Weeds and the soil environment - Ecological Management of Agricultural Weeds - page 229

pp. 19-24) predicts that competitive success between plant species will be a
function of which grows best at the lowest levels of nutrient availability,those
legumes that are best able to maintain growth at the lowest soil inorganic N
levels should be most effective in suppressing the growth of nitrophilous
weed species. Samson (1991) suggested that this approach was effective for
weed management in soybean production systems and noted that winter rye
and other cover crops preceding soybean could be used to sequester and tem-
porarily immobilize soil inorganic N. We examine the impacts of cover crops
on weed dynamics in more detail in the following pages.
Crop residue management
Crop residue constitutes about 65% of the total organic materials
applied to land in the USA(Parr,Miller & Colacicco,1984),and its composition
and management are important factors affecting soil chemical, physical, and
biological characteristics. Through its effects on soil, crop residue also can
affect weed and crop germination, survival, growth, and competitive ability.
In general, it appears that detrimental effects of crop residue are greater for
small-seeded species than larger-seeded species. Because seeds of most major
crops are one to three orders of magnitude larger than the weeds with which
they regularly compete, residue management offers important opportunities
for weed suppression (Mohler, 1996).
Taking advantage of these opportunities requires solving some consider-
able technical challenges, and manipulating and balancing a complex set of
interacting ecological processes. Under some circumstances crop residue can
promote rather than inhibit weeds, and suppress rather than enhance crop
establishment and growth.Moreover,to generate enough residue to influence
weeds appreciably, additional crops may need to be grown within a rotation
sequence, increasing the complexity of the cropping system. Unlike certain
ecologically based management practices that can be implemented with only
modest changes to a farmer's existing cropping system, using residue for
weed management can require substantial system redesign.
Green manures and mulches
Crops whose intended purpose is to alter soil characteristics rapidly
and significantly can have particularly marked effects on weeds. We use the
term cover crop for species grown expressly to add organic matter, maintain or
increase nutrient availability, improve soil physical properties, prevent
erosion, and, in some cases, reduce problems with soil-borne pathogens