Weeds and the soil environment - Ecological Management of Agricultural Weeds - page 251

tics and microbial communities change rapidly and therefore require inten-
sive sampling. Important questions remain unanswered concerning the nec-
essary scope of investigations. For example, because a weed's response to
resource conditions can depend on its genotype (Garbutt & Bazzaz, 1987;
Tardif & Leroux, 1992), how large a pool of genotypes must be used to assess
predictable species-level behavior? If the answer is that many genotypes must
be tested in a range of environments, a significant research effort will be
required.
Despite these challenges, a focus on soil management for weed regulation
provides valuable opportunities to improve overall agroecosystem health. In
particular, such a focus encourages scientists and farmers to integrate weed
management with strategies for soil improvement and conservation. Soil
solarization, flooding, banded nutrient applications, cover cropping with
allelopathic species, and other practices clearly can play desirable roles in reg-
ulating weed populations,but the soil is more than just a medium in which to
suppress and kill weeds — it is a resource that must be enhanced and protected
for long-term crop production. The use of organic materials as soil amend-
ments and the development of weed management machinery for mulch
systems are two areas where weed and soil management could be integrated
and simultaneously improved.
Soil amendment systems
Farmers, soil scientists, and agronomists have long recognized that
regular addition of organic materials can markedly improve soil quality
(Magdoff, 1992, pp. 23-38). As noted previously, cover crop residues can
promote crop growth and yield by improving soil fertility, water availability,
and aeration. Animal manures, composts, and other organic materials offer
similar benefits and can be used in various combinations with cover crops to
provide diversified soil amendment systems adapted to local conditions.
Amending soil with organic materials may also improve crop performance by
reducing pest pressures. Organic matter amendments have been shown to
render crops less attractive to insect pests (Phelan, Mason & Stinner, 1995),
and reduce crop disease problems by promoting better soil structure, more
vigorous root systems, and greater populations of soil organisms that antago-
nize, outcompete, parasitize, or consume crop pathogens (van Bruggen,
1995).
Although the relationship between soil quality and weed dynamics has
received little attention from researchers, data from a cropping systems study
in northern Maine suggest that organic amendments can confer weed man-
agement benefits (Gallandt et al ., 1998 a ), as well as improve soil physical and