by Bill and Mary Weaver
Researchers at several universities are working hard to get “biodegradable” mulches ready for Prime Time. Dr. Carol Miles, horticulturalist at Washington State University, Mount Vernon, is working with a team of 19 university researchers at several locations, including Principal Investigator Doug Hayes, a polymer scientist at the University of Tennessee Institute of Agriculture in Knoxville, to research all aspects of these mulches for use with specialty vegetable crops. Their studies will include the effects of several U.S.-produced mulches on the soil microbes and soil quality, on leachate water quality, and on yield and crop quality. Researchers are also analyzing the effects of temperature, rainfall, humidity, soil type, soil pH, and farming practices on the speed and extent at which these mulches deteriorate and biodegrade after they’re tilled into the soil. “There really is no one-size-fits-all in different locations and climates for these mulches,” commented Miles.
Over a four-five year period, replicated vegetable plots will be laid with a variety of currently available mulches and one experimental mulch, all U.S.-produced. These will be rototilled into the soil at the end of each growing season.
“We are measuring mulch biodegradability both on the soil surface and in the soil following tillage.” ‘Biodegradability’ is the breakdown of the mulch materials into carbon dioxide, water and microbes. While only up to 20 percent of the ingredients of most currently available ‘biodegradable’ films produced in the US are bio-based, they have the potential to completely biodegrade. Researchers hope that their work will spur development of more bio-based formulas and products in the US mulch film industry.
Experiences in the first year show that these biodegradable mulches may not yet be ready for Prime Time.
“For example, there is a phenomenon called ‘mulch adhesion’,” explained Miles. This was shown clearly this year with the ‘Cinnamon Girl’ pumpkins. “Where the pumpkins were sitting on the biodegradable mulch, after a while, if we lifted a pumpkin, we’d see a hole in the mulch under the pumpkin. The biodegradable mulch was stuck to the fruit itself.”
A pumpkin with pieces of biodegradable mulch adhering to it is not going to be salable. “If you wipe it, you’ll maybe get 60 percent of the mulch off the pumpkin,” Miles continued. “Realistically, you can’t get it all off. In my experience — and my colleague Annette Wszelaki in Knoxville, TN agrees — growers will just walk away from that field.
“If crews are paid by the piece, and are told to avoid the pumpkins with mulch adhering, they won’t be able to pick bins quickly, so they won’t pick. Next year, we’re going to try to quantify the mulch adhesion more precisely. (In Washington this year, about 55 percent of the pumpkins had mulch adhered to them.)
Also, researchers found this year that exceptionally heavy rainfalls and strong winds at the beginning of the season made two of the paper-based test products literally start falling apart in the first couple of weeks.
“Our Knoxville, TN site was inundated with over four inches of rain overnight — two times in the first three weeks! However, although the mulch did deteriorate quickly, the weed control remained very good for both products.”
In Mount Vernon, WA, there was very little rainfall through the summer. “The Naturecycle was the most degraded at our site, but weed control was excellent for all products.”
The randomized, replicated treatments included bare ground, standard polyethylene mulch, and three potentially biodegradable mulches, plus an experimental mulch from a company in Cambridge, MA.
“The experimental mulch is the only product for which we are able to know the formula.” Knowledge of the chemical composition of this experimental mulch will allow polymer scientist Doug Hayes and collaborators at Knoxville to monitor chemical changes during the biodegradation process. They will monitor, for example, the uptake of the mulch’s polymer molecules by soil particles, microorganisms, and water in controlled experiments. “With the other products,” Miles continued, “we were given some information about their formulation, but there was a lot of non-disclosure of ‘proprietary’ components.”
At the end of each of the four growing seasons of the study, the standard plastic mulch will be taken up and removed from the field, but the “biodegradable” mulches will be rototilled into the soil, and a cover crop of winter wheat planted.
Five “soil cores” will be taken from each plot every six months, and the amount of mulch in the soil will be measured. The first cores were taken immediately after harvest in 2015.
“My graduate student,” said Miles, “is doing the time-consuming job of screening the soil cores to determine how much of each mulch has deteriorated/biodegraded, and how much remains intact, after repeated mulch applications over the four years.”
The mulch is weighed, cleaned and weighed again, and the area is measured using three different methods. “Our goal is to develop an accurate method for measuring the amount of mulch in the soil that farmers can use without fancy equipment.”
If biodegradable mulches are given the ‘green light’ for organic growers by NOP (as they have been in Europe,) each organic grower will be responsible for measuring the amount of mulch that has not biodegraded in his soil. “The grower may be held in non-compliance if the amount is insufficient. So the methods used to determine the amount of mulch remaining in the soil must be easy to use, while also being reliable and accurate.”
“The simplest is for the grower to take cores, as we do, then sort out the mulch pieces from each core, and place them on pieces of graph paper,” explained Miles. “We’re creating an equation growers can use to calculate the difference between the mulch area remaining and the original mulch area.”
For growers who can download a free app called ‘image J,’ all that’s needed is to lie out the pieces of mulch from each core on a contrasting piece of paper and take a photo. “The app will calculate the area of every mulch sample for you.”
A third method to validate the amount of mulch residue left in the soil compared to the amount of mulch that has been applied is by weight, but this requires a sensitive balance.
Differences in yield from the different mulches are of interest. In the first year at the Knoxville site, the yields in all plots were comparable. At the Mount Vernon site, the standard polyethylene plastic mulch plots had the highest yield, but it was basically equal to the potentially biodegradable mulches. At Mount Vernon, the bare ground treatment had the lowest yields, and the paper-based mulch was in between.
The climate in Mount Vernon is cool to begin with, so the paper product, combined with the drip irrigation, further lowered soil temperatures, a detriment in an already cool area.
In the hot Knoxville climate, on the other hand, cooling the soil can be a positive, so the Knoxville yields on using the same product with drip were higher. “So much depends on climate,” commented Miles. “
“Our results are encouraging manufacturers to create new bio-based formulas”.
Biodegradable mulch research
by Bill and Mary Weaver