II. Factors Affecting Effects of Solarization

              The theory behind solarization is relatively simple: heat the soil to a level that is insupportable to soilborne pathogens.  However, the physical principles of this technique require more explanation.

          A.  Length of Time

The soil must be mulched for an adequate length of time.  What is “adequate?”  The soil should be mulched for at least four weeks; however, a longer duration will improve the effects.  Extra weeks should be added to compensate for loss of radiation time in the event of extended cloud cover.  In addition, a longer mulching time will cause temperature rise in deeper layers of soil.  Summer is the best time for solarization because the highest temperatures and intensities of solar radiation are likely to occur during this time.

          B.  Moist Soil Heats More Effeciently

Solarization is most effective on wet soil that is free of debris.  Water is a good absorber of infrared and ultraviolet radiation (properties of water).  This is because water has a high specific heat capacity, which allows it to contain a great amount of thermal energy.  Also, heat (latent heat of vaporization) is released as water vaporizes.  Due to these properties of water solarization is more effective on moist soil than dry soil.  Soil moisture has also been reported to increase the thermal sensitivity of pathogens (McGovern and McSorley, 1997).  Debris on the soil surface and/or a bumpy site causes the presence of air pockets that slows down solarization heating.  This is because heat moves faster through water than air.  Good contact between the mulch and the soil and an absence of air pockets is essential for thorough and effective solarization. 

          C.  Mulch Selection is Very Important

1.       Transparency is most effective!  Clear, transparent mulch is essential for solarization.  Performance drops as light transmission through the plastic decreases.  Opaque plastics (black, gray, or white) are not suitable for solarization.  Soil temperature will be raised under black plastic mulch, but in most comparisons between black and transparent mulch, transparent plastic was found to be more effective in suppressing soilborne pathogens.  However, black mulch was superior in promoting tree growth when used in a postplant situation (Stapleton et al., 1993).  Thinner is better.  Thin mulch (25 or 50 mm as opposed to 75, 100, or 150 mm) allows more transmission of solar radiation and is therefore more effective in heating the soil (Arun and Matthew, 1993; Meti and Hosman, 1994; Pullman et al., 1981).

2.     A double layer is more effective than a single layer.  Although a thinner layer of mulch is better for solar transmission, a double layer (each layer with a thickness of 40 to 50 mm) is more effective for pathogen destruction and increasing plant biomass.  The apparent inconsistency can be explained by the presence of inter-layer air spaces.  The air is heated during the solarization process and adds to the heat of the soil.  In addition, the air layer acts as an insulator and further prevents heat loss into the atmosphere.  So, using two layers of thin mulch, with an air space between, shows the greatest potential for reduction in soilborne pathogen populations.  Pathogens affected by double mulch layers include: Sclerotinia sclerotiorum, Rhizoctonia solani, Fusarium oxysporum f. sp. lycopersici, and Erwinia spp. (Scannavini et al., 1993; Garibaldi and Tamietti, 1989; El-Shami et al., 1990; AVRDC, 1990).  Some mulches are currently produced with trapped air bubbles, which act similarly to double-layered mulch operations (Berninger et al., 1985).

3.     Mulch material should be considered.  Mulch used for solarization should allow transmission of ultraviolet (UV), short-infrared radiation, and visible light, and should prevent the escape of medium- to long-wavelength infrared radiation.  As shorter-wavelength radiation passes through the mulch, it is transformed into lower-energy, longer-wavelength radiation.  Retention of this energy is critical for soil heating.  Mulch materials that have most commonly been tested in solarization studies are low-density polyethylene (PE), a copolymer of ethyl acetate (EVA), and polyvinyl chloride (PVC).  Polyethylene mulch is most commonly used because of its lower cost, lower minimum thickness, and greater tensile strength.  However, PVC and EVA allow greater radiation transmission and less heat loss.

4.     Stabilizers and additives may also be used.  As water condenses on the undersurface of the mulch, droplets are formed and adhere to the material.  These droplets hinder the transmission of solar energy.  However, the presence of water is desirable since it retains a great deal of heat.  Addition of certain additives to the mulch material may provide a compromise.  These compounds are hydrophilic and cause individual droplets to merge, forming a continuous film of water.  A film, as opposed to droplets, allows radiation transmission while maintaining the insulatory characteristics of water (Jaffrin and Makhlouf, 1990; Lamberti and Basile, 1991).  “Stabilizers” may be added to prevent UV damage to the plastic, and thereby increase the useful life of the material (Bell and Laemmlen, 1991; Brown et al., 1991).  

D.  Methods of Application

      Solarization of narrow strips (row solarization) is less effective in pathogen suppression than are whole-field techniques.  This is because of the cooling “border effects” of the soil outside the treated area (Antoniu et al., 1995; Asworth and Gaona, 1982; Grinstein and Hetzroni, 1991; Grinstein et al., 1995; Hetzroni et al., 1983; Jacobsohn et al., 1980; Mahrer and Katan, 1981).  Also, row solarization increases the chances of treated soil to be reinfested by nontreated border soil (Katan et al., 1976; Katan et al., 1980).  Nevertheless, even row solarization suppresses some pathogens such as Pyrenochaeta terrestris, Rhizoctonia solani, Fusarium spp., Phytophthora cactorum, Verticillium dahliae, and suppresses nematode populations of Meloidogyne javanica, Hirschmanniella mucronata, Helicotylenchus sp., and Rotylenchulus reniformis (Abu-Gharbieh et al., 1991; Hartz et al., 1985; Hartz et al., 1993; Hartz and Bogle, 1989; Heald and Robinson, 1987; Katan et al., 1980; Keinath, 1995; McGovern and Harper, 1996; McSorley and Parrado, 1986; Sivakumar and Marimuthu, 1987).  McGovern and McSorley (1997) suggest using row solarization for short-term crops or crops grown on raised beds in situations where pooling of water on flat solarization tarps could pose a problem.  

 

 

 

 


                                                 To Applications

 

                                                  

                                                   To References

 

                       

                                                   

 

                                                     Back to Homepage