Weeds and the soil environment - Ecological Management of Agricultural Weeds - page 238

the Bolivian Amazon and reported that 50% of the N,P,K,and Mg contained

in 12-month-old plants was released within four weeks after they were
slashed and placed on the soil surface. In an experiment conducted in the
north-central USA (Wisconsin), Stute & Posner (1995) observed that red clover
and hairy vetch green manures released half of their N within four weeks after
incorporation and increased soil inorganic N concentrations to levels similar
to those obtained from ammonium nitrate fertilizer applied at 179 kg N ha 1 .
Because the rate and total amount of germination of certain weed species
are positively correlated with soil nitrate concentration (Henson, 1970;
Roberts & Benjamin, 1979; Taylorson, 1987; Karssen & Hilhorst, 1992), the
use of crop residues that function as slow-release alternatives to early, pulsed
application of synthetic N fertilizer may delay weed emergence and reduce
weed density.Conversely,crop residues that release N quickly should not have
this weed-suppressive effect.Field testing of these hypotheses is needed.
Increased reliance on decomposing crop residues rather than fertilizer
applied at planting may also affect post-emergence weed management. Greater
seed reserves convey greater tolerance of nutrient deficits and other stresses
during early growth (Westoby,Leishman & Lord,1996) and hence low availabil-
ity of nutrients from crop residue early in the growing season may retard seed-
ling growth of small-seeded weeds,but have a neutral effect on the early growth
of large-seeded crops.Alternatively,if residue decomposes quickly and provides
large amounts of nutrients at appropriate weed growth stages, weed problems
might be worse with residue than with synthetic fertilizer, particularly if the
fertilizer were applied in a split and delayed manner in narrow bands close to
the crop row. Collaborative research involving soil, crop, and weed scientists is
needed to resolve these issues and better manipulate residue effects on soil fer-
tility to the advantage of crops and detriment of weeds.

Residue effects on temperature and moisture
Crop residue used as mulch substantially decreases maximum daily
soil temperatures (Mitchell & Teel,1977; Bristow,1988; Fortin & Pierce,1991;
Teasdale & Mohler,1993).Differences between maximum surface soil temper-
ature in mulched and unmulched plots is greatest on hot, sunny days when
the soil is dry, and under such conditions differences as great as 14

C have
been reported (Bristow, 1988). More typically, the difference between maxima
in mulched and unmulched conditions is 2-5

°

C. When weather is relatively
cool, lower maximum temperature in a mulched field will tend to retard the
emergence of both weeds and crops. Under hot conditions, lower maximum
temperature may prevent some weed species from entering secondary dor-
mancy (Forcella et al ., 1997).
However, since mulch has either little effect on daily minimum soil tem-

°