Agriculture Reference
In-Depth Information
Figure 5.7
Phytotoxins leaching from a surface layer oforganic mulch diffuse only
a short distance into the soil.Large crop seeds commonly are planted below the
toxic layer,and thus germinate in a chemically safe environment.Weed seeds that
germinate within the allelopathic zone frequently are poisoned,whereas most
weeds that germinate below the toxic layer exhaust seed reserves before reaching
the surface.
the result of two processes. First, at least from germination until emergence,
the surface-to-volume ratio of a small-seeded species is usually greater, and
therefore its exposure per unit mass to allelopathic substances in the soil is also
greater. Second, when residue is used as a mulch, the allelopathic toxins are
released onto the soil surface and may not diffuse very deeply into the soil
profile. Barnes & Putnam (1986) showed that percent germination and root
elongation of several species decreased as the layer of soil separating seeds
from rye residue decreased from 15 to 0 mm. To have any potential for emer-
gence, a small-seeded crop or weed must germinate near the soil surface, but
under an allelopathic mulch,this is where the toxins are most concentrated.In
contrast, large-seeded crops are planted more deeply, and thus germination
and initial root growth may occur in a less toxic environment (Figure 5.7).Both
hypotheses require testing by careful experimentation.
Production and decomposition of allelopathic compounds is highly vari-
able. Mwaja, Masiunas & Weston (1995) found that rye produced lower con-
centration of BOAand DIBOAin high-fertility conditions relative to medium-
and low-fertility conditions. Similarly, Patrick & Koch (1958) found that soil
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