A comparison of biodegradable mulches to black plastic mulch

A Comparison of Biodegradable Mulches

FNE 05-562
Laura Sorkin
Cave Moose Farm
770 Andrews Road
802 -644-2154
lauraglenn@hotmail.com

Introduction

Both conventional and organic farmers rely on plastic mulch to control weeds, heat the soil and retain nutrients. While it is very effective in all these areas, there is concern over the amount of waste generated when the plastic is pulled out of the fields at the end of the season. A material that provides the above-mentioned benefits but is biodegradable in addition would eliminate the concern of ever-increasing agricultural pollution. To date, some biodegradable materials have been introduced to the market as alternatives to plastic but little has been written comparing their efficacy and, just as important, their cost to a small farmer.
This experiment was designed to compare yield and cost of three materials: black plastic, a cellulose-based biodegradable film called Agrofilm and paper mulch. While the black plastic is less expensive per foot than the other two films, labor hours must be spent to remove the plastic at the end of the season and disposal fees can further reduce profits. A higher yield with the plastic, however, may tip the scales in favor of continuing its use. It is our hope that recording the costs and yields for several crops with all three materials will make the decision of whether to try a biodegradable mulch easier for a farmer who would like an alternative to plastic.

Farm Profile
Cave Moose Farm is a certified organic, diverse vegetable and flower farm. We grow bedding plants in the spring and then field and greenhouse cultivated crops in the summer and fall. In addition we produce log-grown shiitake mushrooms from June through October. We sell to our Community Supported Agriculture (CSA) program, The Burlington Farmer’s market, restaurants and local stores. We have six acres of fields for annual crops and two acres of perennial crops such as berries and flowers. This year we have discontinued the CSA and will be focusing mostly on markets and wholesale; a decision based on the need to reduce labor costs.

Participants
Our technical advisor was Vern Grubinger who visited the site several weeks after the experiment began and gave technical advice throughout the season via email. A crew of three and I carried out the experiment from May through October of 2005.

Project Activities

The experiment was conducted in three different plots randomly selected throughout a six-acre field. The soil type is clay-loam amended with compost in most areas and a pH on average of 6.5. Each plot has one row of plastic adjacent to one row of Agrofilm adjacent to one row of Kraft Paper.
Plot one and two were planted May 20. Plot one consisted of 91 feet of Expert pumpkin planted two plants per hole, every two feet. The remainder of the row (71 feet) was planted with Baby Pam pumpkin, two plants per hole every two feet. The plants were treated with fish emulsion upon planting and ½ cup of Pro-gro was incorporated into the planting hole.
Plot two was planted with 75 feet of Bush Delicata winter squash; two plants per hole, two feet apart; and 95 feet of Metro Butternut squash, two plants per hole, two feet apart. The winter squash were also treated with fish emulsion before planting with an additional ½ cup Pro-gro incorporated into the soil.
Plot three was planted June 3 with 160 feet of Festival watermelon, two plants per hole, two feet apart. A mix of ½ cup Pro-gro and greensand was added to each planting hole upon planting.
Overhead irrigation was provided periodically as needed throughout the summer to each plot. One application of liquid fish and seaweed fertilizer was given in mid-summer.
Hours required to lay each material were recorded as were extra hours needed to fix any of the rows. Given that the machinery always needs to be adjusted many times for the first installment of any new roll, the time taken to lay each material was taken in aggregate for all three plots and divided by three for each of the materials.
Yields from each crop from each material were recorded. Time necessary to remove the plastic at the end of the season was also recorded. Costs per foot for each material as well as disposal fees for the plastic were figured into the final cost analysis.

Economic Results

These figures are based on labor costs (which include hourly wage, taxes, and workers’ compensation) at $20/hr. Disposal of the plastic was $15/yd. The plastic cost $73 for a 4’ x 2000’ roll. The BioBag Agrofilm cost $335 for a 4’ x 4000’ roll. The paper cost $200 for a 4’ x 1500’ roll. All of the above costs include shipping.

Cost per foot = Cost of materials + time in labor for laying and maintaining

Pumpkins: Plastic $0.21/foot
Agrofilm $0.19/foot
Paper $0.35/foot

Winter squash: Plastic $0.20/foot
Agrofilm $0.19/foot
Paper $0.34/foot

Watermelon: Plastic $0.21/foot
Agrofilm $0.20/foot
Paper $0.35/foot

Yield per dollar spent

Pumpkins: Plastic 2.71 lbs/$
Agrofilm 3.19 lbs/$
Paper 0

Winter squash Plastic 2.98 lbs/$
Agrofilm 1.78 lbs/$
Paper 0

Watermelon Plastic 11.16 lbs/$
Agrofilm 6.2 lbs/$
Paper .78 lbs/$

Conditions
Conditions were very good for farming in the summer of 2005. The only factor that may have affected the results was that our soil is still not at a fertility level we would like which resulted in lower yields than ideal.

Assessment

The cost per foot of the Agrofilm and plastic were very close. Though the Agrofilm is more expensive, the hours necessary to remove the plastic plus the disposal fees brought the cost of the plastic to slightly higher than the per-foot cost of the Agrofilm. The per-foot cost of the paper was significantly higher than both materials for several reasons. First, the cost of the material itself was comparatively expensive; one and a half times higher than Agrofilm and more than three and a half times higher than plastic. Second, the paper took a great deal more time to lay than the other two materials due to the fact it tore easily using the mulch layer as intended. In fact, after much frustration and torn paper, we had to remove the rear discs that were designed to throw dirt over the edges and instead hoe dirt by hand to tuck it in. After this effort, the paper tore down the middle on plot one and two within two days after a mild breeze caused it to flap. The paper in plot three tore halfway down the row. No effort was made to weed in these rows and consequently there was zero yield in the paper rows of plot one and two and only a small yield in the paper row of plot three. We would not recommend the paper mulch for any application.
The results are less definitive when looking at the yields from the plastic and Agrofilm. The yield per dollar spent was better in the plastic material for winter squash and nearly twice as high for watermelon but higher in the Agrofilm in the pumpkin patch. We have no explanation for these variances except that it is possible that the plastic retained more heat which resulted in larger fruit in the plastic-watermelon row. A study done by Rangarajan, Ingall and Davis, (Alternative Mulch Products 2003, Cornell Univ, Ithica, NY) that compared the two products showed melon yields comparable to black plastic on one farm and nearly as high on another farm growing melons. Where heat is not as much of a factor in the pumpkins and winter squash, the variances may be attributed to local fertility or weed pressure despite our efforts to ensure the same conditions for all of the crops.

Adoption

For farmers looking for a cost-effective alternative to plastic mulch, the Agrofilm is a good option for some crops such as winter squash. The Agrofilm began to degrade noticeably in mid-July and was nearly disintegrated by September. We noted that by the time it disintegrated, it had eliminated weeds below, most likely from heat and smothering, and weeds were not a problem for the remainder of the season. In addition, rather than allotting time to the dreaded job of pulling up dirty plastic in the fall, we simply tilled the remnant crop under and were able to spend the time on other fall chores.
The results in the watermelon plot may give some farmers pause before switching completely away from plastic. Since our results favored plastic for melons in comparison to the study conducted by Rangarajan, et.al, perhaps more studies are called for in different types of crops to determine which crops are a good match for biodegradable mulches. Though we plan to use the biodegradable mulch for squash and possibly flowers next year, we will continue to use the plastic for our melon crops.
We encourage biotech companies to continue research on this topic using renewable resources or natural by-products of other manufacturing processes. I have always theorized that low-grade wool would be and excellent component of a mulch material in that it is insulating, strong but biodegradable and even contains some nutrients. Currently, there is little or no market for low-grade wool so the manufacturing of such a product could provide extra income for sheep farmers as well as a needed commodity to vegetable farmers.

Outreach
We plan to send our results to various agricultural publications so other farmers can decide if switching from plastic is right for their farm.

Summary
Using large quantities of a petroleum-based product that cannot be recycled is antithetical to the principles of sustainable farming. Even if a farmer chose to switch half of her crops over to biodegradable mulch, this would certainly have a positive effect on reducing agricultural pollution and use of non-renewable resources. We have found that AgroFilm shows comparable yields and nearly the same costs as plastic mulch for two out of the three crops we experimented with. More experiments with other types of crops and other types of materials are needed so farmers can confidently switch from plastic mulch to a biodegradable material. In the meantime, we will switch to the Agrofilm for many of our crops in 2006 and hope to reduce the amount of plastic waste generated at the farm.

Laura Sorkin
March 14, 2006