8. Development in plastics for soil solarization
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Improvement of plastic technology for soil
Influence of different colour plastic mulches used for sol solarization on the effectiveness of soil heating
Improvement in plastic technology for soil heating
Improvement of plastic technology for soil solarization
James E. Brown¹, Clauzell Stevens², Victor A. Khan², George J. Hochmuth³, Walter E. Splittstoesser4, Darbie M. Granberry5, and Brandon C. Early¹
¹Department of Horticulture, Alabama Agricultural Experiment
Station, Auburn University, AL 36849 USA.
²George Washington Carver Agricultural Experiment Station Tuskegee University, Tuskegee Institute, AL 36088 USA.
³Vegetable Crops Department, University of Florida, Gainesville, FL 32611 USA.
4Department of Horticulture, University of Illinois, Urbana, IL 61801 USA.
5Cooperative Extension Service, University of Georgia, Tifton GA 31793 USA.
Most agricultural activities have been modified to Fit within various soil temperature ranges. As technology enables us to modify the microenvironment around the plant, we become less satisfied in tolerating existing undesirable environments. Complete temperature control except in greenhouses, is not presently possible; however, the effect can be modified in limited ways. Regardless of the chemical composition of plastic film, its form or its pigmentation, plastic mulched soil is always warmer, on the average, than bare soil. Many scientists have shown that plastic mulched soil increases in temperature and results in increased early and total yields of certain vegetables. The use of plastic in areas where the production of vegetables is limited by early low temperatures has been promising. A 50 percent yield increase in pineapples was reported in Hawaii when plastic mulch was used. This increase was attributed to an increase in soil temperature during the winter period and not to an increase in soil moisture. The use of clear plastic mulch in cold areas increases soil temperature and promotes germination and emergence.
Rapid healing of the soil is mainly required for spring grown crops where a mulch is used to give a stimulus or al least some degree of protection to the roots. In addition, increased heating of the soil accelerates seed germination in melons; promotes early planting so that the cultural cycle is achieved within the local climatic season; helps with the re-establishment of transplanted tomatoes and peppers, and stimulates vegetative growth as well. Vine crops have characteristically been among the most responsive to microclimate modification with clear plastic mulch, and the response is superior to that of the black plastic mulch treatment. In all cases, accelerated growth rate, resulting from the higher soil temperature obtained with transparent or to a lesser degree with thermal-opaque and photo-selective films, has resulted in an earlier harvest, an extended harvest period and increased yields.
Plasticulture is the science and technology of the use of plastics in the agricultural industry. The science of plasticulture had its beginning in 1924 when Warp (79) developed the first glass substitute for widespread agricultural use. British scientists first made polyethylene (PE) as a sheet film in 1938 (56). The largest volumes of agricultural plastics used today are in the form of plastic films (63).
The earliest method which used organic and inorganic materials for modifying the microclimate of crops was mulching (43). These materials soon gave way to various types of PE films which revolutionized protected cropping as demonstrated by Emmert in Kentucky (23) and Hall in California (33). Emmert is considered by many to be the father of agricultural plastic development in the USA. He detailed the principles of plastic technology with his research on greenhouses, mulches, and row covers (23).
During the past decade there have been many significant developments within the plastic industries. This paper will primarily consider the chemical and physical properties of plastic films, kinds of plastic films used today, and improvements of the heating capacity of plastic.
The Chemical and Physical Properties of Plastic Films
Plastics were first introduced on a commercial scale in 1939 (11). These include PE, polyvinyl chloride (PVC), and ethylene vinylacetate (EVA). The widespread use of PE (the principal type of plastic used today) is due to easy processibilily, excellent chemical resistance, high durability, flexibility and freedom from odour and toxicity (14, 26).
The Chemical Properties of Plastic Films. - The vast majority of plastic films are made from synthetic macromolecular materials based on polymers with the addition of a number of chemical additives. An example additive is a lubricant, which is the metallic salt of a fatty acid and used at low concentrations (less than I percent) in the plastic manufacturing process. In addition to lubricants, a low concentration of a stabilizer (i.e. p-tertiarybutyl phenol) is normally added to plastic film. Polymers of ethylene are based on the monomer ethylene, CH2=CH2, which has a molecular weight of 28. If the degree of polymerization of PE averages "n", then the polymer is formed from "n" units of ethylene. For a long-chain structure and where the average "n" is 1 5(X), the molecular weight is 42 000. These plastic materials are generally made of long-chain molecules with a few short side chains. The individual molecules have a length of up to I um and a width of 2 to 4 nm (22).
PE, softened by heat, can be shaped under pressure and retains its form when cooled. These polymers are classified as thermoplastics and include such materials as PE (high and low density), polyvinyl chloride (PVC), polystyrene, polypropylene and ethylene vinylacetate (EVA) (22).
Chemical Compositions of Plastics used in Agriculture
Low-density Polyethylene. - Originally when anyone referred to PE, it was automatically assumed to be low-density PK. The development of a high density PE and the development of polypropylene opened up the polymer range, all of which belong to a group known as polyolefines. More recently this group has been expanded by the addition of a medium-density PE, a linear low density polymer and a very low density PK. It is the linear low-density polymer which has found widespread use in agriculture (14). Low-density PE is produced by the polymerization of ethylene under very high pressure. The half-finished product or resin comes from the factory in the form of pellets, which are then converted info flexible sheets of plastic film by an extrusion-blowing process (14, 26).
Low-density PE has become the predominant choice for agricultural plastic mulch primarily for two reasons: 1) it has excellent physical properties of tensile strength (required for the mechanical application of plastic mulch to the soil) and of resistance to tearing when exposed to strong wind; and 2) low-density PE film has, over recent years, enabled the film thickness to be considerably reduced while maintaining good tensile strength. This reduction in thickness means that the grower uses less film by weight and consequently the cost is lower (2).
Spunbonded polypropylene plastic films are widely used as vegetable row covers. This material lets in less light than PE but, because it is more porous, it allows less heat build-up during hot days. In this review polypropylene is considered to be a form of PE.
Polyvinyl Chloride and Ethylene VinyIacetate Films. - PVC is the second most widely used plastic material after PE. It is synthesized from acetylene and hydrochloric acid in an autoclave. In contrast to PE, PVC plastic is normally rigid. Plasticizers (normally high boiling point organic esters such as dioctylphthalate or tricresyl phosphate which form a homogeneous mass with the PVC at elevated temperatures) must be added if flexible sheets are to be obtained by the extrusion-blowing process (22, 26).
Another polymer being used by the plastic industry is the co-polymer polyethylene vinylacetatze, also called EVA. This film has increased elastic characteristics compared with PE. When used as a greenhouse film, it shows improved heat retention characteristics (14).
Additives for Special Chemical Properties of Plastic Film. - Several groups of chemicals can be added to polymers of plastics in order to modify or enhance some specific property of finished plastic products. The additives include an array of materials which can be used to improve heat retention, increase wettability of a film surface, ultraviolet (UV) stabilizer and photodegradable additives. Pigments may also be added to produce a wide range of colours. Of these additives, carbon black is the most widely used for the production of black mulch film for agriculture (2, 14).
The Pigmentation of Mulch Films. - Colour determines the energy radiating behaviour of plastic films. Black film (loaded with carbon black filler) absorbs a larger part of the solar energy falling upon it and the film becomes hot. Transparent film (no filler) transmits most of the solar radiation which is in turn absorbed by the soil. White opaque film is produced by incorporating titanium dioxide (2). The white mulch film reflects radiant energy and thus the soil remains cool. There are various formulations of films with different additives aimed at combining the advantages of black such as weed prevention and the thermal advantages of transparent film. These variations include: smoke grey films, thermal opaque films, red or brown and photo-selective films such as green PVC film (2, 41).
Plastic with Ultraviolet Stabilizer. - Transparent plastic degrades when exposed to the sun's UV rays. This degradation is slowed by the addition of stabilizers or additives which will give protection against UV light (2, 22, 26).
Protection against UV attack has been a difficult problem to resolve. Initially, benzophenone was used as an additive. Stabilization was improved through the addition of a complex nickel material. During the past decade, a new type of UV light stabilizer, hindered amine, became available (14). Black films reflect almost all the sun's UV rays resulting in considerably less degrading effect. Carbon black which gives the opacity to the black film plays an equally effective role as an UV stabilizer. Black film last longer as a rule than other types of films, particularly those without a UV stabilizer (2, 26).
Plastic with Anti-condensate Properties. - Plastic film which remains clear in spite of water condensation does so by means of the action of welling agents or anti-condensate properties, that cause the water droplets to coalesce and run down the walls instead of dripping directly onto the plants which could encourage disease when exposed to the high humidity. The anti-condensate properties also help to keep the film transparent for maximum sunlight penetration (22, 65).
Heat-retaining Additives. - Energy is radiated as heat with long wavelengths from approximately 8 to 50 um in the far infrared region of the electromagnetic spectrum. An improved greenhouse effect can be obtained by increasing the absorption in the far infrared (heat) through introducing a thermal barrier to the film. Infrared PE plastic film is modified by an additive which gives it the capacity to absorb infrared radiation so that thermal PE becomes comparable to PVC and EVA in heat retention capacity. The EVA plastic film with its high vinyl acetate concentration acts as a heat-trap to prevent the escape of infrared rays (22). PE has the very simple spectrum of an aliphatic straight chain with absorption bands for C-H stretching and bending and CH2-CH2 skeletal vibration modes. It is almost completely transparent in the far infrared except for one band at 14 um. The filler grades of mineral additives give infrared spectra of increasing complexity of the mineral (34). Les plastiques de Carmaux (77) compared the energies retained by PE films containing a variety of different alluminium silicates, and confirmed that calcined clay gives the best heat retention (over 70 percent). This mineral emits a strong band from 8.3 to 50 um.
Photodegradable and Biodegradable Additives. - Photodegradable plastic films, particularly PE, with controlled degradation characteristics were developed to degrade and disintegrate after serving their purpose. The photodegradable PE films contain a ferric ion complex which accelerates the rate of embrittlement (29, 30, 31). Degradation also can be promoted by incorporation of calcium carbonate into a low-density PK. When exposed to sunlight, the material can disintegrate within one to three months and the residual inorganic materials readily disperse in the soil (58). Recently, Plastigone, a low-density PE, and poly-propylene films have been formulated with an improved activator system. This breakthrough was achieved by varying the relative proportion of a second nickel or cobalt dithiocarbamate which made it possible to achieve a graduated concentration vs. time with the embrittlement relationship (24).
Biodegradable plastic, which incorporates water soluble biodegradable plasticizers and low-density PE into starch ethylene acrylic acid copolymer films, increases the rate of degradation without adversely affecting plastic properties. Because starch is biodegradable, this plastic will disintegrate into small particles much faster than conventional plastics (59, 60, 61).
The Physical Properties of Agricultural Plastic Film
Thermoplastic is the principal material used in plasticulture. Physical properties of thermoplastics relevant IO agriculture are: 1) durability of plastic, 2) permeability to gas vapours and liquids, 3) light transparency, 4) greenhouse effects, 5) modification of soil temperatures under the film and 6) wavelength selective properties.
Durability and Mechanical Strength of Plastic. - Transparent plastics degrade as a result of exposure to direct solar energy light and particularly under the influence of UV rays. This phenomenon is called ageing. The life of transparent film may be doubled if it is not used during the hottest months of the year (26). The factors affecting durability of plastic have been outlined by Dubois (22) and are as follows: 1) thickness, 2) type and grade of plastic, 3) time of exposure to UV rays, 4) oxygen availability (which enhances photo-oxidation causing colour changes and loss of strength by embrittlement), 5) temperature (a temperature increase of 10-20°C causes a doubling of the reaction rates), and 6) humidity.
Mechanical strength of the plastic can vary greatly depending on the quality of the resin and the care with which it has been manufactured. In recent years, mechanical strength has been greatly improved by the use of new resins and by new extrusion machines which provide films with greater homogeneity. Resistance to adverse weather (wind, hail, snow) depends to a great extent on the qualify of the resin (26).
With the exception of certain interior grades of plasticized PVC, the strength of these plastics is acceptable in the normal range of ambient temperature. However, pigmented PE films (black or smoke-grey) absorb solar energy in the same way as a black body. They expand during the day and contract al night If they are stretched too much during application, there is a possibility that the film may tear upon contraction (26).
Permeability to Gas Vapours and Liquids - Under protective covers made of flexible film, the relative humidify is often higher than under covers made of rigid materials. This is because of the reduced air exchange of the covering causing condensation to build up. The permeability of flexible film to water is almost zero. Plasticized PVC is relatively more permeable to water vapour than PK. Under certain conditions, PE is more permeable to oxygen than to carbon dioxide (22, 26).
Light Transparency and Quality of Light. - Light transparency is the degree of permeability of plastic or glass to solar and terrestrial radiation. Most plastic materials are not transparent Since plastics do not allow the eye to see through them clearly, they can be described as translucent. From a practical point of view, glass and plastic materials arc similar in regards to the qualify of light transmitted (26, 48, 57). One of the physical characteristics of transparent materials is high transmittance of solar and terrestrial (thermal) radiation. PE has the greatest transparency to UV rays (wavelengths less than 380 nary), followed by PVC and glass. Glass has a greater permeability to the visible spectrum (380-780 nm) than plastic materials However, differences in actual absorbing capacities is generally less apparent in practice because of losses from reflection which depends on the refractive index of the material. Glass and plastic materials are about equally transparent to thermal short-wave infrared (780-2 500 nm) radiation. Glass and PVC sheets (1 nm thick) obstruct long-wave radiation, while other plastic materials (including black PE) show a transparency to infrared (more than 2 500 to 3 000 nm) ranging from 30 percent for PVC film to about 80 percent for PE (26).
Greenhouse Effect. - The greenhouse effect is produced by the difference in permeability to two categories of radiation: solar and terrestrial radiation. To produce the maximum greenhouse effect and to act effectively as a sun trap, the ideal material should be transparent to solar (280-2 500 nm) but completely opaque to terrestrial radiation (5 000 to 35 000 nm) (26). PE mulch reduces heat convection and water evaporation from the soil to the atmosphere. As a result of the formation of water droplets on the inner surface of the PE film, its transmissivity to long-wave radiation is highly reduced, resulting in better heating of the soil (55, 56).
Soil Temperature Under Plastic Film. - In general, under transparent film, the temperature of the soil rises by several degrees during the day. This temperature rise can vary between 2° and 10°C according to the season, soil type, the level of sunshine and soil moisture (48). At night, the difference in temperature between covered and bare soil is less (2° to 4°C). Under black film, the soil temperature may be 10-15°C higher than the bare soil (17), while in some instances it can be slightly lower than bare soil (22, 78). Under white film the temperature is always lower than uncovered soil. This type of film is used either in regions with a high level of solar radiation or where it is required to reduce the transmitted radiation and soil temperature; or in regions of low luminosity, where there is a need to increase the amount of reflected light on the lower and middle plant leaves (2, 22).
Wavelength Selective Properties of Plastic Film. - Wavelength selective plastic film which absorbs or reflects photosynthetically active radiation (PAR) in the 400 to 700 nm range and transmits near infrared solar radiation in the 700 to 3 000 nm part of solar spectrum, can be considered hybrids between clear and black films (51). This is in contrast to coloured films which reflect different wavelengths of radiation into the plant canopy thereby affecting photosynthesis and/or plant morphogenesis (20).
The theory and benefits of wavelength selective films were discussed al the 21st National Agricultural Plastics Association Congress (NAPA Congress) (51). The concept of wavelength selective films is not new (1, 41, 49, 69) and such films have been referred to as photo-selective films (22), even though part of the selectivity is outside the photometric (visible) region. Both photo-selective and thermally opaque films (those blocking long wavelength infrared radiation) are currently produced and marketed in Europe and Japan (2).
Loy et al. (51) discussed at the 21st NAPA Congress, the evaluation of a PAR-absorbing, IR-transmitting brown film from Israel. Soil temperature under the brown film was intermediate between those of clear and black PE film. Early growth enhancement of muskmelon on brown film also was intermediate between that obtained with clear and black films.
Mulching consists of covering soil with some organic or inorganic materials which form a protective barrier to limit soil water evaporation, control weeds, maintain a good soil structure and protect crops from soil contamination. This is achieved by moderating climatic excesses of sun, rain and wind (19, 25, 20). Materials traditionally used for mulching depend on the region and include: intact or decomposed plant residues, sawdust, livestock waste and stones (10, 42, 46). Non-traditional materials include thin sheets of paper and petroleum or plastic materials, resulting in similar or increased benefits over traditional materials (4, 6, 16, 18, 40, 47, 50, 64, 67, 71, 76).
Based upon the same principle, plastic film has completely revolutionized the age-old technique of mulching. Waggoner et al. (78) described microclimatic changes caused by mulches consisting of straw, paper, PE and alluminium films, and concluded that PE film was the most effective.
Only flexible PVC and PE films are suitable for mulching. Theoretically, PVC should prove better in cases where an increase in soil temperature is a decisive requirement. However, in practice, the difference in recorded temperature under PVC and PE mulch is not significant. Therefore, PE is generally preferred because of its lower price and greater permeability to long-wave infrared radiation which results in a greater amount of radiation flowing from the soil to the aerial parts of plants during the night (26).
Modes of Action and Use of Plastic Mulches
Moisture Conservation. - The high degree of impermeability of plastic films to water vapour prevents the evaporation of soil moisture. The water economy achieved by plastic mulching is substantial; all reserves are available for the plants and consequently, the nutrient supply is also more constant (50, 69, 78).
PE mulch is a definite aid in controlling the infiltration and movement of water through the root zone. Users of micro-irrigation benefit from the use of PE mulch since it reduces moisture evaporation from the mulched soil and decreases irrigation requirements (36). Cannington et al. (13) reported that drip irrigation combined with plastic mulch decreased water consumption of row crops from about 80 to 12 acre-inches.
The capacity to conserve water is shared almost equally by all types of plastics. The movement of water in the soil shows considerable variation associated with the temperature gradients which develop under different types of plastic. Transparent film, which has a relatively large warming effect on the soil, causes the evaporation of a large quantify of water which condenses on the soil surface side of the film. The top layer of the soil dries out to a very shallow depth and most important, the capillary rise of water is accelerated. Black plastic film reduces the variation in temperature and has much less effect on the movement of water in the soil than clear PE (26).
Utilization of Fertilizers. - Soils covered with plastic film mulches usually retain a higher level of soluble minerals (16, 44, 78). Constant moisture content, higher temperature and better aeration of the soil all tend to favour higher microbial populations in the soil thus ensuring more complete nitrification (5, 26, 35).
Plastic mulch prevents leaching of nutrients, particularly nitrogen. The dominant advantage of using PE is that it aids in the retention of nutrients within the root zone, thereby permitting more efficient nutrient utilization by the crop (13, 15, 36, 74).
Gaseous Exchange Between the Air and Soil
The application of plastic film that is almost impermeable to gas over the soil would undoubtedly modify the reciprocal gaseous exchange between air and soil. Carbon dioxide (CO2), given off by the roofs or decomposition of organic matter in the soil, accumulates beneath the plastic mulch (66, 70). It is then channelled through the perforations made at the lime of planting and becomes concentrated around each plant. This small increase of CO2 level around the stomata of foliage promotes growth by enhancing photosynthesis (22, 26).
Modifying the Physical Structure of the Soil. - Plastic mulch protects the soil from water and wind erosion and hail damage (26). Cannington et al. (13) reported that because of the extremely heavy rainfall in south Florida during certain times of the year, vegetables are grown on raised beds covered by plastic mulch to minimize the effect of flooding.
Temperature Modification and its Effect on Earliness and Yield. - Generally, most agricultural activities have been modified to fit within various soil temperature ranges. Complete temperature control, except in greenhouses, is not presently possible; however, the temperature effect can be modified in limited ways. Regardless of the chemical composition of plastic films, their form or their pigmentation, plastic mulched soil is always warmer, on the average, than bare soil (15, 39, 72, 78).
Many workers have shown that plastic mulch increases soil temperature and results in increased early and total yields of certain vegetables (12, 18, 21, 68, 69, 78). Soil temperatures arc generally higher under black PE as compared with non-mulched soils (12, 37). The use of black PE in areas where the production of vegetables is limited by early low temperatures, and throughout the growing season has been promising. A 50 percent yield in pineapples was reported in Hawaii when black PE mulch was used. This increase was attributed to an increase in soil temperature during the winter period and not to an increase of soil moisture (22). The use of clear plastic mulch in cold areas or seasons increases soil temperature and promotes germination and emergence of many crops. The use of clear PE plastic mulches in Iowa and Alaska allowed sweet corn 10 mature four to eight days earlier (62).
Improvement in Yield and Earliness of Crops with Row Covers. - To further modify the microclimate of crops, row covers are used in conjunction with plastic mulches. Conceptually, row covers are mini-greenhouses, with the primary function of modifying the growing environment. Environmental parameters modified by row covers are light, soil and air temperatures and humidity. The forerunners of row covers were bell jars and cloches from Europe and hot tents in the USA (82). The early PE row covers were demonstrated by Emmert in Kentucky (23) and Hall in California (33). A row cover is defined as a flexible transparent covering which is installed over single or multiple rows of vegetables for the purpose of enhancing plant growth and yield. Some are hoop-supported and are referred to as tunnels or row tunnels.
Others are so light in weight that they need no support and are called floating row covers (81, 82).
Since the introduction of slitted row covers in the 1980s a number of new types of row covers have become available for both commercial use and experimentation. These row covers are: slitted PE, perforated PE, unventilated PE, spunbonded polyester, spunbonded polypropylene, extruded polypropylene with 5 percent polyamide, porous PE, laminated non-woven polyvinyl alcohol and non-woven PE (80, 81, 82).
Recognizing a need for more intensive vegetable production in New England, row cover research was initiated on muskmelons at the University of New Hampshire in the early 1970s. Yield of melons with row covers plus black PE mulch was two to three times greater than with black PE mulch alone. Fruit maturity also was three to nine days earlier with row covers than on black plastic alone (53). These results coincide with the findings of many workers from the 1970s to the present who have shown increases in early maturity and yield of warm and cool season vegetables using row covers plus black or transparent plastic mulches (3, 7, 8, 9, 27, 32, 33, 38, 45, 52, 53, 54, 73, 75, 83, 84).
Plastic with Improved Heating Capacity
Increased Heating Retention in Plastic Films. - In greenhouses, row tunnels and row covers, if climatic and soil conditions (moisture saturation) are optimum, these climatic conditions constitute another version of solarization. Recently, several workers have used infrared trapping greenhouse films for intensifying heating during the cool winter season (19). There is now a considerable number of commercially available thermal barrier films with many reported advantages over unfilled or unmodified PK. Earlier and increased yield of flowers, ornamentals and vegetables was obtained with walk-in row tunnels and plastic houses using infrared film in comparison to standard PE film (65). Earliness was enhanced and tomato yields were greater under infrared film than clear PE film. Also air and soil temperatures were greater under infrared film than under clear PE film. Similar field results were obtained for peppers, watermelons, eggplants and strawberries in Israel using infrared film row covers (65).
Infrared PE, PVC and EVA films are examples of infrared trapping plastic film (65). The EVA and other infrared films act as a heat- trap which prevents the escape of infrared rays reflected from the warm ground during the night. The EVA films maintain 2-5° C above outside temperatures. With infrared PE film, day and night temperatures average 3.5°C and 6.5°C higher, respectively when outside temperatures are low (65).
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