Weeds and the soil environment - Ecological Management of Agricultural Weeds - page 245

Figure 5.9 Mid-season density ofweeds above the potato canopy (a) and final
biomass ofweeds in potato crops (b) following fallow,sudangrass,and rapeseed
green manure treatments in a field experiment conducted in Washington,USA.
(After Boydston & Hang,1995.)
Rapeseed and many other cruciferous species contain glucosinolate com-
pounds that hydrolyze to isothiocyanates. These have potent inhibitory
effects on plant growth and seed germination (Brown & Morra, 1995).
Boydston & Hang (1995) suggested that isothiocyanates were responsible for
weed suppression by rapeseed residue,and that the residue's positive effect on
potato growth may have been due to improved crop nutrition and suppres-
sion of soil-borne pathogens. They also suggested that potato's tolerance of
rapeseed allelochemicals may have resulted from its large propagule size;
small-seeded crops might be susceptible to rapeseed residue.
The ability of a legume green manure to suppress weed growth while sup-
plying N to a crop was examined in field experiments investigating effects of
crimson clover residue and ammonium nitrate fertilizer on Chenopodium
album and sweet corn (Dyck & Liebman, 1994; Dyck, Liebman & Erich, 1995).
Live clover plants were incorporated into soil shortly before planting C. album
and sweet corn, and the synthetic N source was applied immediately after
planting. Based on measurements of N uptake, the estimated fertilizer equiv-
alency value of the clover green manure for the succeeding sweet corn crop
was about 55 kg N ha 1 (Dyck, Liebman & Erich, 1995).
When C. album and sweet corn were grown in single-species stands at fixed
densitites, biomass production by the weed species was significantly reduced