- François-Xavier Branthôme
Opinion trends and political decisions taken in favor of limiting chemical inputs (fertilizers, pesticides, etc.), particularly in Europe, have had a positive impact on the use of agricultural plastics, in addition to drastically (and sometimes problematically) reducing the number of substances available for agriculture. Widely criticized for their consequences on the environment and health, “aerial” input treatments have increasingly given way to precision inputs, delivered to crops by means of fertigation (by micro-irrigation or drip-irrigation).
Today's processing tomato crops rely heavily on fertigation in almost all countries where they are produced, on the one hand because of the increased efficiency of input supplies (directly and precisely where the plants need them), and on the other hand because of the savings made possible by the accuracy of dosages (precise matching of the quantities delivered to the exact needs of the crop, leading to less unnecessary spreading of input substances).
Processing tomatoes also use biodegradable plastic mulch films, the use of which has been developed particularly in northern Italy (Piacenza), and to a lesser extent in Spain. Mulching is known to improve yield but also earliness (by around 10 to 15 days), the latter effect being linked to the warming of the soil and to the root development that it promotes. Plastic mulching also has the advantage of saving water since it reduces evaporation, of limiting leaching phenomena by promoting flow towards the interrow, and of reducing weeding operations (now carried out mechanically and not by chemical means) to the interrow alone thanks to its herbistatic properties.
Different specific properties expected from mulching correspond to different compositions and color codes (black means herbistatic and "warming", black-green means "double-sided" for herbistatic properties with less thermal effect, while blue, red, brown, etc. have other meanings). In the 90s, additive companies like Ampacet explored the different pigments that could be incorporated into the manufacturing resin in addition to charcoal black. Consequently, plastic mulching was produced on an experimental basis in the following colors: blue, red, orange, yellow, green, brown and metallic silver (very reflective). The response of plants to some colors has been very positive while other colors have had absolutely no positive effect.
The color of plastic mulch was initially black or clear when it was introduced in the sixties. Black was the preferred color due to the resulting increase in soil temperature compared to the absence of plastic mulch and because it prevented the photosynthetically active radiation (PAR) from reaching the soil, thereby eliminating weed growth. Clear plastic mulch increased soil temperature by at least 5°F to 8°F (9 to 14°C) compared to black film, but transparent mulch allowed weed growth under the film. Over the past decade, other colors have been introduced to the industry, including red, infrared thermal green (IRT), IRT brown, blue, metalized or reflective silver, white, and co-extruded films with two colors, black on one side and white on the other, for example.
The change in vegetable crop yields in response to the different mulch colors is due in part to an increase in soil temperature and the range of wavelengths reflected by the different mulch colors. Different colors absorb and reflect different wavelengths of light, and plants are particularly sensitive to the color of light their leaves catch from the sun and from reflective surfaces. The pigment in plants that controls the effect of the red and far-red light response is called phytochrome. Phytochrome can exist in different states. When phytochrome is exposed to red light, it is in the form that absorbs far-red light, and when exposed to far-red light, it is in the form that absorbs red light. Red and far-red light (between 600 and 800 nanometers) produce the greatest growth response in plants. Light that has a lower far-red to red (FR/R) ratio will cause shorter stems and larger roots to develop in plants. Conversely, a higher FR/R ratio will encourage plants to produce more new shoots and new buds, with the end result being taller, more leafy plants. Different mulch colors reflect different wavelengths of light and thus create different FR/R ratios, which in turn induce different responses that can be targeted by the choice of colors.
The mulches available today typically include three and up to seven layers depending on the cultivation goals and the needs of the plant. But not all types of film are useful for all plants and in all growing conditions, so the quality and nature of the film must be carefully adapted. In processing tomato crops, for example, only biodegradable films are used, as their fragmentation before harvesting allows mechanical harvesters to function unhindered. The biodegradability of films for processing tomatoes is linked to this one constraint: conventional recyclable polyethylenes would be perfectly suited to cultivation (moisture retention, herbistatic effect, thermal effect, etc.), but their excessive resistance prevents their use as it would interfere with mechanical harvesting or even prevent it.
Oxo- and bio-degradable: European standard EN 17033
Plasticultors have long been concerned about the fate of heavily soiled mulch films. They have also sought to develop technical solutions for satisfactory field degradation. However, the communication made around biodegradable, oxo-degradable, photo- and thermo-degradable products can create confusion that is damaging to their reputation, both with the public and with environmental authorities. So professional partners must be able to use a common and justifiable vocabulary to avoid the pitfalls of an ideological discourse that is purely for show... This is the reason for the standards that are gradually being put in place.
Paul Colonna, Research Emeritus Director at the National Institute for Agricultural Research (INRA)
However, it is rare to obtain real scientific data on oxo-degradable and "enzyme-degradable" plastics that are completely biodegradable in the environment. Up to now, this data has been irreproducible and not easily available. Often, parameters like the carbonyl index, reduction in molecular weight, microbial growth and the ADP/ATP ratio have been used to demonstrate the potential biodegradability of these materials. However, these parameters can be considered only as evidence of microbial activity and are not direct evidence of biodegradation. The conversion of carbon to CO2 is the only scientifically correct parameter to quantify biodegradation in a particular environment. The fragmentation of conventional plastics containing additives and the disappearance of visual pollution linked to plastic cannot be considered a relevant solution if no biodegradation of these fragments can be guaranteed, or if only partial biodegradation occurs. Since the EN 17033:2018 standard requires full biodegradability in the soil, conventional plastics containing additives cannot be considered as complying with it, notably due to the presence of metal salts, which are added to enhance the degradation process, but which make them unsuitable for recycling. Once they are present in the environment as small non-biodegradable particles (microplastics), their accumulation in the environment is practically inevitable. This is a kind of “double penalty”, as the material is then neither recyclable nor perfectly assimilable in the soil.
To avoid any confusion, a better term designating these conventional polymers enriched in additives could be "thermo-fragmentable" or "photo-fragmentable plastics". To call them biodegradable is obviously an error.
A clear distinction must therefore be made between biodegradable plastics and plastics containing additives such as oxo-degradable plastics and enzyme-degradable plastics. The complete biodegradability of a material used in and on soil is crucial to being able to claim compliance with standards such as EN 17033 (2018). The enrichment of conventional plastics with specific additives facilitates fragmentation into smaller pieces. However, if complete biodegradation does not take place, it will lead to a build-up. Furthermore, collection for recycling will no longer be possible. In order to prevent microplastics from escaping and posing a risk to the environment, legal action is currently underway in Europe to implement specific restrictions on products intended for consumers and professionals. In addition to Directive 2019/904 and the ECHA proposal relating to oxo-degradable plastics, it should be noted that biodegradable plastics are also being targeted for more restrictive regulations. For uses where well-established collection and good waste management schemes are in place, biodegradable plastics are not the best solution. However, for some products, such as mulch films, biodegradable plastics may be considered an interesting option.
The EN 17033 standard defines the characteristics of biodegradable film in the field, including the duration of use by type of crop. On a technical level, a film (polymer + plasticizer + filler (starch or chalk)) that is biodegradable, and conventional polyethylene to which no stabilizer has been added, degrade the moment they are manufactured – in fact plastic is naturally biodegradable. The effective lifespan of a plastic is directly conditioned by the nature and quantity of stabilizing product it contains (determined to withstand a given quantity of thermal radiation (degree days) and light radiation (kilolumens). It remains an obvious fact that weather conditions are difficult to predict. The nature of the soil and especially the microbial activity (degradation of polylactic acid (starch) which makes up the plastic) also play an important role, causing variations in the degradation speed of the mulch. However, all mulches do not degrade in the same way, depending in particular on the compositions linked to the different colors used. In the increasingly green context of European agricultural policy, colorless "crystal" mulch remains "politically incorrect" insofar as it must contain an anti-UV additive (traces (a few ppm) of component based on metal oxides). However, this type of mulch film is not used for processing tomato crops, as its use is reserved for the melon sector.
Benefits of mulching: the case of processing tomatoes
Biodegradable polymers allow operators to increase the mulched surface area. Mulching that treats tomato crops with conventional polyethylene (PE) mulch films has been tested in the past, but on a small scale and for manual picking only, making the harvest quite expensive. Nevertheless, this test was carried out in the south of Italy (Foggia) some twenty years ago, and constitutes a reference point.
Then, with the advent of harvesting machines, it became clear that PE mulching was difficult to handle during harvesting, with high risks of crop contamination and problems inside the harvesting machines due to interference of plastics with the rotating equipment parts. The mechanical properties of the films were not sufficiently weakened at the end of the crop cycle.
The first biodegradable mulches were tested in Spain (Navarra) and in Northern Italy, in addition to better crop management and easier and safer harvesting methods.
The advantages of biodegradable mulch films have been the subject of an abundant literature from Spain (Institute of Agrarian Technology) and Italy (CIO, Consorzio Italiano Orticola), most of which date back to the 1980s... Technicians from these research organizations (Juan Ignacio Macua, Marco Dreni) quickly demonstrated the positive impact on processing tomato yields, as well as better weed control and a reduced need for irrigation water.
The Tomato News editorial team wants to thank Mr. Olivier de Beaurepaire, agronomist for BASF France in charge of the CIPA technical committee for biodegradables, and Mr. Xavier Ferry, of the International Committee for Plastics Used in Agriculture (Assistant to the General Secretary), for their help in providing documentation and assistance in writing this article.
Some complementary data
Sources: Interview, N° 139 de Plasticulture, FreshPlaza
More information about technical and economic benefits of biodegradable mulch films in the second part of this dossier which will be published very shortly