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Are Biodegradable Mulches Actually Better for the Environment?

Henry Sintim, PhD student at Washington State University, is investigating whether biodegradable mulches are, in fact, what they claim to be.

Biodegradable

Application of plastic mulches conserves water, and helps in weed, pest, and disease control.

He and his research team want to understand what leaches into the soil as the mulches degrade and which ones perform as well as polyethylene-made plastic mulches (PEs) at weed, pest, and disease control.

Plastic Mulch

Application of plastic mulches in agriculture is a common practice by specialty crop producers worldwide. It conserves water, and helps in weed, pest, and disease control, subsequently improving crop yield and quality. Because PE is durable and does not degrade in the soil, you cannot leave it in the field, which ultimately leads to the question of disposal.  When PE is buried in the field, it becomes contaminated with soil and can’t be recycled but instead requires transport to a landfill, increasing production costs. Another problem arises when landfill facilities are not available. When this is the case, growers stockpile PE on their farm, where the rain can wash the mulch down to streams and water bodies. Henry Sintim and his team are investigating whether or not biodegradable plastic mulches (BDMs) could be a viable alternative.

Biodegradable

The team installs a lysimeter beneath the mulches.

Biodegradable Alternatives

Substituting PE with BDM could alleviate the need for disposal. However, Sintim says the potential impact on agricultural soil ecosystems needs to be assessed before adopting biodegradable mulch for field use. For instance, do biodegradable mulches really degrade?  Sintim explains, “By BDM, we mean it is plastic mulch, but it has been made from pure or partial biobased materials. Though there are plastic mulches advertised as biodegradable, none have actually been proven to biodegrade, so the team is examining degradation of different commercial BDM types over time. They have also included an experimental BDM, in which the constituents were specified by the team.”

Sintim is monitoring the degradation of BDM by assessing the material properties and measuring the particle size and surface area via photography: digitizing and analyzing them using Image J software.

Biodegradable

There are indications that some of the BDMs are performing well.

How Well Do the Mulches Compare?

Sintim also wants to find out how well BDMs maintain microclimate in comparison to PE. Since soil temperature and moisture content are important parameters that govern chemical reaction rates and microbial activity, and are likely to vary among the different BDM treatments, he is monitoring soil moisture dynamics using soil moisture and temperature sensors installed at 10 cm and 20 cm depths. In addition, the team has installed sensors directly underneath the mulches to measure surface temperature and light penetration. Reduction of light penetration is the attribute that helps plastic mulches to control weeds. The team is also assessing soil quality using the USDA Soil Quality Test Kit.  

Sintim says so far one of the commercial BDMs and the experimental BDM had the same yield performance as PE.  He adds, “We don’t have final results yet, and there are a lot of variables that could come into the picture. But I will say there is an indication that some of the BDMs are performing well.”

Next week:  Find out how Sintim will determine what’s leaching into the soil and another alternative for polyethylene plastic mulch.

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