Using Plastic Mulch In Commercial Vegetable Production
Mulching is the act of spreading a protective layer of material on top of soil in crop production. The activity offers numerous benefits, including the conservation of soil moisture, the limiting of soil erosion, and the maintenance of an even soil temperature.
There are a wide variety of materials that may be used — they include organic options such as bark chips, grass clippings, and straw, and inorganic options such as brick chips and stones (Natural Resources Conservation Service).
However, in the world of commercial vegetable production, no alternative is as high-profile as plastic mulch.
The story of plastic mulch begins in the 1950s. During that period, Dr. Emery Emmert of the University of Kentucky experimented with using polyethylene film as a cover for greenhouses instead of glass. Emmert then extended this application to vegetable production, finding that the film helped warm the soil. His work lead directly to the usage of plastic as mulch material in commercial plant production in the 1960s (Plastic Mulch Films). Since then, its adoption has become widespread — in fact, approximately 2,500 square miles of agricultural land worldwide makes use of polyethylene mulch for crop production (TechAgro).
Plastic mulch is popular because it provides a number of advantages over other varieties. These include the following (Schonbeck; Samuel Roberts Nobel Foundation):
- Earlier Crop Growth — Plastic mulch has a “greenhouse” effect on soil, effectively trapping heat to raise its temperature. This enables growers to begin planting sooner in growing season than they could with organic varieties, and also results in more rapid crop development and earlier yields. Many consider this the biggest value of using plastic.
- Of note, there is a circumstance in which a higher temperature may not be desired. If plants are being produced during the summer, they do not require more heat. In this situation, it is ideal to use a white plastic mulch, as the color reflects heat instead of absorbing it.
- Stronger Weed Control — Sunlight helps foster the growth of weeds by compacting soil; plastic mulch blocks sunlight from reaching the soil around the plant, thus preventing weeds from appearing. Black polyethylene and other opaque films in particular are useful for this purpose, blocking a wider range of weeds than organic mulches. The only exception is clear plastic, which lets sunlight in. The presence of fewer weeds also reduces the necessity for mechanical cultivation, leading to less root damage.
- Better Moisture Retention — Plastic mulch helps limit evaporation of water from soil, leading to better moisture retention. Because of this, less water is required for irrigation. It also facilitates more even distribution of water through the soil.
- Minimized Fertilizer Leaching — Plastic mulches prevent an excess of water from going into the soil, helping to limit the loss of plant nutrients through leaching. Because the mulch is restrictive in this sense, it is most ideal to use it in conjunction with the drip irrigation method, which allows the addition of water and fertilizer in small amounts.
- Higher Crop Quality — Plastic mulch helps crops avoid direct contact with soil; this results in the plants being cleaner, which is desirable for fruits such as strawberries. It also works to prevent rot as mud from irrigation is not splashed on the crops.
As the list above shows, using plastic mulch for commercial crop production yields a number of positive outcomes. Growers encouraged to adopt the practice based on these benefits should be aware of certain guidelines that will ensure the success of the activity in its four key areas: preparation, application, transplantion, and removal.
In preparing a surface for mulch, there are some conditions that need to be met. Plant beds should be loose and free of clods, stones, and organic residue. The presence of these materials can result in insufficient contact between the soil and the mulch, reducing its effectiveness. Soil should be sufficiently moist to ensure firm beds. The Natural Resources Conservation Service recommends an available water capacity (AWC) of 60 — 80 percent; its websites offers guidelines on how to make field estimates (NRCS in Alabama).
Once the field is prepared, the surface is ready for mulch application. In the commercial world, the simplest way to accomplish this process is through the use of a mechanical mulch layer. (Plastic Mulch Films). The device rolls over the field, laying down a sheet of plastic mulch over the plant beds. The sheet can be flat (meaning it is level with the soil surface) or raised (meaning that it is thicker so that the top of the sheet is higher off the soil). The machine also facilitates drip irrigation by placing drip tubing under the plastic during the laying process. Drip irrigation is beneficial for plastic mulching because it delivers water and fertilizer to the soil in small amounts, preventing an excess of the former and leaching of the latter. Placement of the tubing is important and varies by soil type. In heavier soils, movement of water tends to be lateral, so it is fine to place the tubing 1-2 inches below the surface. However, in lighter sandy soils, downward movement of water is more likely, so tubing should not be placed more than 1 inch below the soil surface, to ensure that it receives enough moisture (NRCS in Alabama).
After the mulch has been applied, next comes transplanting, or planting. Commercially, this process also utilizes special equipment — a waterwheel-type transplanter. The machine has a rotating drum with spikes that, when rolled over the plastic mulch, puncture its surface, creating holes. The drum simultaneously fills the holes with water and fertilizer. The individuals operating the transplanter then place a plant in the hole. The transplanter can puncture the mulch at specified intervals to ensure the preferred amount of spacing for the crops (Coolong).
After the plants have grown and the harvest has been completed, the plastic mulch should be removed. There are number of equipment types available for this purpose; however, at their most basic, they cut the plastic down the middle and pull it up from each side. Following this activity, the plastic should be disposed of in accordance with local laws (Plastic Mulch Films).
An alternative to plastic mulch aimed at eliminating the need for removal and disposal is biodegradable plastic. This variety theoretically works by breaking down over time through exposure to the environment, so that no mulch remains. The ultimate goal is to save growers the time and expense posed by having to remove and discard plastic mulch at the end of a season; these tasks can represent a particularly significant cost for medium and large operations. Biodegradable plastic mulch has been in development for decades. Initial versions of the material were photodegradable, meaning that they decomposed when placed under light. These were spurned by growers because they left pieces of undecomposed mulch in the field. In the past few years scientists have introduced another option, black plastic films made of plant starch; these films are torn down by soil microbes into water and carbon dioxide. They compare favorably to traditional black plastic in terms of soil warming and weed control, and appear to experience thorough decomposition. Even though they can be priced as much as two to three times the cost of black plastic, these expenses would be offset by the avoidance of disposal costs. Unfortunately, the USDA National Organic Program (NOP) is yet to approve this option because it features synthetic components whose residues may stay in the soil once the mulch has broken down. The most prominent biodegradable mulches being studied currently are BioTelo by Dubois Agrinovations and Eco-One by Heartnut Groves (Schonbeck; Plastic Mulch Films).
Another oft-discussed alternative to plastic mulch is paper mulch. This variety has been studied for use in commercial crop production for decades, and was once highly used; however, it was largely abandoned once the advantages of plastic mulch were discovered. Recent years have seen renewed interest in the material due to three qualities: it is biodegradable, it is manufactured from a source that is renewable, and it does not require disposal following harvest. Further, the aforementioned NOP has approved several versions of the product. However, paper mulch also presents many disadvantages. For one, it is pricier than plastic mulch. These expenses include higher transportation costs, as it is heavier. Paper mulches can also degrade too quickly for them to be effective. Moreover, some studies have shown that the mulches do not warm soil sufficiently, leading to smaller crop yields. Despite these realities, some researchers believe that combining paper with biodegradeable plastic could address some of these issues (Schonbeck; Coolong).
In summary, plastic mulch offers a wide number of significant advantages over organic mulches and many of its inorganic counterparts in commercial vegetable production. These benefits touch all parts of the mulching process, from soil protection to temperature control to plant quality preservation. In pursuing this activity, it is in the best interests of growers to follow the guidelines previously outlined. Doing so will help to ensure that the process runs as smoothly and efficiently as it can, and that one’s crops are in the best possible shape they can be.
Bibliography
Coolong, Timothy. “Performance of Paper Mulches Using a Mechanical Plastic Layer and Water Wheel Transplanter for the Production of Summer Squash.” Hort Technology 20.2 (2010): 319-324. Website. 13 July 2015. <http://horttech.ashspublications.org/content/20/2/319.full>.
Samuel Roberts Nobel Foundation. “Permanent Raised Bed Gardening: Plastic Mulch Application.” n.d. Website. 13 July 2015.
Natural Resources Conservation Service. Mulching. n.d. Website. 13 July 2015.<http://www.nrcs.usda.gov/wps/portal/nrcs/detail/az/home/?cid=nrcs143_023585 >.
NRCS in Alabama. “Plastic Mulch and Drip Irrigation.” n.d. 13 July 2015.
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Plastic Mulch Films. n.d. Website. 13 July 2015. <http://nevegetable.org/cultural-practices/plastic-mulch-films>.
Schonbeck, Mark. “Synthetic Mulching Materials for Weed Management.” Organic Agriculture (2012). Website. 13 July 2015. <http://www.nrcs.usda.gov/wps/portal/nrcs/detail/az/home/?cid=nrcs143_023585 >.
TechAgro. Plastic Mulch. 12 July 2009. Website. 14 July 2015. <http://techagro.blogspot.com/2009/07/first.html>.
