also increases yields, conserves moisture and maximizes nutrient applications. Another important characteristic is the mulch’s ability to warm soil, encouraging root development and ultimately enhancing above-ground growth, contributing to earliness and higher yields.

Laying plastic on raised beds (foreground) with a bed pre-shaper (background). Photo: Ron Goldy/MSU Extension.
Laying plastic on raised beds (foreground) with a bed pre-shaper (background). Photo: Ron Goldy/MSU Extension.

To get the full warming effect, it is important that plastic make tight contact with the soil. Soil warming occurs when the sun warms the plastic and the plastic transfers heat to the soil through physical contact. Loose fitting plastic will not have the same amount of heat transfer ability as tight fitting plastic. There are some steps producers can take to ensure good heat transfer.

Soil preparation

Good heat transfer starts with proper soil preparation. All organic matter must be turned over and there should be no sod clumps on or near the surface. These clumps interfere with a smooth, sharp edged bed. This means moldboard plowing and then being sure subsequent soil preparation doesn’t bring clumps back to the surface. Some growers use rotary harrows as the final step since they mix soil horizontally, not vertically.

Another helpful practice is to pre-shape beds (see photo). This is as simple as attaching two coulters to a three-point hitch toolbar on the back of a tractor. Set the coulters so they throw the soil into a mound the same width as the final bed. Doing this helps fill the bed by getting more soil to the center. Not enough soil in the center causes plastic to be suspended above the soil. The air in that space will be heated, but air will not allow as much heat to be transferred.

Proper adjustment

Getting enough soil into the bed is helped by pre-shaping, but proper adjustment to the bed shaper, plastic layer itself is also important to getting enough soil into the bed. If there is a dip in the bed center, not enough soil is going into the shaper. Lowering the bed shaper a bit deeper or adjusting the angle of the shaper will help get more soil into the implement.

Warming the plastic

This is something most producers don’t think about. Plastic is often laid when air temperatures are cool but the sun is shining brightly. Rolls of plastic are usually stored in a pole building and brought into the sun just before it is put into the laying implement. So the plastic is laid when it is cold and then exposed to the bright sunshine. Like most things, plastic shrinks when it is cold and expands when it warms. Laying plastic when it is cold and shrunk and then exposing it to the sun where it can heat up allows the plastic to loosen on the bed. Loose plastic can be caught by the wind and eventually pulled off if the wind is strong or prolonged. In the meantime, the loose plastic during the day isn’t conducive to good heat transfer.

If possible, store the plastic in a heated area rather than having it heat and expand when applied. At the very least, the plastic could be brought out and placed in the direct sun for several hours where it can heat up before it is applied to the bed.

Infrared transmitting (IRT) plastic

IRT plastics are plastics that allow infrared light to pass through while excluding photosynthetically active light. This inhibits weed growth but allows heat generating light wavelengths to reach the soil and warm it directly. These are useful for early plantings and in more northern, cooler areas, but are not generally used for large acreage due to increased expense over black. With IRT plastic, tightness isn’t as critical from a heat generating standpoint since the heating process does not require physical contact.

Michigan State University Extension encourages growers to utilize plasticulture technology, but understanding how the technology works will help producers get the most out of it.

Ron Goldy, Michigan State University

Source: Michigan State University Extension

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