Agriculture Reference
In-Depth Information
Figure 5.3 Green surface area ofbarley and Brassica hirta grown in mixture without
(a) and with (b) N fertilizer.Data were collected in 10-cm strata,160 days after
planting,in a field experiment conducted in California,USA.Ninety kg N ha 1
(as ammonium sulfate) was applied to N fertilized treatments at planting and
again 94 days later.(M.Liebman,unpublished data.)
experiment, N application greatly increased the weed's ability to shade the
crop.
Not surprisingly, when weeds exhibit stronger height and leaf area
responses to fertilizer than do crops, fertilizer application may have a neutral
or even negative effect on crop yields. In Oregon, Appleby, Olson & Colbert
(1976) observed that wheat yields were no higher or were slightly reduced by
N application when the crop grew in association with high densities of Lolium
multiflorum . In northern California, Carlson & Hill (1985) found that applica-
tion of N fertilizer to wheat infested with Avena fatua increased crop yield only
when A. fatua density was
1.6% of the total weed plus crop density. At high
weed densities,Napplication increased A. fatua panicle production by as much
as 140%, and decreased wheat yield by as much as 49%, compared with unfer-
tilized treatments.
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