Weeds and the soil environment - Ecological Management of Agricultural Weeds - page 252

chemical characteristics (Gallandt et al .,1998 b ).In this experiment,potato was
planted in a two-year rotation sequence with either barley grown for grain or a
mixture of oat, pea, and hairy vetch grown for green manure. In an
“amended” soil management treatment, beef manure, cull potato compost,
and low rates of synthetic fertilizer were applied to potato following the green
manure crop. In an “unamended” soil management treatment, a high rate of
synthetic fertilizer but no beef manure or compost was applied to potato fol-
lowing barley. Three contrasting weed management treatments were used
during the potato phase of the green manure-potato-and-barley-potato rota-
tions: full labeled rates of herbicides (metribuzin and paraquat) and zero or
one cultivation other than standard hilling operations (“conventional”); half-
rates of herbicides and one cultivation (“reduced input”); and one to three cul-
tivations but no herbicides (“mechanical”).
Applications of organic amendments significantly increased soil organic
matter and water-stable aggregate content,cation exchange capacity,available
P, K, Mg, and Ca, and potato leaf area and tuber yield (Gallandt et al ., 1998 a ,
1998 b ). By the fourth and fifth years of the study (1994 and 1995), weed
growth in potato was strongly affected by an interaction between weed man-
agement and soil management systems (Figure 5.12) (Gallandt et al ., 1998 a ).
When herbicides were applied (i.e., the conventional and reduced input treat-
ments), weed growth in potato was minimal regardless of the soil manage-
ment treatment. However, when herbicides were not applied (i.e., the
mechanical treatment), significant differences in weed growth were evident
between soil management treatments: amendments reduced weed biomass by
72% to 77%. Reductions in weed biomass occurred even when weed densities
in the contrasting soil management treatments were equal, leading Gallandt
et al . (1998 a ) to conclude that improvements in soil quality due to use of
organic amendments promoted a more vigorous potato crop that was better
able to compete with weeds.
Before generalizations can be drawn, the impact of organic matter amend-
ments on crop-weed interactions needs to be examined in other cropping
systems. Research is also needed to understand how soil amendments affect
weed seed survival, seedling recruitment, and growth. Amendment-related
factors that might affect weed dynamics include changes in communities of
microbes and insects that attack weed seeds and seedlings, alterations of soil
physical properties influencing safe sites for weed germination and establish-
ment, increased concentrations of amendment-derived phytotoxins and
growth stimulants, shifts in the timing of nutrient availability, and differen-
tial responses between crop and weed species to these factors (Gallandt,
Liebman & Huggins, 1999; Liebman & Davis, 2000). The diversity of issues