The Ubiquitous Use of Plastic Mulch in Organic Systems
Organic Agriculture Centre of Canada
Consumers are sometimes shocked to find that organic standards allow for the use of plastic mulch on organic farms. After all plastic polymers are synthetic petrochemical products and not “natural”.
Organic standards allow the use of polyethylene, polypropylene and other polycarbonate-based plastic mulches but only if they are not incorporated into the soil, and that they are lifted off and disposed of in an environmentally responsible manner. After lifting the plastic a soil building cycle must be completed to help rebuild soil health.
So why do some organic farmers persist? Because plastic mulches do have their advantages, and many farms already have the tools to work them.
| Alternatives to using plastic mulch for soil warming |
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Not only are a lot of farmers familiar with their use but plastic mulches warm up the soil which gets plants off to a quick start. Plastic mulches are also synonymous with cost effective weed control. For weed control there are other options, but for warmth, plastic mulch can’t be beat. For example, clear plastic and dark coloured infrared transmitting (IRT) films can raise soil temperatures 4-8°C over soil without plastic. Black plastic less (only 1.5– 4°C). But clear plastic and some shades of IRT have drawbacks. Neither can suppress weeds and soil temperatures get too warm underneath them especially in regions with intense solar radiation.
Compared to bare soil there is less water evaporation when plastic mulches are used. Crops are usually cleaner with plastic mulches, reducing post harvest handling requirements. Cleaner produce might mean lower risk of food pathogens.
Conversely there are disadvantages associated with using plastic mulches. Number one is the cost of the mulch and the associated drip irrigation materials. Neither rain nor irrigation penetrate through the plastic making under-plastic irrigation systems necessary. There is also the expense of laying (manually or with the aid of equipment), removing, and possible disposing of  the plastic. Few places can recycle agriculture plastic, requiring tipping fees to be paid and the plastic left to accumulate in land fills.
Another serious consideration is run off. As the material is impervious and covers a large portion of a field, water runs off the plastic causing varying degrees of soil erosion and possible contamination of nearby waterways. Planting cover crops between the rows to catch and slow run off can help, but is rarely done.
In addition, some studies show that there is a greater likelihood of disease outbreaks (e.g. early blight tomatoes) in plastic mulch plantings vs. those with vetch mulch. And the greatest drawback is the inability to build soil health while plastic mulch is in place.
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Use plastic mulch in the row, and a tall cover crop between rows. As the cover crop grows, cut it This cools the plastic slowing its degradation. Producers report re-using plastic for up to 5 years. |
But reasonable alternatives are available. Comparative production including in-row weed control have been achieved using kraft paper mulch (bought in rolls) and live or killed cover crop mulch instead of plastic. Biodegradable mulches are also becoming popular and are allowed if the composites of the mulch fully comply with organic requirements. Finding a compliant biodegradable mulch that most certifiers will accept is challenging and currently they are prohibitively expensive. Â
All these alternatives have the advantage that they do breakdown and in the process contribute to soil and ecological health; crop residues more so than paper. Unfortunately, none of these alternatives consistently contribute to early soil warming.
Paper mulch is heavier than plastic so more costly to transport and harder to handle. It may disintegrate prematurely and can be harder to install and hold in place. Crop mulches don’t have the same problems as paper but create good slug habitat and are definitely slower to warm up in the spring. Crop mulches can become a challenge if allowed to go to seed.
Paper and living mulches or killed cover crop mulches can be excellent alternatives to plastic mulches unless early spring soil warming is a must. More growers should consider making the switch.
This article was written by Rochelle Eisen on behalf of the OACC with funding provided by Canada’s Organic Science Cluster (a part of the Canadian Agri-Science Clusters Initiative of Agriculture and Agri-Food Canada's Growing Forward Policy Framework). The Organic Science Cluster is a collaborative effort led jointly by the OACC, the and industry partners. For more information: oacc@dal.ca or 902-893-7256.
Posted May 2011