Influence of different colour plastic mulches used for sol solarization on the effectiveness of soil heating

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A.W. Alkayssi and A.A. Alkaraghouli
Solar Energy Research Center, Baghdad, Iraq

Abstract

The effect of different coloured plastic polyethylene mulches (black, blue, green, yellow, red, transparent) on soil temperature was studied. The highest soil temperature was recorded under the red mulch followed by transparent, green, blue, yellow and black mulches. The soil heat flux under each mulch was found to be closely related to the surface energy balance. Thus it decreased with decreasing energy balance in the order of red > transparent > green > blue > yellow > black. The heating coefficient was derived by using the linear regression equation correlating net total and net short wave radiation. It was consistently negative and their values varied between -0.27 and -0.05.

Keywords

Solar energy; soil healing; plastic mulches; solarization.

Introduction

Soil temperature is an important edaphic factor that can influence plant biological activity. It could be also used as a lethal agent for the control of soilborne pathogens and weeds by soil solarization (1-8). The method has been described in detail by Katan et al. (9). Much attention has been focused on evaluating the effect of transparent, black and white polyethylene mulches on soil temperature, while few studies are concerned with the effect of other colours of polyethylene mulches. In a previous study, Al-Karaghouli et al. (10) evaluated the photometric properties (absorptance, transmittance and reflectance in the ultraviolet-visible and infrared regions of the radiation spectrum) of black, blue, yellow, green, red and transparent polyethylene mulches. These measurements which were carried out both in the field (with pyranometers) and in the laboratory (with spectrophotometers), showed that the photometric properties varied significantly as the colour of the polyethylene mulch changed. It was found that transmittance of the coloured polyethylene mulches to global solar radiation was arranged in the order of: c blue < yellow < green < red < transparent. Moreover, red and transparent polyethylene mulches have an even transmittance to global solar radiation and to infrared radiation. Whereas the absorptance to global solar radiation of the red polyethylene mulch was more than the transparent one.

The goal of the present study was to evaluate the influence of different coloured polyethylene mulches on soil temperature during the solarization period.

Materials and Methods

Site and Climate.- The experiment was carried out at the Agrometeorological Research Station of the Solar Energy Research Center during the period from 1 June to 30 September 1987. The station is located 35 km east of Baghdad at 33°14'N latitude, 44°14'E longitude and at elevation of 34 m above mean sea level. The climate of the study area is semi-arid, sub-tropical with a very low rain fall. The soil of the experiment site is silty clay loam, composed of 14 percent sand, 58 percent silt and 28 percent clay.

Soil mulching.- Low density polyethylene sheets 100 m m thick containing five types of pigments with 1 percent concentration were used in this study. These sheets were red, yellow, green, blue and black. Transparent polyethylene (PE) of 180 ?m thickness and without pigments was used as control. Each treatment was replicated three times. The effective area of each plot was 1 m², and was covered on the outside with thermal insulating material to a depth of 1 m. The soil was welled to a depth of about 1 m, and no additional irrigations were given after mulching.

Measurements and instrumentation.- The total quantities were continuously monitored:
3a - Net total radiation (0.4-60 m m).
3b - Incident and reflected solar hemispherical radiation (0.3-3 m).
3c- Soil temperature at the depths 0.00, 0.05, 0.10, and 0.30 m beneath each mulch.

Net total radiation was measured with a miniature net radio-meter. The instrument was mounted horizontally with its sensing surface 0.05 m above the ground surface, while the mulch was maintained with a wooden frame at a height of approximately 0.01 m. Measurements of the incident and reflected shortwave radiation were made with a set of black-and-white Epply Pyranometer. For the latter measurement, the instrument was converted downward and held level to face the reflected surface. Both instruments were mounted horizontally with their sensing surface at the same height as the net radiometer. Soil temperature was monitored by means of a shielded copper-constantant hermocouples at the centre of each plot; thus, no edge "effect" was expected (Mahrer and Katan, 11). The analogue signals from the sensors were converted into digital signals for the Hewlett-Packard automatic data acquisition control system model 3054A. The output data were printed continuously (24 hours) on an hourly basis using a Hewlett-Packard model 9845B computer connected on-line with the data acquistion system.

Bulk density as well as gravimetric soil moisture content were measured at the depth of interest during the experimental period.

Calculation procedures.- Since the soil heat flux was not directly measured, calculations were made using the temperature integral-method, where the soil heat flux, G. was computed from the change in heat storage in the soil profile according to:

G = (Q₂ - Q₁)/(t₂ - t₁) ---------------(1)

The heat storage, Q. was computed at given time, t, as:

z₂
Q = z₁ C(z) T(z)dz --------------(2)

where the volumetric heat capacity (c) and the temperature (T) are both functions of depth z. Using equation 3, C(z) can be computed from measurements of bulk density and moisture content at the interested depths (De Vries, 12):

C(z) = 1.92 X_m(z)+2.51 X_o(Z) + 4.18 X_w(z) ---(3)

where X_m, X_o, X_w are the volume fractions of minerals, organic matter and water, respectively, for each depth z.

Results and Discussion

The maximum and minimum soil temperatures under the different coloured mulches for a representative day are shown in Figure 1.

As it is shown in the figure the differences in maximum and minimum temperatures for the various mulches are evident especially at depths 0.00 m and 0.05 m. Corresponding differences for the depth 0.01 m and 0.3 m are relatively smaller. The red mulch has the highest temperatures followed by transparent, green, blue, yellow and black mulches. This result is achieved due to the amount of heat flux transmitted through each mulch.

Heat flux is one of the components of the earth energy balance. In order to evaluate the amount of energy transferred in the soil for each mulch, a relationship between weekly average values of total net radiation Rn (at wavelength 0.4-60 m m), measured under each mulch and the soil heat flux (0.00-0.05 m) is presented in Table 1 and illustrated in Figure 2.

As the figure shows, the soil heat flux is closely related to the amount of radiation transmitted through the mulches. The best-fit line to the data points was obtained by the Hewlett-Packard linear regression analysis package. High values and even distribution of soil heat flux were found under red and transparent mulches. The soil heat flux under the rest of the mulches was strongly skewed toward lower values in the order of green > blue > yellow > black.

The relationship between the resultant energy (EG+) for each mulch, which is calculated by integrating the positive heat fluxes and the rise in soil temperature T for the depths 0.05, 0.10 and 0.30 m, is plotted in Figure 3. Each point represents the weekly mean values throughout the experiment period. A quick look at the figure reveals that there is a linear relationship between T and KG+. The slope of the line was projected to decrease at deeper depths, due to the fact that the amplitude of temperature variation will be less at deeper depth. The higher temperature prevailed under the mulch could play an important role in a [aster killing of various soilborne plant pathogens as compared with the rest of the PE mulches. Many researchers emphasized that the pathogens death lime tends to decrease in a logarithmic scale with the temperature increase during soil solarization under field conditions (2, 14, 15).

The heating coefficient (ß) is defined as the rate of change in longwave radiation loss (R_n) for a unit change in net radiation (R_s). It was computed from the slope of the regression line (a) of Rn plotted against Rs for the data of Figure 4. (ß = (1-a)/a). A good linear regression equation was deduced for the mulches. Generally all the regressions had correlation coefficients confined between 0.96 and 0.99, and pertinent results are summarized in Table 2. The values of the heating coefficient showed a good agreement with the soil temperature data observed under each mulch as it could be noticed from Figures 1 and 2.

Conclusion

From the previous discussion, it is apparent that the photometric properties of the plastic soil mulch is an important applicable parameter which affects the soil temperature during soil solarization. The values of the temperature observed under the various mulches were in the following order: red > transparent > green > blue > yellow > black. It could be seen from the above results that the transparent mulch which is widely used by farmers in agriculture applications provides a better condition than the other mulches except the red one. Therefore according to the results, the red mulch is the most appropriate for soil solarization to control diseases caused by soilborne pathogens.

References

1. Katan, J. 1981. Solar heating (solarization) of soil control of soilborne pests. Ann. Rev. Phytopath. 19: 211-236.

2. Pullman, G.S., J.E. DeVay and R.H. Garber. 1981. Soil solarization and thermal death: A logarithmic relationship between time and temperature for four soilborne plant pathogens. Phytopathology 71: 959-964.

3. Stapleton, J.J. and J.E. DeVay. 1982. Effect of soil solarization on populations of selected soilborne micro-organisms and growth of deciduous fruit tree seedlings. Phytopathology 72: 323-326.

4. Stapleton, J.J. and J.E. DeVay. 1984. Thermal components of soil solarization as related to changes in soil and root microflora and increased plant growth response. Phytopathology 74: 255-259.

5. Porter, I.J. and P.R. Merriman. 1985. Evaluation of soil solarization for control of root diseases of row crops in Victoria. Plant Pathology 34: 108-118.

6. Jacobsohn, R., A. Greenberger, J. Katan, M. Levi and H. Alon. 1980. Control of Egyptian broomrape (Orobanche aegyptiaca) and other weeds by means of solar heating of the soil by polyethylene mulching. Weed Science 28: 312-316.

7. Horowitz, M., Y. Regev and G. Herzlinger. 1983. Solarization for weed control. Weed Science 31: 17()-79.

8. Standifer, L.C., P.W. Wilson and R.P. Sorbet. 1984. Effects of solarization on soil weed seed populations. Weed Science 32: 569-573.

9. Katan, J., A. Greenberger, H. Alon and A. Grinstein. 1976. Solar heating by polyethylene mulching for the control of diseases caused by soil-borne pathogens. Phytopathology 66:683-388.

10. Al-Karaghouli, A., A.W. Al-Kayssi and A.M. Hasson. 1990. The photometric properties of different colored plastic mulches used for soil solarization. Solar and Wind Technology (In Press).

11. Mahrer, Y. and J. Katan. 1981. Spatial soil temperatures regime under transparent polyethylene mulch - numerical and experimental studies. Soil Sci. 131: 82-87.

12. De Vries, D.A. 1963. Thermal properties of soils. pp. 210-233. In: W.R. Van Wijk (ed.) Physics of Plant Environment, North Holland, Amsterdam.

13. Goheen, A.C. and J.T. McGrew. 1954. Control of endoparasitic root nematodes in strawberry propagation stocks by hot-water treatment. Plant Dis. Reptr. 38: 818-825.

14. Bigelow, W.D. 1921. The logarithmic nature of thermal death time curves. J. Infect. Dis. 28: 528-536.

15. Fernow, K.H., I.C. Peterson and R.L. Plaisted. 1962. Thermotherapy of potato leafroll. Am. Potato J. 39: 445-451.

Table 1. Results of linear regression analysis of net total radiation and soil heat flux data (wm-2) under PE mulches for G+ = abR_n.

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Table 2. Results of linear regression analysis of radiation data (wm^-2) for Rn = a+bR_s.

Figure 1. Maximum (A) and minimum (B) soil temperature variations for different soil depths.

Figure 2. Relationships between weekly average values of net radiation and soil heat flux al the soil surfaces.

Figure 3. Relationship between the energy at the soil surface (E_G+) and the rise in soil temperature (WT) for various depths.

Figure 4. Relationship between the rate of change in longwave radiation loss (R_n) and a unit change in net radiation (R_s).

Improvement in plastic technology for soil heating

Franco Lamberti and Martino Basile
Istituto di Nematologia Agraria, C.N.R., 70126 Bari, Italy

In the 1950s, plastic materials previously widely used in a variety of industries and by the general packaging of goods began to be used in agriculture, particularly for soil solarization. As this latter use increased, manufacturers gave greater attention to the production of plastic films that suited the specific agricultural needs with such characteristics as mechanical resistance, slow chemical degradation, photoselectivity, and transmission of light.

One of the most important characteristics relates to the capacity of transmission of short and medium-long infrared radiation because the permeability to the former and the impermeability to the latter limits the loss of heat. There are now available various infrared plastic materials which can he used as a barrier to heat (5).

Main physical processes involved in soil solarization. - Solar light (white) has a spectrum which passes from violet to blue-green, green, yellow, orange, red to deep red. In the spectrum the colours are separated because they represent different wave lengths gradually increasing from violet to red. The spectrum visible to the eye is from 400 m m (violet) to 700 m m (deep red). Beyond these two extremes there are invisible radiations in the ultra violet (UV) and infrared (JR) bands. As the wave length increases the radiations change from thermic to radio waves.

The passage of light through a semi-transparent body depends on the characteristics of the material that constitutes it, and on the angle of incidence of the ray: the angle formed by the luminous ray and the perpendicular to the surface of the point of penetration (Figure 1).

The law that governs the passage of a non-polarized light ray from a medium with a refraction index of N₁ to a medium with a refraction index of N₂ is the following:

where q ₁ is the angle of incidence of the ray and q ₂ the angle of refraction.

Reflexivity, r , the ratio between the radiation reflected from a surface and that normally incident to it, is expressed by Fresnel's law:

Transmissibility, t , the ratio between the radiation that goes through a material (R_s) and that normally incident to its surface (R_o), follows Bouger's law:

where m is the attenuation coefficient dependent upon the material and S is its thickness.

In the case of soil solarization, the plastic material that covers the soil has a certain thickness and two interfaces which cause two successive refractions of the light luminous ray and thus:

total transmittance

total absorbance

total reflectance

These formulae show that transmittance, absorbance and reflectance of the tarps (plastic covers) transparent to the solar energy vary as the sun position varies during its movement around the earth.

The trend of the variation of transmittance, as the incidence angle of the solar radiation varies; experimentally calculated (3), indicates that its reduction is negligible (around 5 percent) within an incidence angle from 0° to about 40°, but then with a rapid reduction of 30 percent at an incidence angle of 70° (Figure 2).

When both direct and diffuse solar radiations strike the plastic tarp on the soil, they are in part reflected back toward the atmosphere, in part adsorbed by the plastic material, and in part penetrate the underlying soil. The material used for tarping is therefore the main clement in the phase of capturing and storing the solar energy during the day, but it also acts as a barrier to dispersion of the heat energy during the night when the flux of the solar energy has ceased (Figure 3). During the night the soil is the only source of heat energy and compensates for the flux of heat (flux that in this second phase goes from soil to atmosphere) which is dispersed by irradiation after a temporary phase when the presence of other phenomena, e.g. the latent heat of vaporization, contribute to balance this loss.

The transfer of energy from the soil depends on its internal conductivity which is a function of the soil type and its humidity content.

The modalities of toss of energy are:

1) Thermic conduction which depends on the coefficient of thermic transmission (l ) of the tarping plastic material on its thickness:
where m = thickness

2) Thermic convection which consists in the exchange of heat between the soil, the surfaces of the tarping material and the atmosphere. These losses are affected by wind and its spread.

3) Irradiation which is the loss of the medium-long infrared radiation (irradiated energy) through the surface of the plastic tarp.

Irradiation is the most important form of loss and is linked to the greenhouse effect and to the thickness of the plastic tarping material.

Solar radiation, which is transmitted through the tarp to the soil is partially rejected by the soil within the wave length of the medium-long IR and is called calorific radiation. The balance between the heat developed from the soil irradiation and that lost because of the permeability to heat of the tarp is called the greenhouse effect.

The soil acts as a black body releasing radiations between 4 and 30 m m with a maximum of 10 m m and with 90 percent of them between 5 and 14 m m A black body presents the maximum of emission of temperatures around 28 K and wave length of 11 000 nm. Therefore for a tarp material to have a high greenhouse effect it must possess a good transparence to the luminous radiation (wave length within the visible and short IR) and a good opacity to the IR radiation released by the soil (medium-long IR, calorific radiations).

Thermic properties of the soil. - The soil temperature in field conditions is never constant, the most important source of variation being the thermic energy caused by the absorption to the soil surface of the radiant energy of sun. Soil colour is important in the capture of the radiant energy; the lighter the colour and the drier the soil, the more it reflects. Maximum absorption occurs with a black, moist but not saturated, soil. The amplitude of the variations of the temperature along the soil profile is maximum at the surface and decreases rapidly in depth (Figure 4). The daily fluctuation is reduced by fractions of a degree already at a few dm deep (2).

The spread of heat in the soil towards layers at lower temperature may occur by thermic conduction (transmission of kinetic energy between molecules as a result of the thermic turbulence), by convection (movement of fluid masses containing thermic energy), and by irradiation (irradiation from the body surface of low frequence electromagnetic waves which may be captured by the surface of other bodies). However, it may be generally postulated that conduction is most important.

Optical and thermic characteristics of plastic films available for soil solarization. - Descriptions of the characteristics of the available plastic films help to illustrate the advantages and disadvantages of the materials which can be used for soil solarization. They are: polyethylene (PE), polyvinyl chloride (PVC), copolymer of ethylenevinyl acetate (EVA), low density polyethylene (LDPE) and coextrose with different percentage of vinylacetate (LOPE/EVA). Some of these can be linked in the enhancing substances e.g. LDPE + inorganic silicate salts, LDPE + inorganic phosphate salts, LDPE + silicates + EVA, EVA + silico-alluminates, LDPE + inorganic silico-aluminate salts.

PE first appeared in 1978 and was promptly adopted in agriculture because of some of its important characteristics such as mechanical resistance, durability at high temperatures, filmability and the possibility of reducing its thickness, but also for various practical and industrial reasons. It is cheaper than PVC, not so much for its production costs but more for greater convenience in the process of transformation.

Based on the chemical and physical structure linked to the technology of polymerization and to the asset of the implement, there arc three different types of PE: linear molecules with very rare short ramifications (HDPE), linear molecules with short ramifications (LLDPE) and molecules with long and short ramifications (LDPE).

LDPE has a limited greenhouse effect which can however be enhanced (Table 1) with the addition of inorganic substances or through manufacturing processes such as co-extrusion (4, 10) which results in the production of bio-o-polyextruded films consisting of two or more layers of compatible and complementary materials. PVC is the plastic material which possesses the best characters with regard to transparence to luminous rays and thermic capacity. Its consumption is paramount in Japan where it constitutes, about 65 percent of the plastic material used, with 25 percent LDPE and 10 percent EVA. Conversely in Italy the most used is LDPE, 90 percent, compared to 7 percent of EVA and 3 percent of PVC. According to Pacini (8), this difference is due to a more rational use of PCV in Japan of thinner (0.06-0.12 mm) and smaller sheet sizes (0.50-2.204.20 m). PVC is of particular value because of its high transparence to the visible and short IR (>=90 percent) and the transmittance to the long IR (=< 15 percent) as shown in Table 2. However, because of the complexity of its formulation, PVC must be manufactured utilizing prime materials of high qualify transformed according to very rigorous processes to obtain a product responding to the required characteristics (1). EVA films, obtained from ethylene by copolymerization with acetate vinyl, have characteristics similar to PVC with 89 percent transparence to the visible spectrum starting and less than 25 percent transmittance to the long IR; they therefore present high thermic capacity. According to Zanon (12) the thermic films (PVC and copolymers EVA) are more interesting in comparison to LDPE. The choice of the optimal characteristics of thermic films aim to have a transmittance of >= 80 percent in the visible spectrum and =< 25 percent in the long IR. The present values of transmittance are around 90 percent for the visible spectrum and up to 10 - 15 percent (it differs according to the film) for the long IR.

Finally, one should consider also the possibility of adopting for soil solarization after careful evaluation, the following materials:

a) films treated on the surface with hydrophilic products which allow the humidity to condense in a thin continuous film of water to enhance the thermic effect without reducing the luminous transmittance;

b) plastic films that are biodegradable by soil micro-organisms, constituted by a particular mixture of cellulose with natural collants, which thus avoid the problems of their disposal.

In general the criteria to adopt in the selection of tarping material for soil solarization consist of the maximum penetrability to solar radiation and the greenhouse effect, or the opacity to the medium-long (l >2 000 nm) IR radiations which enable the film to retain the heat during the coldest period of the day.

To pretect quality of plastics for agricultural use the Istituto Italiano dei Plastici (IIP) has introduced the "marchio di qualita", (Figure 5) which ensures that the following characteristics are incorporated:

i) LDPE films: total luminous transmittance (T) 90 percent for the films between 0.05 and 0.15 mm thick and 88 percent for those of 0.20 mm thickness, barrier effect to the medium-long IR radiations (greenhouse effect) (7 420 - 12.500 m m of 24 percent for the films 0.10 mm thick and 38 percent for those of 0.20 mm thickness;

ii) PVC Films: T = 90 percent for any thickness; greenhouse effect 91 percent for 0.10 mm and 94 percent for 0.20 mm thick films, respectively;

iii) EVA (10-18 percent V.A) films: T = 91 percent for those 0.05-0.010 mm thick and 89 percent for those 0.15-0.18 mm thick, greenhouse effect 65 percent for the first and 73 percent for the second ones.

Thermic transmittance (K_t) is defined as the sum of the heat losses which occur for thermic conduction, thermic convection and irradiation. Since losses by irradiation are the most important and depend on the greenhouse effect of the tarp material and its thickness, the only system to increase the resistance to the dispersion of heat is the reduction of K_t by creating an air interspace (double film) between IWO layers. But this will reduce light transparence (11). For example if we consider the case of IWO films with T = 80 percent the total transmittance will become

80 X 80
------------ = 64 %
100

Total transmittance (K_t=W/m²°C) and total light transmission values experimentally calculated, under different climatic conditions for plastic materials widely used in agriculture, are reported in Table 3. They indicate that the single layered PVC films are better than those of PE and EVA which are in an intermediate position. Double films with or without interspace cause a reduction of K_t which reflects the decrease of light transmission. The combinations are PVC (0.08 mm) internally and PVC (0.20 mm) externally or PE (0.20 mm) externally and either PVC or EVA (0.08 mm) internally with values pf K_t of 3-3.54.5 and T 80-79-79 percent. However, it mut be considered that K_t is related to the wind speed and that a speed variation from 0 to 6 m/see will double the K_t value (9).

Conclusion

In conclusion, a plastic film for soil solarization must possess a high greenhouse effect. This effect is much higher as the transparency of the film increases to the visible solar rays and short IR and more opaque to the calorific radiations (long IR). Comparative curves of the greenhouse effect of various plastic films are reported in Figure 6.

The films that are defined thermic and available on the market have a transmittance below 35 percent with regard to the long IR and higher than 82 percent for the visible and short IR (Figure 7). In PVC films transmittance to long IR is less than 15 percent and to short IR more than 92 percent. EVA films also have a transmittance to the long IR of less than 25 percent and to the short IR of about 89 percent. However, its greenhouse effect depends on the percentage of vinyl acetate (Figure 8) which varies between 12 and 14 percent in the films available on the market. The thickness of the film is also of importance (Figure 9).

Other films with high greenhouse effect are the LOPE/EVA. Comparing two films of the same thickness, one single layered EVA and the other LDPE (50 percent) / EVA (50 percent) the second shows an inferior greenhouse effect because the concentration of vinyl acetate is reduced by 50 percent (4). This reduction can be recovered either by additivation or by reducing the thickness of LDPE in favour of EVA. The transmittance of the visible and IR radiations for such films are shown in Figures 10 and 11, respectively (6, 7).

References

1. Boldrin, L. 1985. Film termici nelle coperture delle serre. Colture Protette 11:59-62.

2. Cavazza, L. 1981. Le proprieta termiche del terreno agrario. In: Cavazza, L. (Editore), Fisica del terreno agrario. Publ. UTET Torino, pp. 511-541.

3. Dal Sasso, P. and C. Manera. 1985. Misure spettroradiometriche su alcuni material) di coperture per serre. Colture Protette 2:3743.

4. Fabbri, A. 1984. Film in LDPE ad effetto serr. Colture Protette, 12:39-42.

5. Garnaud, J.C. 1986. Lo sviluppo delle materie plastiche: un fenomeno mondiale. Colture Protette 5:35-38.

6. Magnani, G. 1 987. Film "termici" di recente formulazione: risultati agronomici di due anni di ricerche. Seleplast 3:89-110.

7. Magnani, G. and E. Nesti. 1988. Effetti delle cariche minerali caratteristiche e sul comportamento agronomico di film LDPE et EVA. Colture Protette 2:77-84.

8. Pacini, L. 1987. 11 "progetto di norma 482" per il miglioramento delle corture protette. Seleplast 3:114-119.

9. Pacini, L. 1988. La norma UNI 9298 per il miglioramento delle colture protette. Colture Protette 4:51-56.

10. Raviv, M. and Y. Allingham. 1983. Films de polyéthylène modifié caractéristiques. Plasticulture 59:3-12.

11. Zanon, D. 1987. Conoscere le materie plastiche ed il loro utilizzo: moralizzazione del mercato. Seleplast 3:120-121.

12. Zanon, D. 1988. Film plastic): progress) e sviluppi. Colture Protette 4:4345.

Table 1. Values of the percentage of IR radiation absorbed from 7.5 to 12.5 m (thermic effect) and of film transmittance in the visible solar radiation range (Magnani, G. and E. Nesti, 1988)

Table 2. Comparison among optical properties of LDPE, PVC, and EVA films (Pacini, L. 1988)

Table 3. Total thermic transmittance as W/m²° C. (Pacini, 1988)

Figure 1. Transmission and reflection of a sun ray through S-thick and n₂ reflection indice material.

Figure 2. Transmittance variation in the wavelength range 400 - 1.300 nm versus incidence angle of solar radiation on the flat surface of a film (Dal Sasso P. and Manera C., 1985).

Figure 3. Capture of solar energy (day lime) and of infrared radiation (night time) omitted from the soil.

Figure 4. Day time soil temperature variation at different soil depths (Cavazza, 1981).

Figure 5. Trade mark adopted by the Istituto Italiano dei Plastici.

Figure 6. Greenhouse effect curves of three materials according to their thickness (Boldrin L., 1985).

Figure 7. Transmittance curve of a thermic film in the visible and mid-long IR radiations (Boldrin L., 1985).

Figure 8. Variation of the greenhouse effect in relation to vinyl acetate content of EVA films (Boldrin L., 1985).

Figure 9. Greenhouse effect between 7.5 - 12.5 ? radiations, of films having different VA content, according to film thickness (Pacini L., 1988).

Figure 10. Solar radiation transmittance in the visible wavelength range of different film types during a two-year investigation (Magnani G., 1987).

Figure 11. IR radiation transmittance for different films, compared during the two-year trials.

Figure 11. IR radiation transmittance for different films, compared during the two-year trials (cont'd).

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