Gardening Without Irrigation: or without much, anyway | Page 7

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 1.0 Sandy loam 1.4 Loam 2.0 Clay loam 2.3 Silty clay 2.5 Clay
2.7
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._
How Soil Loses Water
Suppose we tilled a plot about April 1 and then measured soil moisture
loss until October. Because plants growing around the edge might
extend roots into our test plot and extract moisture, we'll make our
tilled area 50 feet by 50 feet and make all our measurements in the
center. And let's locate this imaginary plot in full sun on flat, uniform
soil. And let's plant absolutely nothing in this bare earth. And all season
let's rigorously hoe out every weed while it is still very tiny.
Let's also suppose it's been a typical maritime Northwest rainy winter,
so on April 1 the soil is at field capacity, holding all