Weeds and the soil environment - Ecological Management of Agricultural Weeds - page 236

pH and water content affected production of toxins in several species, and
that toxin concentration and rate of release varied with plant age.Since toxins
are released from crop residue by temperature- and moisture-sensitive pro-
cesses of leaching, volatilization and decomposition, the rate of release also
varies with environmental conditions.Cochran,Elliot & Papendick (1977) bio-
assayed water extracts of soil taken weekly from beneath residues of several
crop species and found that toxin production was usually preceded by condi-
tions favorable for microbial activity.Patrick,Toussoun & Snyder (1963) found
that the degree and duration of the toxic effect varied with location, even
within a given field. Toxicity may also vary depending on the growth condi-
tions of the target plant. Thus, Bradow (1993) found that ketones of the sort
volatilized from decomposing legume residue were more toxic to cotton
when the root zone temperature was warmer, and Einhellig (1986) showed
synergism between ferulic acid and moisture stress in the suppression of
sorghum seedling growth.
In general, allelochemical effects of crop residue are short lived. Patrick,
Toussoun & Snyder (1963) and Kimber (1973) found that phytotoxicity of
several crop species declined markedly after two to three weeks of decomposi-
tion. The transitory nature of allelopathy and its dependence on soil and
weather conditions represent major challenges to the use of crop residues for
weed management. Fortunately, the toxic properties of crop residues are only
one aspect of their inhibitory action on weeds. Even for allelopathically active
materials such as rye residue, physical modifications of the environment can
be responsible for a large portion of the weed suppression obtained (Teasdale
& Mohler, 1992).
Nutrient availability from crop residue
Patterns of nutrient release from crop residue are important for weed
management because, as explained below, they may affect weed density, the
timing of weed emergence, and interactions between crops and weeds.
Decomposition of crop residue and subsequent changes in soil nutrient
status are determined by multiple factors, including residue age and quality
(e.g., C:N ratio, lignin and polyphenol contents), loading rate, temperature
and moisture conditions, soil aeration and pH, and soil microbial, meso- and
macrofaunal populations (Fox, Myers & Vallis, 1990; Honeycutt, Potaro &
Halteman, 1991; Palm & Sanchez, 1991; Honeycutt et al ., 1993; Honeycutt,
Clapham & Leach, 1994; Killham, 1994). In general, crop residue retained on
the soil surface decomposes and releases nutrients more slowly than residue
incorporated into soil (Blevins, Smith & Thomas, 1984; Wilson & Hargrove,
1986; Sarrantonio & Scott, 1988; Dou, Fox & Toth, 1995). Application of crop