Plants Are Water
Like all other carbon-based life forms on earth, plants conduct
their chemical processes in a water solution. Every substance that
plants transport is dissolved in water. When insoluble starches and
oils are required for plant energy, enzymes change them back into
water-soluble sugars for
movement to other locations. Even cellulose
and lignin, insoluble structural materials that plants cannot
convert back into soluble materials, are made from molecules that
once were in solution.
Water is so essential that when a plant can no longer absorb as much
water as it is losing, it wilts in self-defense. The drooping leaves
transpire (evaporate) less moisture because the sun glances off
them. Some weeds can wilt temporarily and resume vigorous growth as
soon as their water balance is restored. But most vegetable species
aren't as tough-moisture stressed vegetables may survive, but once
stressed, the quality of their yield usually drops markedly.
Yet in deep, open soil west of the Cascades, most vegetable species
may be grown quite successfully with very little or no supplementary
irrigation and without mulching, because they're capable of being
supplied entirely by water already stored in the soil.
Soil's Water-Holding Capacity
Soil is capable of holding on to quite a bit of water, mostly by
adhesion. For example, I'm sure that at one time or another you have
picked up a wet stone from a river or by the sea. A thin film of
water clings to its surface. This is adhesion. The more surface area
there is, the greater the amount of moisture that can be held by
adhesion. If we crushed that stone into dust, we would greatly
increase the amount of water that could adhere to the original
material. Clay particles, it should be noted, are so small that
clay's ability to hold water is not as great as its mathematically
computed surface area would indicate.
Surface Area of One Gram of Soil Particles
Particle type Diameter of particles in mm Number of particles per gm
Surface area in sq. cm.
Very coarse sand 2.00-1.00 90 11
Coarse sand 1.00-0.50 720 23
Medium sand 0.50-0.25 5,700 45
Fine sand 0.25-0.10 46,000 91
Very fine sand 0.10-0.05 772,000 227
Silt 0.05-0.002 5,776,000 454
Clay Below 0.002 90,260,853,000 8,000,000
Source: Foth, Henry D., _Fundamentals of Soil Science,_ 8th ed.
(New York: John Wylie & Sons, 1990).
This direct relationship between particle size, surface area, and
water-holding capacity is so essential to understanding plant growth
that the surface areas presented by various sizes of soil particles
have been calculated. Soils are not composed of a single size of
particle. If the mix is primarily sand, we call it a sandy soil. If
the mix is primarily clay, we call it a clay soil. If the soil is a
relatively equal mix of all three, containing no more than 35
percent clay, we call it a loam.
Available Moisture (inches of water per foot of soil)
Soil Texture Average Amount
Very coarse sand 0.5
Coarse sand 0.7
Sandy loam 1.4
Clay loam 2.3
Silty clay 2.5
Source: _Fundamentals of Soil Science_.
Adhering water films can vary greatly in thickness. But if the water
molecules adhering to a soil particle become too thick, the force of
adhesion becomes too weak to resist the force of gravity, and some
water flows deeper into the soil. When water films are relatively
thick the soil feels wet and plant roots can easily absorb moisture.
"Field capacity" is the term describing soil particles holding all
the water they can against the force of gravity.
At the other extreme, the thinner the water films become, the more
tightly they adhere and the drier the earth feels. At some degree of
desiccation, roots are no longer forceful enough to draw on soil
moisture as fast as the plants are transpiring. This condition is
called the "wilting point." The term "available moisture" refers to
the difference between field capacity and the amount of moisture
left after the plants have died.
Clayey soil can provide plants with three times as much available
water as sand, six times as much as a very coarse sandy soil. It
might seem logical to conclude that a clayey garden would be the
most drought resistant. But there's more to it. For some crops, deep
sandy loams can provide just about as much usable moisture as clays.
Sandy soils usually allow more extensive root development, so a
plant with a naturally aggressive and deep root system may be able
to occupy a much larger volume of sandy loam, ultimately coming up
with more moisture than it could obtain from a heavy, airless clay.
And sandy loams often have a clayey, moisture-rich subsoil.
_Because of this interplay of factors, how much available water your
own unique garden soil is actually capable of providing and how much
you will have to supplement it with irrigation can only be
discovered by trial._
Previous: Source: _The Water Encyclopedia_
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