Weeds and the soil environment - Ecological Management of Agricultural Weeds - page 250

exceptionally heavy cover crop production (10-14 Mg ha 1 - Creamer et al .,
1996 b ).
Adequate weed control from mulch alone has often been observed when
the mulch was applied from an outside source (Midmore, Roca & Berrios,
1986; Daisley et al ., 1988; Niggli, Weibel & Gut, 1990; Singh, Singh & Singh,
1991; Roe,Stoffella & Bryan,1993; Davis,1994),usually because of high appli-
cation rates. Nevertheless, several studies found that supplemental weeding
was needed, particularly when the mulch was applied at rates similar to those
used in studies with in situ production (Obiefuna, 1986; Baryeh, 1987; Okugie
& Ossom, 1988).
Effectiveness of residue for weed suppression often declines substantially
after four to six weeks (Wallace & Bellinder,1992; Creamer et al .,1996 b ; Smeda
& Weller, 1996; Vidal & Bauman, 1996), probably due to loss of mass through
decomposition, and the breakdown of allelopathic compounds. In addition,
condensed mulches that tend to retain water, such as sawdust, compost, and
rotted baled hay, may provide a seed bed for the establishment of wind-borne
seeds (Niggli, Weibel & Gut, 1990). This is rarely a problem with loose straw
mulches that dry quickly following rain.
The foregoing discussion suggests three reasons why mulch-based weed
management is a more viable strategy in tropical and warm temperate regions
than in cool temperate regions. First, a warm fallow season provides better
opportunities for production of more mulch biomass. Second, a greater range
of cover crops are winter-hardy in warmer regions. Third, in cooler climates,
mulch will generally lower soil temperatures to the detriment of the crop,
whereas in hotter regions, lowered soil temperatures will pose less of a
problem, and may be advantageous to crop production (Midmore, Roca &
Berrios, 1986; Obiefuna, 1986).
Toward the integration of weed and soil management
Examples have been presented throughout this chapter illustrating
how manipulations of soil temperature, moisture, nutrient, and residue con-
ditions can alter weed density, growth, competitive ability, and community
composition. In some cases the outcomes obtained are beneficial; in others
they are undesirable. If soil management practices are to be used to regulate
weeds in a consistently successful manner, a better understanding of the
mechanisms is needed.
Serious relevant technical challenges confront researchers studying
soil-weed relationships: fine roots are hard to recover, soil spatial heterogene-
ity introduces variability into field experiments,and soil chemical characteris-