Biodegradable Mulch: Does it Make the Grade?

 Biodegradable mulch and bioplastics are buzz words heard fairly often in today’s social climate of reducing waste and replacing some of our traditional materials with more eco-friendly options. There are many products out there claiming to be compostable or biodegradable, but what does that actually mean?

One of the biggest concerns with some of the plastic products claiming to break down in the environment is that they never fully break down and instead result in microplastics, which can cause even more problems down the line. So, what is a biodegradable mulch?

The major indicator of what constitutes a biodegradable mulch is the degradation process and the resulting products. Degradation is the measurable conversion of mineralized carbon to CO_2, resulting in a change in the chemical structure, physical properties, or appearance of the material. This is a very important point since the degradation of biodegradable mulch results in the complete breakdown of the material into biomass and CO₂ rather than microplastics. It’s important to note that biodegradable is not the same thing as photodegradable (breaks down with light) or oxodegradable (breaks down by oxidation) plastics, both of which are made up of conventional plastics.

Photo: Dubois Agrinovation

Biodegradable mulch is designed to be tilled into the field after use and relies on certain material properties and environmental conditions to facilitate degradation such as temperature and presence of microorganisms.

A study conducted through Washington State University Extension looked at the impact of soil-biodegradable plastic mulch on soil health and quality. The soil health assessment was based on indicators relating to structure, hydrology, biology, chemistry, fertility and salinity/sodicity. Field plot treatments were four different biodegradable plastic mulches, a biodegradable paper mulch, a conventional polyethylene mulch and bare ground. The biodegradable mulches were applied annually in the spring and tilled into the soil in the fall for 2-4 years. The overall findings indicated that the various biodegradable mulch products did not negatively impact overall soil health. However, it should be noted that long-term evaluations are required to form more solid conclusions.

A separate study looked at the presence of visible mulch fragments after four years of mulch applications and one year of rest as a measure of initial mulch degradation. As little as 20% of the original volume of mulch remained in the soil in a recoverable form (fragments large enough to be separated from the soil with a 2mm sieve), proving that degradation is taking place. Although this measurement wouldn’t indicate the rate or extent of degradation, the materials that make up the mulch will eventually degrade into CO₂ and biomass, so would not be at risk for contributing microplastics to the soil environment.

There are some important things to consider when thinking about switching to biodegradable mulch from traditional plastics. Biodegradable mulch typically has less elasticity and strength than polyethylene mulch, so field conditions need to be appropriate so there isn’t extra strain being placed on the mulch, which could cause it to tear. Appropriate field conditions would include a smooth seedbed that isn’t saturated with water to form clods, and no big rocks. The mulch should be laid looser than you would a plastic mulch as it will tighten up a little under field conditions. Trials in Washington State showed that vegetable yields under biodegradable mulch were equal to those under plastic mulch and generally better than on bare ground. Weed suppression with the biodegradable mulch did not always measure up, however. Since the mulch should be fairly brittle by the end of the season, it should easily break into pieces during rototilling and not clog up the equipment.

For more information about biodegradable mulch or for more detail on the studies mentioned in this post, visit Washinton State’s Small Fruit Horticulture Research and Extension Program and University of Tennessee Institute of Agriculture’s Biodegradable Mulch page.