Organic Broadcaster

Organic certification guidelines require polyethylene (plastic) mulch to be removed from the field at the end of the growing season. Photo by Elise Reid

New materials in biodegradable mulches hold promise for vegetable production

By Elise Reid

Plastic mulches in vegetable production improve water retention in the soil, warm the soil, reduce weed pressure, and may improve yields. However, difficulties in recycling or reusing these materials result in massive quantities of plastic going to landfills. For the organic farmer, plastic mulches represent a particularly uneasy balance between economic and environmental sustainability.

Mulches currently allowed by the National Organic Program (NOP) include non-synthetic, untreated materials and paper, as well as plastic films if the plastics are removed from the field at the end of the season.

An ideal solution would be development of a 100% biobased biodegradable mulch. There are several biodegradable mulch options on the market. Most have one or two of the desired qualities but lack others. Some new materials under testing may become promising alternatives.

Biodegradable Paper Weed Barrier

Early season winds can rip apart paper mulch such as WeedGuard Plus (shown here). Photo by Elise Reid

Paper is one of the few available biodegradable mulches to meet NOP standards. Available paper-based mulches include products such as WeedGuard Plus (SunShine Paper Co.), Planters Paper (Garden Trends Inc.), and others. Paper-based mulches fall into the NOP’s standard of 90% or greater degradation within two years.

Most paper mulches are actually overachievers and tend to degrade before the end of the field season. In windy Midwestern states, this can be a problem, as once paper edges degrade wind can airlift the mulch to a neighboring field. Paper mulches are also much heavier than plastic alternatives, which adds to cost and fuel use in their application.

In both field studies and high tunnels, WeedGuard Plus did not perform as well as plastic or other biodegradable materials measured by produce yield and quality.1 Additionally, after 18 months of WeedGuard Plus incorporation into the soil, soil in contact with WeedGuard Plus had a lower soil quality index than the bare soil control.2 Overall, paper is not the most effective mulch, although it is allowed in organic production.

Biodegradable Plastics
Biodegradable plastics on the market include BioAgri (BioBag Americas), Bio360 (Dubois Agrinovation), EcoFilm (Cortec Corp.), and Naturecycle (Custom Bioplastics). Numerous studies have found biodegradable plastics to have similar benefits to polyethylene mulches in vegetable yield and weed control, although rips and tears diminished both in some cases.3–5 Mater-Bi plastics, such as BioAgri and Bio360 products, are produced from non-GMO European cornstarch and vegetable oil. However, Mater-Bi and other biodegradable plastics still contain petroleum-based products to provide elasticity to the films. As they are non-biobased, petroleum complexing agents make biodegradable plastics ineligible for NOP certification. Similar to paper mulches, biodegradable plastics can have a very short lifespan, reducing some of their functionality.6

There is concern over microplastic or similar pollution from biodegradable plastic mulches. After two seasons of use and incorporation of biodegradable plastics, microplastics accumulated in the soil in a vegetable production system.7 Microplastics are 1-5,000 microns or smaller in size; for reference, a human hair is approximately 100 microns in diameter.8 Microplastics have been shown to affect the soil environment. However, depending on the structure of the microplastic and type of plastic, they can either improve or hinder properties such as evapotranspiration, root biomass, microbial activity, and plant dry weight.9 Benefits from these microplastics likely are due to the release of compounds from newer microplastics that have a fertilizing effect on the soil.9 The conflicting evidence gives good cause to further investigate whether the remains of biodegradable plastics will be detrimental or beneficial to soil and plant health.

Polylactic Acid-Based Mulches
Polylactic acid-based biodegradable mulches are being intensely researched as an alternative to biodegradable plastics and paper mulches. Polylactic acid (PLA) can be 100% plant-based and cheap to produce. It is derived from fermentation of corn, sugar beets, cassava, and sugar cane. However, the polymers are polymerized synthetically, which makes it ineligible for NOP certification as “biobased.” (See number 5 on NOP certification rule.)

PLA-based mulches are produced as fabrics, which can be either meltblown or spunbond. These two methods alter the rate of degradation of the mulches.10 Meltblown is typically more delicate, while spunbond is more durable. Material engineering solutions are important for this biodegradable mulch, as it also does not degrade quickly enough for NOP standards.

Innovations to improve the rate of degradation include incorporating agricultural byproducts such as alfalfa particles, soy particles, or wood particles.11 When agricultural byproducts are added to PLA fabrics they exhibit faster degradation than PLA alone. These new biodegradable mulches can also retain soil moisture and prevent weeds.6

PLA mulches do not increase soil temperature, which could be useful in hotter areas or for cool weather crops.6 Additionally, PLA mulch offers a yield boost, in some cases up to 148% compared to a bare-ground control.12 While there is potential for this material, as of now it would not be suitable for annual production and still faces hurdles for organic certification.

Polyhydroxyalkanoate
One other bio-based polymer is polyhydroxyalkanoate (PHA). PHA is too brittle on its own to produce a fabric or film mulch, therefore it is more often added to PLA mulches in a blend.13 However, PHA can be produced from microbial fermentation in one to two steps and is, therefore, allowable/non-synthetic according to the NOP.14 The production of this polymer is costly though, making it less attractive at this time.

Impact on Soil
Not much is known about what happens in the long-term to the soil, microbial community, or crop yields after biodegradable mulch incorporation. In a two-year field study of residual Bio360 biodegradable plastic mulch and a wood-fiber embedded PLA mulch, there was no yield difference between plots that had incorporated mulch of either type and control plots.15 This suggests that in vegetable production the residual mulch in the soil did not affect future crop yields.

One soil physical property was affected differently between residual mulch types, however. Soil water-stable macroaggregates were higher in plots that had incorporated PLA mulches whereas macroaggregates decreased in one location with incorporated Bio360.15 The increase in soil macroaggregates could in fact be due to a higher fungal population in soils in contact with PLA.16–18 Fungi are major players in soil aggregate formation.19,20 Other soil physical properties such as soil tensile strength, compaction, and soil sorptivity (the tendency to absorb and transmit liquids by capillarity) were unaffected by incorporation of either mulch.

Another concern with the incorporation of biodegradable mulches into the soil is the immobilization of nitrogen (N) due to the high-carbon content of these mulches. As organic systems are low input, incorporated mulch mustn’t tie up N. In one study, neither Bio360 nor the PLA-wood fiber mulch immobilized N (measured as nitrate), nor altered potassium or phosphorous balances in the soil after two years from initial mulch incorporation.15 In addition, a lab study also found no N immobilization in connection with Mater-Bi degradation.21

Take-Aways
There may not be organic certified biodegradable mulch available for farmers today, but there are several options that are inching closer to becoming safe, effective products. Ideally, these will soon eliminate plastic mulch from organic agriculture. If PLA-based mulches can be certified in the future, they may be more useful in perennial systems. In perennial systems, their slow degradability would be a benefit rather than a hindrance. As of yet, biodegradable plastics such as Mater-Bi have shown no ill effects on soil or plant health. However, many of these mulches have not had the long-term (two years or greater) research on their residual effects after soil incorporation. Continued work in this area is necessary before organic certification to ensure that there are no harmful effects.

Elise Reid is a graduate research assistant in agronomy and horticulture at the University of Nebraska-Lincoln.

 

NOP Guidelines for Biodegradable Mulch

1. Meets compostability specifications of one of these standards: ASTM D6400, ASTM D6868, EN 13432, EN 14995, or ISO 17088 (see §205.3)
2. At least 90% degradation absolute or relative to microcrystalline cellulose in less than two years, in soil, according to one of the following: ASTM D5988 or ISO 17556 (see §205.3)
3. Biobased with content assessed using ASTM D6866 (§205.3)
4. Is produced without organisms or feedstocks derived from excluded methods such as GMO (Section 205.601 (b)(2)(iii)
5. Be produced without the use of non-biobased synthetic polymers: minor additives such as colorants and processing aides are not required to be biobased (NOP Policy Memo 15-1)

 

References

1. Cowan JS, Miles CA, Andrews PK, Inglis DA. Biodegradable mulch performed comparably to polyethylene in high tunnel tomato ( Solanum lycopersicum L.) production. J Sci Food Agric. 2014;94(9):1854-1864. doi:10.1002/jsfa.6504
2. Li C, Moore-Kucera J, Lee J, et al. Effects of biodegradable mulch on soil quality. Appl Soil Ecol. 2014;79:59-69. doi:10.1016/j.apsoil.2014.02.012
3. Ghimire S, Wszelaki AL, Moore JC, Inglis DA, Miles C. The Use of Biodegradable Mulches in Pie Pumpkin Crop Production in Two Diverse Climates. HortScience. 2018;53(3):288-294. doi:10.21273/HORTSCI12630-17
4. Touchaleaume F, Martin-Closas L, Angellier-Coussy H, et al. Performance and environmental impact of biodegradable polymers as agricultural mulching films. Chemosphere. 2016;144:433-439. doi:10.1016/j.chemosphere.2015.09.006
5. Costa R, Saraiva A, Carvalho L, Duarte E. The use of biodegradable mulch films on strawberry crop in Portugal. Sci Hortic (Amsterdam). 2014;173:65-70. doi:10.1016/j.scienta.2014.04.020
6. Wortman SE, Kadoma I, Crandall MD. Assessing the potential for spunbond, nonwoven biodegradable fabric as mulches for tomato and bell pepper crops. Sci Hortic (Amsterdam). 2015;193:209-217. doi:10.1016/j.scienta.2015.07.019
7. English M. The role of biodegradable plastic mulches in soil organic carbon cycling. 2019.
8. Hayes D. Micro-and Nanoplastics in Soil: Should We Be Concerned?; 2019. https://ag.tennessee.edu/biodegradablemulch/Documents/Microplastics-soil-Factsheet-formatted.pdf. Accessed September 16, 2019.
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16. Janczak K, Hrynkiewicz K, Znajewska Z, Dąbrowska G. Use of rhizosphere microorganisms in the biodegradation of PLA and PET polymers in compost soil. 2018. doi:10.1016/j.ibiod.2018.03.017
17. Karamanlioglu M, Robson GD. The influence of biotic and abiotic factors on the rate of degradation of poly(lactic) acid (PLA) coupons buried in compost and soil. Polym Degrad Stab. 2013;98:2063-2071. doi:10.1016/j.polymdegradstab.2013.07.004
18. Samuelson MB, Drijber R, Wortman SE. Microbial response to biodegradable mulch: Can degradation rate be accelerated by management? 2019.
19. Beare MH, Hu S, Coleman DC, Hendrix PF. Influences of mycelial fungi on soil aggregation and organic matter storage in conventional and no-tillage soils. Appl Soil Ecol. 1997;5(3):211-219. doi:10.1016/S0929-1393(96)00142-4
20. Bossuyt H, Denef K, Six J, Frey S., Merckx R, Paustian K. Influence of microbial populations and residue quality on aggregate stability. Appl Soil Ecol. 2001;16(3):195-208. doi:10.1016/S0929-1393(00)00116-5
21. Bettas Ardisson G, Tosin M, Barbale M, Degli-Innocenti F. Biodegradation of plastics in soil and effects on nitrification activity. A laboratory approach. Front Microbiol. 2014;5:710. doi:10.3389/fmicb.2014.00710

From the November | December 2019 Issue

 

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