Weeds and the soil environment - Ecological Management of Agricultural Weeds - page 247

cated that weed competition reduced crop growth by an average of 8% in the
clover treatment, but by 28% in the synthetic fertilizer treatment. Thus, C.
album was smaller and inflicted less damage on sweet corn when green
manure,rather than synthetic fertilizer,was used as the major N source.Dyck,
Liebman & Erich (1995) attributed these results to two factors: (i) lower levels
of soil inorganic N present in the green manure treatment during the early
portion of the growing season, which retarded growth of the weed but had
little effect on the crop,and (ii) selective phytotoxic effects of clover residue on
the weed.
In contrast to the reasonably high level of C. album suppression obtained
from fresh crimson clover in the experiments described above, less effective
and less consistent suppression was obtained from winter-killed, partially
decomposed clover residue in an experiment with C.album and maize (Dyck &
Liebman, 1995). Dyck & Liebman (1994) observed that fresh crimson clover
residue incorporated into soil reduced C. album emergence, whereas Blum et
al . (1997) found that it stimulated emergence of three other weed species.The
factors responsible for these different outcomes might include differences in
residue age,chemical composition,time of incorporation,environmental con-
ditions, and weed identity, but more research is needed to understand their
actual importance, especially if useful management practices are to be iden-
tified and damaging practices avoided.
The effects of crop residue used as mulch have been clarified by examining
dose-response relationships (Figure 5.11) (Mohler & Teasdale, 1993; Teasdale
& Mohler, 2000). For some weed species, such as Amaranthus retroflexus,
Chenopodium album , and Panicum capillare , emergence declined monotonically
as mulch rate increased (Figure 5.11). For others, such as Abutilon theophrasti,
Rumex crispus, Stellaria media , and Taraxacum officinale , a low rate of mulch
increased weed emergence. Because most effects of mulch on weed germina-
tion are negative,the enhancement of emergence at low mulch rates was prob-
ably due to improved water uptake by seeds in the moister environment
under the mulch. Probably some of the difference between species in the
shape of the mulch dose-response curve was due to propagule size, since
surface-to-volume ratio favors the water uptake of small propagules relative
to larger ones.
When soil moisture conditions are favorable for seed germination, the pro-
portion of seedlings emerging through mulch relative to the number emerg-
ing in the unmulched condition, E ,can usually be well described by a negative
exponential curve (Teasdale & Mohler, 2000):

E e b •MAI