Environmental Engineering Reference
In-Depth Information
and is a good thermal insulator, serving the role particularly well (Liu et al.,
2009). The polyethylene (LDPE) and PVC films keep the soil temperature
high during cold season and reduce the evaporation of soil water (Li et al.,
2013) allowing economical drip irrigation to be used. The retention of soil
moisture and the buildup of relatively high concentrations of CO
2
under
impermeable plastic mulch films result in a microclimate conducive to crop
growth (Bonachela et al., 2012). Not only do plastic mulch films produce
crops earlier, they can also be optimally colored (Decoteau et al., 1998) to
reflectorscatterappropriate wavelengths oflighttomatchneedsofthecrop
to ensure higher yield of produce.
However, the capital cost of plastic mulch is generally higher than that for
conventional natural mulch, and it also has to be removed from the field
annually and either reused or replaced in the next growing season. This
labor-intensive and expensive practice is avoided by using environmentally
photodegradable mulchfilm; thesefilms leftonthefieldafter theseason are
broken down by photodegradation (see
Chapter 6
) into small fragments and
are mixed in with the soil. The plastic fragments generated are too small to
be discernible against the soil background.
Commercial use of greenhouse technology increases crop yield by providing
anextendedgrowingseasonaswellbyallowingclosercontrolofthegrowing
environment. Vegetables grown in greenhouse environments required less
irrigation compared to conventional vegetable cultivation and provide a
higher yield (Chang et al., 2011).
Rigid plastics such as polycarbonate (PC), fiber-reinforced polyester (GRP),
impact-modified acrylics, as well as flexible films such as polyethylene (PE),
PVC, and sometimes PET are used as glazing material in commercial
greenhouses (Giacomelli, 1999; Govind et al., 1987). Glass by comparison
is fragile, expensive to install, and heavy and may not withstand the weight
of accumulated snow/ice. Even though plastics have a shorter service life
compared to glass and the 4-6 ml flexible films are easily prone to tears,
they are still a popular and economical choice of greenhouse material. A
comparison of the optical and thermal performance of these glazing
materials with glass is given in
Table 5.5
. The transmittance of
plastics is about the same, but rigid plastics transmit less of UV radiation
(UV-B radiation that is considered detrimental to plant growth), while
flexible plastics transmit approximately 50% of the IR radiation as well.
