Weed life history: identifying vulnerabilities - Ecological Management of Agricultural Weeds - page 50

Why tillage promotes the germination of weed seeds
Tillage promotes germination of most agricultural weeds, provided
the soil disturbance comes at a time of year when the seeds are not innately
dormant.Agricultural weeds have adapted to respond to cues associated with
soil disturbance because their small seedlings make them poor competitors
early in life. Vigorous, well-established plants are unlikely to be present
immediately after soil disturbance, and hence weedy species have been
selected for germination under conditions that indicate soil disturbance.
Relative to undisturbed soil with established vegetation, recently tilled
ground tends to be warmer, have higher diurnal temperature fluctuations,
higher nitrate concentration, and better aeration (Gebhardt et al ., 1985; Cox et
al ., 1990; Dou, Fox & Toth, 1995). Perhaps most importantly, when tillage or
natural processes stir soil, exposure to light prompts seed germination (Sauer
& Struik, 1964).
Each of these factors promotes the germination of some common agricul-
tural weeds (Table 2.3). For example, Rumex crispus , Chenopodium album , and
Panicum dichotomiflorum have a higher percentage germination when exposed
to fluctuating temperatures than when exposed to a constant temperature
with the same mean (Henson, 1970; Totterdell & Roberts, 1980) or when sep-
arately tested against the two temperature extremes (Fausey & Renner, 1997).
Amaranthus retroflexus germinates best at 30-40

C (McWilliams, Landers &
Mahlstede, 1968; Weaver & Thomas, 1986), a soil temperature that is unlikely
to occur under the shade of established vegetation.
Tillage and other soil disturbances stimulate decomposition of organic
matter and nitrification of the ammonium released by decomposition. The
presence of nitrate thus indicates not only enhanced availability of mineral
nutrients, but also the elimination of competing vegetation. In any case,
nitrate indicates favorable growing conditions, and germination of several
weed species,including Chenopodium album and Plantago lanceolata ,increases in
response to elevated nitrate concentrations (Williams & Harper, 1965; Pons,
1989).
Gas exchange in the soil during tillage probably prompts germination of
many weed species. Although oxygen concentration influences germination
(Edwards, 1969; Popay & Roberts, 1970; Brennan et al ., 1978), oxygen levels
near the soil surface are rarely low enough to directly inhibit germination,
except when the soil is saturated with water (Egley,1995).Several studies have
shown, however, that flushing the soil with air substantially increased germi-
nation or emergence of buried seeds (Wesson & Wareing, 1969 b ; Holm, 1972;
Benvenuti & Macchia, 1995). Since flushing with nitrogen also enhanced ger-

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