Weeds and the soil environment - Ecological Management of Agricultural Weeds - page 225

can reduce the negative effect of V. hederifolia on wheat yield by benefiting the
crop more than the weed.
In cases where weed species are more synchronous with crops in their pat-
terns of nutrient capture, delayed fertilization may provide no advantage
with regard to weed suppression and crop yield. In field experiments with
winter wheat infested by the winter annual grass Bromus tectorum , Ball,
Wysocki & Chastain (1996) observed that delaying application of N fertilizer
until spring had either no effect or a stimulatory effect on weed biomass pro-
duction, and either no effect or a negative effect on wheat yield, compared
with N application at planting. Nitrogen fertilizer increased wheat yield
without increasing B. tectorum biomass only when it was applied during the
fallow period preceding crop production. The latter result was also observed
by Anderson (1991), who attributed it to two phenomena: movement of ferti-
lizer N into lower layers of the soil profile before growth of the crop and weed
began, and differences in their rooting habits; the shallow-rooted weed was
unable to extract fertilizer N at lower depths, while the deeper-rooted crop
gained access to that source of nutrients. Ball, Wysocki & Chastain (1996) con-
cluded that pre-planting N application may limit growth of B. tectorum and
benefit the crop, but may also increase N leaching and water contamination.
Other approaches for managing soil fertility and B. tectorum are therefore
needed.
Synthetic nitrogen source effects
The form in which nutrients are provided can have differential effects
on weed and crop performance. An illustration of this phenomenon can be
seen in the results of Teyker, Hoelzer & Liebl (1991), who fertilized maize and
Amaranthus retroflexus with nitrate or ammonium N sources. Shoot weight of
the crop was unaffected by N source (Figure 5.4a),but use of ammonium (with
the addition of a nitrification inhibitor) reduced the weed's shoot weight by
75%, compared with the nitrate N source (Figure 5.4b). Although the investi-
gators did not examine the effects of different synthetic N sources on compet-
itive interactions between the crop and weed, and did not compare growth of
the two species over the same time interval, the observed effects suggest that
use of ammonium N with a nitrification inhibitor could greatly enhance
maize's ability to suppress growth of A.retroflexus.
Differences in synthetic N sources may also affect the species composition
of weed communities. Pysek & Leps (1991) compared the weed floras of fields
in the Czech Republic that had been cropped with barley for seven years and
fertilized with ammonium sulfate (AS), calcium-ammonium nitrate (CAN), or