Agriculture Reference
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and Helianthus annuus have shown increased survival when seeds were incor-
porated into the soil (Banting, 1966; Wilson, 1972; Wilson & Cussans, 1972,
1975; Robinson, 1978), and recent work on several broadleaf species (Mohler,
1999) showed consistent increase in seed survival with depth. In general, bio-
logical activity increases toward the soil surface, and action of seed predators
and pathogens should be greatest there. In addition, desiccation and exhaus-
tion of dormant seeds is more likely near the surface than in the cooler,
moister soil below. Nevertheless, the pattern of seed mortality with depth
apparently varies among species, and probably with soil and cultural condi-
tions as well.
Plowing and other tillage that raises deeply buried seeds will bring many
sufficiently near the surface to prompt germination. However, only some of
these successfully emerge, and the tillage operation has essentially killed
those that fail. This effect is highly dependent on the germination biology of
the species present. Moreover, it cannot be studied simply by placing seeds at
different depths, since seeds in the soil are likely to have different dormancy
status than seeds from the laboratory shelf or refrigerator.
In summary, tillage decreases the weed seed bank. Frequent tillage
decreases the seed bank more rapidly than infrequent tillage.Tillage that pro-
duces more vertical displacement of seeds probably creates more true seed
mortality, though shallow tillage may decrease the seed bank more rapidly
through seedling emergence.Except for a few studies that have distinguished
death by germination or disappearance from loss of viability (Sanchez del
Arco, Torner & Fernandez Quintanilla, 1995), few data are available on the
causes of seed mortality in field conditions.
Synthesis of the effects of tillage on seeds and seedlings
Mohler (1993) used an analytical model to explore the multiple
effects of tillage on emergence. The model assumed that emergence declined
exponentially with depth of the seed and that seed survival increased with
depth to an asymptote. The model consisted of equations that predicted for
various tillage regimes the proportion of an initial seed bank that emerged as
seedlings in each successive year,assuming no additional seed input.Depth of
tillage and species properties including dormancy, ability to emerge through
soil,and near-surface seed survival were varied to determine conditions under
which one tillage regime resulted in fewer weed seedlings than another.
Emergence was greater from no-till than from plow or rotary tillage during
the initiation year for most realistic parameter values (Mohler, 1993).
Assuming no innate seed dormancy, in the next year tillage had more seed-
lings than no-till, unless the species only emerged when near the surface and
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