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 CO2, 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.
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| 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.
