Weeds and the soil environment - Ecological Management of Agricultural Weeds - page 214

Figure 5.1 Effects ofsoil solarization on weed seedling emergence from natural
seed populations in a field experiment conducted in Mississippi,USA.Plots were
covered with transparent polyethylene tarps for 1 to 4 weeks; an untarped control
treatment (0 weeks ofsolarization) was included in the experiment.(After Egley,
1983.)
Second, weed seedling emergence decreases as the solarization period
increases in duration. Egley (1983) observed that relative to an unsolarized
treatment, emergence of annual grass species was reduced by 64%, 95%, 98%,
and 99% when soil was covered with polyethylene for 1, 2, 3, and 4 weeks,
respectively (Figure 5.1a). Emergence of other annual species — Amaranthus
spp., Ipomoea spp.,and Trianthema portulacastrum — was also reduced strongly as
the length of solarization increased (Figure 5.1a, b). However, none of the
solarization treatments significantly affected emergence of the perennial
sedge Cyperus rotundus (Figure 5.1b).
The latter result illustrates a third aspect of solarization: weed species can
differ greatly in their susceptibility to the technique. Annual weeds tend to be
more susceptible to solar heating than perennial species (Horowitz, Regev &
Herzlinger, 1983; Kumar et al ., 1993), probably because perennials can arise
from roots and rhizomes that are buried too deeply to be affected by surface
heating. Similarly, large-seeded annual species that can emerge from greater
soil depths generally suffer less than small-seeded annual species that can