Soil Degradable Bioplastics for a Sustainable Modern Agriculture

Since its appearance in agriculture, plastic films have revolutionized this sector with huge benefits in terms of quality and quantity of crops. In the last decades, agricultural films gained not only a great deal of interest and attention but also a big market more and more extended in any country of Europe, America, and Asia, with a constant positive trend that does not know crisis. Plasticulture is a term that indicates the world of plastic films applied in agriculture, ranging from greenhouse covers to mulches, from low tunnels to solarization films, from Totally Impermeable Films (TIF) to photo-selective films. It includes raw material (several kinds of polymers), different types of films, applications, agronomical performances, and the recycling problems. In this chapter, a review of Plasticulture is presented, from historical origins to the films extrusion technique, from their physical properties to the different typologies and uses. In particular, a new generation of agricultural films is described with characteristics, performances, and employment modalities. Finally, the new frontier of plastic films for greenhouse, with open window to UV-B radiation, is presented, considering the huge potentiality for improving further the crops quality and the increase of different nutraceutical contents in fruits and vegetables.

Plastic mulch film and sheets, rods, and tubing find increasing use in agriculture. Current polyethylene plastic mulch film is not biodegradable and therefore cannot be plowed back into the soil. It may undergo fragmentation, and the small fragments are blown all over and find its way into ocean and other pristine environments. This causes irreparable harm to ecosystems and the habitats. Completely soil-biodegradable plastics or compostable plastics offer an environmentally responsible end-of-life solution for plastic mulch film and plasticulture products. Claims of biodegradability should be qualified by the disposal environment (soil or compost), 90% + biodegradability as measured by the evolved CO₂ from the microbial process using international standards for soil biodegradability and/or compostability. However, one has to be careful of misleading claims that are prevalent in the marketplace, particularly additive-based polyolefin plastics. Using biobased carbon in place of petro-fossil carbon in the products offers a reduced carbon footprint, empowers rural agrarian economy, and reduces dependence on fossil resources.

Many applications of traditional plastic in agriculture have a short life, which on average do not exceed 2 years and for precisely this characteristic generates large quantities of waste that must be adequately disposed. It has been estimated that 30% of the plastic waste produced from agriculture originates from short-life applications such as clips, wires, nets, pheromone dispensers, and geotextiles with a high risk of remaining in the agricultural system thereby causing pollution. In this chapter the state of the art of these commercial applications is described, together with research and development for these applications. Some successful case histories in European countries are reported, along with how these solutions came about. Biodegradable and compostable bioplastics have physico-chemical and mechanical characteristics suitable to substitute traditional polymer plastic applications, thereby reducing waste generation at the end of life cycle. Biodegradable and compostable materials, compliant with the main international standards can be left directly in the field on the soil where they are biodegraded by microorganisms present either in the soil or in the compost heaps, together with crop residues, producing organic matter that can be recycled to the soil.

This chapter describes the state of the art of the agronomic effects of degradable bioplastics used as agricultural films. Current use of bioplastics and certified commercial biodegradable materials, both as granulates and as final products, are introduced. Following, agronomic effects on crops are reported and compared to the routinely used oil-based nondegradable plastics, basically the polyethylene films. Biodegradable films for agriculture were initially developed mostly for mulching application, which still remains the most significant one. Since last reviews published in 2011, new progress and perspectives have mainly arisen regarding the agronomic effects of biodegradable mulching on vegetable crops, not only as films but also as nonwoven biobased mulches. The film mechanical laying and the effects on yield, earliness, product quality, weed control efficacy, microclimatic improvement and film soil coverage and degradation are presented in detail for tomato crops and for other crops where mulching is a common technique (pepper, melon and other cucurbits, strawberry, lettuce,…). Some information is provided for crops not so frequently mulched (broccoli, sweet potato, sweet corn). New findings published on the use of biodegradable films for solarisation are also reviewed, while no significant progress on the use of films for low tunnel covers has been made. Recent proposals for vineyards and future potential application of bioplastics for orchard crops are also addressed. Finally, pros and cons for the adoption of biodegradable films for cultivating crops are discussed.

Agricultural activities need plastics for many applications such as films for soil mulching and pots for plants transplanting. The use of plastic products, made of fossil raw materials, such as polystyrene, polyethylene, and polypropylene results in huge quantities of plastic wastes to be disposed of. In the past two decades, the growing environmental awareness strongly encouraged researchers and industries toward the use of biodegradable polymers for solving the plastic waste problem. Researchers have made strong efforts to identify new biopolymers coming from renewable sources as valid ecosustainable alternatives to petroleum based plastic commodities. The main research results and current applications concerning the biodegradable plastics in agriculture, such as thermo-extruded Mater-Bi and sprayable water-born polysaccharides based coatings, are described in this chapter. A lineup of biopolymers coming from raw and renewable sources, such as polysaccharides, are reported; the intrinsic chemico-physical properties of polysaccharides, responsible for the realization of dry water stable hydrogels, suitable for the formation of both soil mulching coatings and transplanting biopots, are investigated. A description of the natural additives, fillers and cellulosic fibers included in the polymeric matrices, able to enhance the mechanical performance of coatings and pots is provided, together with the outputs in the specific applications.

The main standard test methods for biodegradation of plastics in soil (ISO 17556, ASTM D5988, NF U52-001 and UNI 11462) determine the rate of biodegradation under normalized conditions. The standard testing procedures are designed to determine the inherent biodegradability of plastics in soil under optimal controlled conditions that may not be necessarily representative of any specific environmental conditions but they ensure repeatability. The normalized conditions defined by the standard test methods differ in several aspects. A comparative analysis is presented. Besides the biodegradation test methods, pass levels and a time frame also need to be defined in order to determine whether bio-based products will biodegrade sufficiently under soil conditions. There is currently no European or international specification that defines criteria for biodegradation of bio-based products in soil. Criteria for biodegradation of materials used in agriculture and horticulture are only defined in standard specifications NF U52-001 and UNI 11462, together with criteria for environmental safety. However, the evaluation of the biodegradation in soil is not obligatory in the French specification. The main requirements for mulching films are: (1) biodegradation at least 90% within 24 months; (2) material shall not contain heavy metal, no ecotoxicological effects. The same requirements have been adopted by the USDA-AMS National Organic Program (NOP) for mulching films allowed for organic crop production. The constraints, gaps, and limitations of existing relevant testing methods and the new developments are identified and analyzed in this chapter. Functional barriers with respect to standards and labeling for soil biodegradable plastics are analyzed.

The study of the life cycle of products for the quantification of their environmental impacts, in each of their production and utilization stages, is a well-established and scientifically recognized methodology. This approach is known as Life Cycle Assessment (LCA) and it is the base for several product and service certifications. This chapter focuses on the strengths and weakness of the LCA approach to biodegradable plastic in agriculture, thorough the description of main issues emerged from an extensive literature search and key case studies. In particular, studies which apply LCA on biodegradable mulching films and nursery pots are presented and discussed. Results of the study are somehow controversial. Despite the fact that LCA is the most systematic way to understand the interrelation between a product and the environment (including biodegradable products), some specific issues, related to the own nature of biodegradable products, require a more detailed way to be properly addressed. In particular crucial issues are related to the modelling of realistic waste management scenarios, the development of more appropriate impact categories (e.g. effects of littering) and the assessment of biorefineries which represent the only non-fossil carbon source for bio-based plastic materials.