Weeds and the soil environment - Ecological Management of Agricultural Weeds - page 212

Temperature management

Seed germination and plant growth occur only within a range of tem-
peratures, and often are retarded by temperature reductions within the lower
end of this range. On a field scale, the most practical means of reducing soil
temperature involves the retention of crop residue on the soil surface. The
consequences for weeds and crops of using residue to lower soil temperature
are discussed later in this chapter.
Elevation of soil temperature can be used to kill weeds,and may be particu-
larly useful for attacking seeds and other structures that resist control by other
means. A common example in temperate regions is post-harvest burning of
cereal straw and stubble, which can raise soil surface temperature above
200 C and kill or reduce the germination of weed seeds there and within

plant debris (Young, Ogg & Dotray, 1990; Giovanninni et al ., 1993).In tropical
slash-and-burn systems, burning vegetation can raise temperatures above
200

°

C at 1 cm depth, which lowers total
seed density, though it favors the emergence of heat-stimulated species (Ewel
et al ., 1981; Uhl et al ., 1981). These effects not withstanding, the resulting air
pollution, degradation of soil organic matter, and potential for creating wild-
fires argue against broadscale burning as a weed management practice.

° C at the soil surface and above 100 °

Solarization
Soil solarization (also called solar heating, plastic mulching, or soil
tarping) is an approach for thermal weed suppression that is suited to regions
with seasonally high temperatures and intense sunlight. Under such condi-
tions, soil is covered for several weeks with a polyethylene sheet that traps
solar energy and raises soil temperature substantially above ambient levels
(Stapleton & DeVay, 1986; Katan, 1987; Bell, Elmore & Durazo, 1988).Because
crops as well as weeds are susceptible to heat stress, solarization is performed
before crops are sown. Irrigation is generally used to wet the soil before or
shortly after polyethylene tarps are laid because (i) weed seeds and perennat-
ing structures are physiologically less tolerant of high temperatures under
moist,rather than dry conditions; (ii) moist soil conducts heat more effectively
than dry soil; and (iii) adequate moisture promotes biological activity in the
soil that may increase weed mortality (Horowitz, Regev & Herzlinger, 1983;
Katan, 1987; Elmore, Roncoroni & Giraud, 1993).To maximize heating, poly-
ethylene is laid close to the soil surface and sealed with soil at the edges
(Stapleton & DeVay, 1986). Transparent mulch is superior to black mulch for
raising soil temperature and reducing weed populations (Horowitz, Regev &
Herzlinger, 1983; Standifer,Wilson & Porche-Sorbet, 1984).