The Environmental Complex - Agroecology: The Ecology of Sustainable Food Systems - page 165

History of the Plot
The prior history of the field,
including crops grown, soil
treatments, frequency of
disturbance, etc. will all have
impacts on the factors that
directly influence
germination

History of the Seed
The age of the seed and
the conditions it has been
exposed to (such as abrasion)
are important determinants
of how other factors will
influence germination

Light
Some seeds require light to
germinate, and others are
inhibited by light. The amount
of light received by the seed is
determined by its history and
by its depth and the character
of the soil surface cover.

Seed Depth
Conditions change with
soil depth; for each type of
plant, there is an ideal
depth at which conditions
are most conducive to
germination.

Precipitation
Precipitation or irrigation
preceding seed germination is
the primary factor influencing
soil moisture. The effect of
precipitation is modified by
soil type, wind, fire, soil
surface cover, and temperature.

Temperature
Each species of plant has
slightly different temperature
optima for germination, and the
temperature of the soil varies
with depth, moisture,
topography, and other factors.
Light and temperature are
closely related.

Biotic Factors
A wide range of organisms,
from viruses to mammals,
can have varying impacts on
seed germination. Their
presence in the soil changes
depending on the site, its
history, the soil, and cropping
patterns.

Soil Cover
Organic matter on the soil
surface alters water
movement, soil temperature,
light availability, and the
potential for disease. Toxins
can be released from
decomposing plant residues.

Wind
Wind can influence seed
germination by altering soil
temperature and soil
moisture, and also by
changing the structure of
the soil surface cover.

Soil Moisture
Germination only occurs after the
seed has imbibed sufficient water.
Soil texture, structure, color, and
microrelief will all impact
moisture availability, as will
irrigation or rain, temperature,
and wind.

Soil
Soil type affects seed
germination by controlling how
moisture is added and retained
in the soil, and by creating a
chemical environment that may
enhance or retard germination
processes.

Fire
If it occurs, fire may alter soil
moisture, soil quality, the
ability of the soil to absorb
solar energy, and the
structure of the soil surface
cover. Fire may also have the
direct effect of killing seeds.

FIGURE 12.2 Environmental factors affecting seed germination. Factors immediately surrounding the seed affect it most directly;
factors in the outer perimeter mostly affect the intensity, level, and presence of the direct factors. The importance of each factor will
vary depending on the species of the seed.

In multiple cropping systems, variations in the vertical
dimension must also be taken into account because one
crop or canopy layer will generally create strata of varying
conditions for other crops or canopy layers. This is espe-
cially true if a new crop is being planted into an already
established canopy, such as into an agroforestry or tree-
dominated home garden agroecosystem. To complicate
matters even more, a large mature plant member of such
a system is occupying a range of microhabitats simulta-
neously. Which portion of the habitat and combination of

microenvironmental conditions are affecting the organism
the most?
Because of the difficulty involved in creating abso-
lutely uniform conditions in farm fields, especially in
resource-limited or small-scale traditional agroecosys-
tems, farmers often plant multiple species or a variety of
crop mixtures, with the idea that a diverse combination of
crops with a range of adaptations will do better in a vari-
able environment (Smith and Francis, 1986). It is a real
challenge in experimental agronomic studies to adequately