Mechanical management of weeds - Ecological Management of Agricultural Weeds - page 158

Figure 4.5 Proportion of Chenopodium album , Amaranthus retroflexus ,and Abutilon
theophrasti emerging after planting into either tilled or untilled soil with an
apparatus that produced minimal soil disturbance.See Mohler & Galford (1997).
Tillage and seed survival
Within a given season, the density of seedlings is the integral over all
depths of the product of the number of seeds at a given depth and the prob-
ability of a seed at that depth producing an emerged seedling (Mohler, 1993).
However, to understand the effects of tillage over several seasons, the survival
of seeds in the soil must be considered as well.
Roberts & Dawkins (1967) performed an early experiment relating seed
survival to tillage.They turned the soil at three-month intervals,at six-month
intervals, or left it undisturbed. No seed production was allowed in the plots.
Annual sampling of the seed bank indicated that the rate of decline in number
of seeds present was relatively constant over years and increased with fre-
quency of soil disturbance (Figure 4.6a). Other studies corroborate this
finding (Roberts,1962; Roberts & Feast,1973 a ,1973 b ).In the study of Roberts
& Dawkins (1967), the decrease in viable seeds with tillage was largely
accounted for by an increase in the number of emerged seedlings (Figure
4.6b), but the generality of this result is unknown.
Tillage affects seed survival in three ways (Figure 4.7). First, the tillage
operation itself may stimulate germination, for example, by exposing the
seeds to a light flash or by scarifying them,and if germination then occurs at a
depth that does not allow emergence or at a time of year or in weather condi-
tions that do not allow establishment, then germination will lead to death.
Second, changes in soil conditions due to tillage may also stimulate germina-
tion in conditions unsuitable for establishment. Finally, action of seed preda-
tors, pathogens, and damaging physical influences generally decreases with
greater depth in the soil, and, as discussed above, tillage redistributes seeds in
the soil profile. For simplicity, germination under conditions that do not
allow establishment is referred to below as inappropriate germination and
treated as a type of seed mortality, although technically, death occurs in the
seedling rather than seed stage.