Weeds and the soil environment - Ecological Management of Agricultural Weeds - page 213

Solarization with transparent polyethylene sheets has multiple effects on
the soil environment. Typically, increases are observed in soil ethylene and
carbon dioxide concentrations, nutrient availability (nitrogen, phosphorus,
calcium, and magnesium), and moisture content of upper soil layers (Egley,
1983; Horowitz, Regev & Herzlinger, 1983; Rubin & Benjamin, 1984;
Stapleton & DeVay, 1986). The increase in daily maximum temperatures in
surface soil layers is illustrated by data from a field experiment conducted in
central India by Kumar et al . (1993). Solarization with transparent polyethy-
lene tarps (100 m thickness) increased mean maximum soil temperatures at

5, 10, and 15 cm depth by 9, 7, and 7

C, respectively. Over a 32-day period
when mean maximum air temperature was 39

°

° C,maximum soil temperature

at 5 cm depth exceeded 50 ° C on 32 days, exceeded 55 ° C on 23 days, and

exceeded 60 C on 7 days; soil temperature at 5 cm depth in the untarped

control treatment never exceeded 50

°

C. In the southern USA (Mississippi),
Egley (1983) noted that maximum soil temperature at 1.3 cm depth under
clear polyethylene sheets reached 65-69

°

° C for 3-4 hours, compared with

43-50 C in uncovered soil; at 5.1 cm depth the increase in maximum temper-

ature due to tarping was about 10

°

C. Horowitz, Regev & Herzlinger (1983)
and Rubin & Benjamin (1984) observed similar tarping effects on maximum
soil temperature in field experiments in Israel.
Solarization is thought to increase weed seed mortality through direct
thermal damage to cell structure and metabolism, toxic effects of gases pro-
duced within soil by decomposing organic matter and metabolizing seeds, and
microbial attack on seeds and perennating structures weakened by elevated
temperature (Horowitz, Regev & Herzlinger, 1983; Rubin & Benjamin, 1984).
Field experiments conducted in the southern and western USA, Israel,
India, and other locations have demonstrated that soil solarization can sub-
stantially reduce viable seed densities of many weed species and weed emer-
gence in subsequent crops (Horowitz, Regev & Herzlinger, 1983; Standifer,
Wilson & Porche-Sorbet, 1984; Bell, Elmore & Durazo, 1988; Kumar et al .,
1993).Several important points emerge from such studies.
First, mortality of weed seeds due to solarization is usually greater close to
the soil surface than deeper in the soil profile. For example, in a solarization
experiment conducted by Standifer, Wilson & Porche-Sorbet (1984), reduc-
tions in numbers of germinable Poa annua seeds were largely restricted to the
upper 6 cm of soil. Reductions in annual Cyperus species occurred mostly in
the upper 4 cm of soil, though negative effects on Echinochloa crus-galli seeds
were apparent to 15 cm depth. Greater weed seed mortality near the soil
surface and lower mortality with depth reflect the distribution of heat in the
soil profile created by solarization (Rubin & Benjamin, 1984).

°