PE plastic mulch is used widely in specialty crop production to control weeds, conserve soil moisture, increase crop yields, modify soil temperature, and shorten the time to harvest (Hill et al., 1982; Schonbeck, 1998; Schonbeck and Evanylo, 1998; Shogren, 2000). These benefits provide farmers worldwide with significant horticultural and economic advantages (Takakura and Fang, 2001). However, the widespread use of PE mulch creates removal and disposal costs to growers as well as costs to the environment due to waste plastics being buried in landfills and on farm, or burned. As an alternative, plastic mulches that are biodegradable have been developed. These products first appeared on the market in the 1980s, and may be made from renewable, natural, or sustainable feedstocks (Hayes et al., 2012; Miles et al., 2009). They provide the benefits of PE mulch, such as weed suppression and improved crop yields, and are purported to completely degrade between crop seasons (Cowan et al., 2014; Miles et al., 2012; Minuto et al., 2008; Moreno and Moreno, 2008). Because farmers must bear the annual cost of mulch removal and disposal, estimated at ≈$250 per hectare (Shogren and Hochmuth, 2004), the ability to till mulch into the soil following harvest with confidence that biodegradation will occur is an economic incentive (Olsen and Gounder, 2001). Additionally, the environmental benefit of a mulch that degraded completely in soil without producing toxic byproducts would be highly desirable.
Biodegradable plastics typically undergo a two-phase degradation process: disintegration or weathering during use in the field, and biodegradation, which occurs after being incorporated into the soil (Krzan et al., 2006; Kyrikou and Briassoulis, 2007). Moisture, temperature, and light are key factors that impact disintegration, and interactions among these factors may further enhance degradation (Hakkarainen, 2002; Ho et al., 1999; Krzan et al., 2006). Disintegration can lead to decreased molecular weight and increased water solubility of the plastic, which subsequently facilitates biodegradation (Lucas et al., 2008). During biodegradation, biotic processes mineralize the polymer fragments to carbon dioxide, water, and microbial biomass (Lucas et al., 2008). In this study, the word “deterioration” is used to reflect a reduction in the function or visual intactness of the mulch without respect to any particular mode of action or biological process.
Parameters typically measured in the field and laboratory to evaluate deterioration of biodegradable mulch films include reduction in soil coverage, weight loss, and changes in mechanical properties. Measuring the decrease in soil coverage provided by mulch is relatively inexpensive and can be performed in the field without the use of special equipment. Measuring changes in mulch weight requires either large samples, or sensitive laboratory-quality balances, and cannot be reliably performed in the field. Although measuring mulch weight is less subjective than a visual assessment, weight measurements are subject to error because of soiled samples. Measuring changes in mulch mechanical properties, including breaking force and elongation to break, among others, are objective and accurate, but requires specialized laboratory equipment and climate-controlled facilities. If visual assessment of soil coverage reliably predicts degradation of biodegradable plastic mulches, the need for more expensive and less accessible methods may be obviated, despite the potential for greater accuracy.
To quantify biodegradable mulch deterioration and compare mulch performance in the field, Miles et al. (2012) measured the number of rips, tears, and holes (RTH), as well as the PVD in three distinct climates of the United States. However, the authors found RTH to be an unreliable measure of deterioration due to the coalescence of RTH over time, leading to underestimation of deterioration. Other studies have used similar observations to visually rate or evaluate loss of integrity of biodegradable mulch on the soil surface (Minuto et al., 2008; Moreno and Moreno, 2008; Moreno et al., 2009; Ngouajio and Ernest, 2005; Ngouajio et al., 2008), but none have been adopted as a standard of measurement. In addition, other investigators have evaluated changes in mulch mechanical properties over time to evaluate deterioration (Briassoulis, 2006, 2007; Candido et al., 2006; Cascone et al., 2008; Kijchavengkul et al., 2008; Scarascia-Mugnozza et al., 2006; Tocchetto et al., 2001). Martín-Closas et al. (2007 and 2008) implied that visual observations of biodegradable mulch deterioration were related to changes in mechanical properties, though no statistical comparisons were made between the measures.
Studies that statistically analyze the relationships between visual assessments of mulch deterioration in the field and mechanical properties in the laboratory have not yet been reported. However, an increased awareness and understanding of such relationships could contribute to more accurate interpretations about visual measures of deterioration. The research reported here builds on the work of Miles et al. (2012), and seeks to determine whether visual assessments of biodegradable mulch intactness during the growing season (independent variable) predict statistically significant changes in mechanical properties (dependent variables). Additionally, the study was designed to contrast mulch deterioration in open-field and high tunnel production environments to expand on potential agricultural settings. Tomato (Solanum lycopersicum cv. Celebrity) was selected as the model crop for this study because it is an important vegetable crop throughout the United States and plastic mulch is a common and important component of tomato production systems (Lamont, 1993).
ASTM 2006 Standard test method for breaking strength and elongation of textile fabrics (strip method). ASTM D5035-06. ASTM International, West Conshohocken, PA
Briassoulis, D. 2006 Mechanical behaviour of biodegradable agricultural films under real field conditions Polym. Degrad. Stabil. 91 1256 1272
Briassoulis, D. 2007 Analysis of the mechanical and degradation performances of optimised agricultural biodegradable films Polym. Degrad. Stabil. 92 1115 1132
Candido, V., Miccolis, V., Castronuovo, D., Margiotta, S. & Manera, C. 2006 The effect of soil solarization and protection techniques on yield traits of melon in unheated greenhouse Acta Hort. 710 415 420
Cascone, G., D’Emilio, A., Buccellato, E. & Mazzarella, R. 2008 New biodegradable materials for greenhouse soil mulching Acta Hort. 801 283 290
Cowan, J.S., Miles, C.A., Andrews, P.K. & Inglis, D.A. 2014 Biodegradable mulch performed comparable to polyethylene in high tunnel tomato (Solanum lycopersicum L.) production J. Sci. Food Agr. 94 1854 1864
Dharmalingam, S., Hayes, D.G., Wadsworth, L.C., Dunlap, R.N., DeBruyn, J.M., Lee, J. & Wszelaki, A.L. 2015 Soil degradation of polylactic acid/polyhydroxyalkanoate-based nonwoven mulches J. Polym. Environ. 23 302 315
Hablot, E., Dharmalingam, S., Hayes, D.G., Wadsworth, L.C., Blazy, C. & Narayan, R. 2014 Effect of simulated weathering on physicochemical properties and inherent biodegradation of PLA/PHA nonwoven mulches J Polym. Environ. 22 417 429
Hayes, D., Dharmalingam, G.S., Wadsworth, L.C., Leonas, K.K., Miles, C. & Inglis, D.A. 2012 Biodegradable agricultural mulches derived from biopolymers, p. 201–223. In: K.C. Khemani and C. Scholz (eds.). Degradable polymers and materials: Principles and practice, 2nd ed. (ACS Symposium Series, Volume 1114). American Chemical Society, Washington, DC
Hill, D.E., Hankin, L. & Stephens, G.R. 1982 Mulches: Their effects on fruit set, timing and yields of vegetables. Conn. Agr. Expt. Sta. Bul. 805
Ho, K.G., Pometto, A.L. III & Hinz, P.N. 1999 Effects of temperature and relative humidity on polylactic acid plastic degradation J. Polym. Environ. 7 83 92
Kijchavengkul, T., Auras, R., Rubino, M., Ngouajio, M. & Fernandez, R.T. 2008 Assessment of aliphatic–aromatic copolyester biodegradable mulch films. Part I: Field study Chemosphere 71 942 953
Kirk, R.E. 1982 Experimental design: Procedures for the behavioral sciences. 2nd ed. Brooks/Cole, Monterey, CA
Klungland, M.W. & McArthur, M. 1989 Soil survey of Skagit County area, Washington. U.S. Dept. Agr. Soil Conservation Serv., Washington, DC
Krzan, A., Hemjinda, S., Miertus, S., Corti, A. & Chiellini, E. 2006 Standardization and certification in the area of environmentally degradable plastics Polym. Degrad. Stabil. 91 2819 2833
Lucas, N., Bienaime, C., Belloy, C., Queneudec, M., Silvestre, F. & Nava-Saucedo, J.E. 2008 Polymer biodegradation: Mechanisms and estimation techniques Chemosphere 73 429 442
Martín-Closas, L., Pelacho, A.M., Picuno, P. & Rodríguez, D. 2007 Biodegradable mulching in an organic tomato production system Acta Hort. 801 275 282
Martín-Closas, L., Bach, M.A. & Pelacho, A.M. 2008 Biodegradable mulching in an organic tomato production system Acta Hort. 767 267 274
Miles, C., Beus, C., Corbin, A., Wallace, R., Wszelaki, A., Saez, H., Walters, T., Leonas, K., Brodhagen, M., Hayes, D. & Inglis, D. 2009 Research and extension priorities to ensure adaptation of high tunnels and biodegradable plastic mulch in the United States. p. 102–108. (Proc. 35th Natl. Agr. Plastics Congr)
Miles, C., Wallace, R., Wszelaki, A., Martin, J., Cowan, J., Walters, T. & Inglis, D. 2012 Deterioration of potentially biodegradable alternatives to black plastic mulch in three tomato production regions HortScience 47 1270 1277
Minuto, G., Pisi, L., Tinivella, F., Bruzzone, C., Guerrini, S., Versari, M., Pini, S. & Capurro, M. 2008 Weed control with biodegradable mulch in vegetable crops Acta Hort. 801 291 298
Moreno, M.M. & Moreno, A. 2008 Effect of different biodegradable and polyethylene mulches on soil properties and production in a tomato crop Sci. Hort. 116 256 263
Moreno, M.M., Moreno, A. & Mancebo, I. 2009 Comparison of different mulch materials in a tomato (Solanum lycopersicum L.) crop Span. J. Agr. Res. 7 454 464
National Resources Conservation Service 2010 Web soil survey. 12 Aug. 2010. <http://websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx>.
Ngouajio, M. & Ernest, J. 2005 Changes in the physical, optical, and thermal properties of polyethylene mulches during double cropping HortScience 40 94 97
Ngouajio, M., Auras, R., Fernandez, R.T., Rubino, M., Counts, J.W. Jr & Kijchavengkul, T. 2008 Field performance of aliphatic-aromatic copolyester biodegradable mulch films in a fresh market tomato production system HortTechnology 18 605 610
Olsen, J.K. & Gounder, R.K. 2001 Alternatives to polyethylene mulch film—a field assessment of transported materials in capsicum (Capsicum annuum L.) Austral. J. Expt. Agr. 41 93 103
Saxton, A.M. 2010 DandA.sas: Design and analysis macro collection version 2.11. Univ. of Tenn., Knoxville, TN. 17 Sept. 2013
Scarascia-Mugnozza, G., Schettini, E., Vox, G., Malinconico, M., Immirzi, B. & Pagliara, S. 2006 Mechanical properties decay and morphological behaviour of biodegradable films for agricultural mulching in real scale experiment Polym. Degrad. Stabil. 91 2801 2808
Schonbeck, M.W. 1998 Effects of mulches on soil properties and tomato production I. Soil temperature, soil moisture and marketable yield J. Sustain. Agr. 13 13 33
Schonbeck, M.W. & Evanylo, G.K. 1998 Weed suppression and labor costs associated with organic, plastic, and paper mulches in small-scale vegetable production J. Sustain. Agr. 13 55 81
Shogren, R.L. & Hochmuth, R.C. 2004 Field evaluation of watermelon grown on paper-polymerized vegetable oil mulches HortScience 39 1588 1591
Takakura, T. & Fang, W. 2001 Climate under cover. Kluwer Academic Publishers, Dordrecht, Germany
Tocchetto, R.S., Benson, R.S. & Dever, M. 2001 Outdoor weathering evaluation of carbon-black-filled, biodegradable copolyester as substitute for traditionally used, carbon-black-filled, non-biodegradable, high-density polyethylene mulch films J. Polym. Environ. 9 57 62