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2. Soil Fundamentals
How does water move through soil?
Water enters and flows through a soil’s pore
space. Macropores are large enough for plant
roots to penetrate, and therefore the water in
them is available to plants. Plants suck water
from soil space through their roots and into
their stems and leaves, and the water then
returns to the atmosphere as water vapor.
Water in macropores is also susceptible to
leaching through the downward force of
gravity. As a soil dries, water is held tightly in
micropores, where it is not available to plants
because even fine root hairs are too large to
penetrate tiny micropores.
We can follow the changing conditions in soils
as they lose water. It is especially important to
note that plants are like Goldilocks in the story
of the three bears—too much water in the soil
is not suitable for them; too little is not suitable
for them; but the middle amount is just right.
Glossary
Saturation
occurs when all of a soil’s pore space is filled
with water; a saturated soil contains no air.
Glossary
Field Capacity
full saturation of the soil, minus water lost
to free drainage by gravity.
Glossary
Permanent Wilting Point
the condition in which there is still water in
the soil but it is held so tightly to soil physical
components (especially small particles like
clay) that plants are not able to extract it fast
enough to meet their needs.