Studies on Degradable Plastics for Agriculture in Taiwan
In February 1991, the Tainan District Agricultural Improvement Station (TDAIS) began a series of experiments to evaluate the feasibility of using degradable polyethylene (PE) plastic films incorporated with starch in cultivating horticultural crops. Results showed no difference in the growth, yield, total soluble solids, and heavy metal content of cabbage mustard, processing tomatoes, cantaloupes, and head lettuce grown on beds mulched with various kinds of disintegradable, biodegradable, or traditional PE films. No difference was observed in yields of these crops and in the heavy metal (Fe, Pb, Ni,Cu, Cd, and Cr) content of the edible parts of cabbage mustard and head lettuce grown in the field incorporated with debris of used disintegradable PE films in 10 consecutive years. The same was observed for rice. As well, no difference was observed in the microflora and physical properties of the soil incorporated with disintegradable PE debris.The degradation rate was influenced by the mulching date, content of starch incorporated into the films, and the kind of soil. The macro- and micro-environmental changes in different seasons affected the degradation time of the tested degradable plastic films. The more starch incorporated, the faster the films degraded. Degradation rate reached 93.5% for the biodegradable film buried in slate alluvial soil, but it was less for the film buried in red soil.
Abstract
Introduction
Plastic films are widely used in the cultivation of crops and vegetables particularly in covering plants at seedling stage, sunscreening and rainproofing them, and field mulching. Plastics are also used as protective bags for vegetables and fruits. Traditional plastic materials used earlier in agriculture caused pollution problems especially when plastic wastes were burned, producing irritant air like hydrogen chloride and other poisonous gases like methyl aldehyde. When buried, these plastic wastes required large areas, affecting the physical and chemical features of the soil and polluting it. Moreover, discarding undegradable plastics carelessly resulted in blocked irrigation ditches and clogged drainages. None of these contributed to or benefited the environment and the living things on it.
In view of the changes that have taken place in recent years such as the decreasing rural labor that is increasingly aging, the labor-saving and environment-friendly use of degradable plastic films in agriculture has been on the rise. A pioneer in this effort, Taiwan's Tainan District Agricultural Improvement Station (TDAIS) initiated a feasibility study in February 1991 to evaluate the application of degradable plastics in agriculture.
Mulching Films and Debris
Effects on Yield and Quality
No difference was observed in the growth, yield, total soluble solids, and heavy metal contents of cabbage mustard, processing tomatoes, cantaloupes, and head lettuce grown on beds mulched with various kinds of disintegradable, biodegradable, or traditional polyethylene (PE) film.
Effects of Disintegradable Pe Debris on Yield and Quality of Crops
No difference was observed in the yields of cabbage mustard, cantaloupe, head lettuce, processing tomatoes, and rice (Table 1, Table 2, and Table 3) and in the heavy metal (Fe, Pb, Ni,Cu, Cd, and Cr) contents of the edible parts of cabbage mustard and head lettuce grown in fields incorporated with used distintegradable PE debris (Table 4).
Effects of Various Amounts of Disintegradable Pe Debris on the Growth, Yield, and Heavy Metal Content of Rice Plants
Various amounts (0 g, 4 g, 8 g, 16 g, 32 g, and 64 g) of used disintegradable PE films were separately mixed with field soil. The mixed soil was used to grow paddy rice in clay pots. No difference among the treatments was observed in plant height, length, and weight at harvesting. No difference was also observed in the content of heavy metals (except Cu and Zn) in brown rice subjected to the different treatments. Heavy metal contents were less than the critical level of tolerance limits in foods.
Effects of Used Disintegradable Pe Debris on the Properties of Soil
No difference was observed in the microflora (bacteria, fungi, and actinomyces) and physical properties of the soil incorporated with disintegradable PE debris (Table 5 and Table 6).
Accumulative Weight of Debris
Weight of debris of the biodegradable film was less than that of the biodisintegradable film (with incorporated starch in regular film) after being plowed back into the field (Table 7). Besides, the practical accumulative weight of disintegradable debris in the field was only 13.7% in 10 consecutive years in the Tainan location (Table 8).
Degradation Rate
The degradation rate was influenced by the mulching date, content of starch incorporated into the films, and the kind of soil.
Degradation Rate at Different Mulching Dates
Macro- and micro-environmental changes in different seasons affected the degradation time of the tested degradable plastic films (Table 9 and Table 11). The bio/photo-degradable silver-black PE films containing 20% starch from USI Far East Corporation degraded after 56, 83, 38, and 33 days when they were used as mulch in autumn (October 1991), winter (December 1991), spring (April 1992), and summer (August 1992), respectively (Table 9). The more starch incorporated, the faster the films degraded (Table 10).
Degradation Rate in Different Kinds of Soil
Degradation rate reached 93.5% for biodegradable film buried in slate alluvial soil, but it was less for the film buried in red soil (Fig. 1).
Conclusion
The studies in Taiwan that began in 1991 have shown that degradable PE films can help lessen the problem of plastic wastes in agriculture. By using degradable plastics, farmers can protect their horticultural crops from harsh elements like too much sun, wind, rain, and diseases, without significant negative effect on yield, quality, and heavy metal content of these crops as well as on soil properties. Meanwhile, the starch content of the degradable PE films is a major factor in their degradability, and as such, future studies may focus on this aspect.
Index of Images
Table 1 Yields of Head Lettuce and Cabbage Mustard Grown in the Field Incorporated with Used Disintegradable Pe Debris (Ecolene) in 10 Consecutive Years (Tainan Location)<BR>
Table 2 Yields of Cantaloupe and Paddy Rice Grown in the Field Incorporated with Used Disintegradable Pe Debris (Ecolene) in Eight Consecutive Years (an-Nan Location)<BR>
Table 3 Yields of Processing Tomatoes and Paddy Rice Grown in the Field Incorporated with Used Biodegradable or Disintegradable Pe Debris
Table 4 Heavy Metals Content of Head Lettuce and Cabbage Mustard Grown in the Field Incorporated with Used Disintegradable Pe Debris (Ecolene)
Table 5 Microflora of Soil Incorporated with Used Disintegradable Pe Debris (Ecolene)
Table 6 Physical Properties of Soil Incorporated with Disintegradable Pe Debris (Ecolene)
Table 7 Weight of Debris of Biodegradable and Biodisintegradable Films after Plowing Back to the Field
Table 8 Accumulative Weight of Disintegradable Films after Plowing Back to the Fields (1992_2002)
Table 10 Degradation of Mulching Films Incorporated with Different Starch Contents
Table 11 Degradation of Various Mulching Films after Exposure to Natural Solar Radiation at Different Periods
Figure 1 Weight Loss of Mater-Bi Biodegradable Films Buried at Different Types of Soil
Table 9 Days from Mulching to Degradation at Different Mulching Dates
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