Water-Wise Gardening Science
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 Number of
particles particles Surface area
in mm per gm 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 the
moisture it can. From early April until well into September the hot
sun will beat down on this bare plot. Our summer rains generally
come in insignificant installments and do not penetrate deeply; all
of the rain quickly evaporates from the surface few inches without
recharging deeper layers. Most readers would reason that a soil
moisture measurement taken 6 inches down on September 1, should show
very little water left. One foot down seems like it should be just
as dry, and in fact, most gardeners would expect that there would be
very little water found in the soil until we got down quite a few
feet if there were several feet of soil.
But that is not what happens! The hot sun does dry out the surface
inches, but if we dig down 6 inches or so there will be almost as
much water present in September as there was in April. Bare earth
does not lose much water at all. _Once a thin surface layer is
completely desiccated, be it loose or compacted, virtually no
further loss of moisture can occur._
The only soils that continue to dry out when bare are certain kinds
of very heavy clays that form deep cracks. These ever-deepening
openings allow atmospheric air to freely evaporate additional
moisture. But if the cracks are filled with dust by surface
cultivation, even this soil type ceases to lose water.
Soil functions as our bank account, holding available water in
storage. In our climate soil is inevitably charged to capacity by
winter rains, and then all summer growing plants make heavy
withdrawals. But hot sun and wind working directly on soil don't
remove much water; that is caused by hot sun and wind working on
plant leaves, making them transpire moisture drawn from the earth
through their root systems. Plants desiccate soil to the ultimate
depth and lateral extent of their rooting ability, and then some.
The size of vegetable root systems is greater than most gardeners
would think. The amount of moisture potentially available to sustain
vegetable growth is also greater than most gardeners think.
Rain and irrigation are not the only ways to replace soil moisture.
If the soil body is deep, water will gradually come up from below
the root zone by capillarity. Capillarity works by the very same
force of adhesion that makes moisture stick to a soil particle. A
column of water in a vertical tube (like a thin straw) adheres to
the tube's inner surfaces. This adhesion tends to lift the edges of
the column of water. As the tube's diameter becomes smaller the
amount of lift becomes greater. Soil particles form interconnected
pores that allow an inefficient capillary flow, recharging dry soil
above. However, the drier soil becomes, the less effective capillary
flow becomes. _That is why a thoroughly desiccated surface layer
only a few inches thick acts as a powerful mulch._
Industrial farming and modern gardening tend to discount the
replacement of surface moisture by capillarity, considering this
flow an insignificant factor compared with the moisture needs of
crops. But conventional agriculture focuses on maximized yields
through high plant densities. Capillarity is too slow to support
dense crop stands where numerous root systems are competing, but
when a single plant can, without any competition, occupy a large
enough area, moisture replacement by capillarity becomes
significant.
How Plants Obtain Water
Most gardeners know that plants acquire water and minerals through
their root systems, and leave it at that. But the process is not
quite that simple. The actively growing, tender root tips and almost
microscopic root hairs close to the tip absorb most of the plant's
moisture as they occupy new territory. As the root continues to
extend, parts behind the tip cease to be effective because, as soil
particles in direct contact with these tips and hairs dry out, the
older roots thicken and develop a bark, while most of the absorbent
hairs slough off. This rotation from being actively foraging tissue
to becoming more passive conductive and supportive tissue is
probably a survival adaptation, because the slow capillary movement
of soil moisture fails to replace what the plant used as fast as the
plant might like. The plant is far better off to aggressively seek
new water in unoccupied soil than to wait for the soil its roots
already occupy to be recharged.
A simple bit of old research magnificently illustrated the
significance of this. A scientist named Dittmer observed in 1937
that a single potted ryegrass plant allocated only 1 cubic foot of
soil to grow in made about 3 miles of new roots and root hairs every
day. (Ryegrasses are known to make more roots than most plants.) I
calculate that a cubic foot of silty soil offers about 30,000 square
feet of surface area to plant roots. If 3 miles of microscopic root
tips and hairs (roughly 16,000 lineal feet) draws water only from a
few millimeters of surrounding soil, then that single rye plant
should be able to continue ramifying into a cubic foot of silty soil
and find enough water for quite a few days before wilting. These
arithmetical estimates agree with my observations in the garden, and
with my experiences raising transplants in pots.
Lowered Plant Density: The Key to Water-Wise Gardening
I always think my latest try at writing a near-perfect garden book
is quite a bit better than the last. _Growing Vegetables West of the
Cascades_, recommended somewhat wider spacings on raised beds than I
did in 1980 because I'd repeatedly noticed that once a leaf canopy
forms, plant growth slows markedly. Adding a little more fertilizer
helps after plants "bump," but still the rate of growth never equals
that of younger plants. For years I assumed crowded plants stopped
producing as much because competition developed for light. But now I
see that unseen competition for root room also slows them down. Even
if moisture is regularly recharged by irrigation, and although
nutrients are replaced, once a bit of earth has been occupied by the
roots of one plant it is not so readily available to the roots of
another. So allocating more elbow room allows vegetables to get
larger and yield longer and allows the gardener to reduce the
frequency of irrigations.
Though hot, baking sun and wind can desiccate the few inches of
surface soil, withdrawals of moisture from greater depths are made
by growing plants transpiring moisture through their leaf surfaces.
The amount of water a growing crop will transpire is determined
first by the nature of the species itself, then by the amount of
leaf exposed to sun, air temperature, humidity, and wind. In these
respects, the crop is like an automobile radiator. With cars, the
more metal surfaces, the colder the ambient air, and the higher the
wind speed, the better the radiator can cool; in the garden, the
more leaf surfaces, the faster, warmer, and drier the wind, and the
brighter the sunlight, the more water is lost through transpiration.
Dealing with a Surprise Water Shortage
Suppose you are growing a conventional, irrigated garden and
something unanticipated interrupts your ability to water. Perhaps
you are homesteading and your well begins to dry up. Perhaps you're
a backyard gardener and the municipality temporarily restricts
usage. What to do?
First, if at all possible before the restrictions take effect, water
very heavily and long to ensure there is maximum subsoil moisture.
Then eliminate all newly started interplantings and ruthlessly hoe
out at least 75 percent of the remaining immature plants and about
half of those about two weeks away from harvest.
For example, suppose you've got a a 4-foot-wide intensive bed
holding seven rows of broccoli on 12 inch centers, or about 21
plants. Remove at least every other row and every other plant in the
three or four remaining rows. Try to bring plant density down to
those described in Chapter 5, "How to Grow It: A-Z"
Then shallowly hoe the soil every day or two to encourage the
surface inches to dry out and form a dust mulch. You water-wise
person--you're already dry gardening--now start fertigating.
How long available soil water will sustain a crop is determined by
how many plants are drawing on the reserve, how extensively their
root systems develop, and how many leaves are transpiring the
moisture. If there are no plants, most of the water will stay unused
in the barren soil through the entire growing season. If a crop
canopy is established midway through the growing season, the rate of
water loss will approximate that listed in the table in Chapter 1
"Estimated Irrigation Requirement." If by very close planting the
crop canopy is established as early as possible and maintained by
successive interplantings, as is recommended by most advocates of
raised-bed gardening, water losses will greatly exceed this rate.
Many vegetable species become mildly stressed when soil moisture has
dropped about half the way from capacity to the wilting point. On
very closely planted beds a crop can get in serious trouble without
irrigation in a matter of days. But if that same crop were planted
less densely, it might grow a few weeks without irrigation. And if
that crop were planted even farther apart so that no crop canopy
ever developed and a considerable amount of bare, dry earth were
showing, this apparent waste of growing space would result in an
even slower rate of soil moisture depletion. On deep, open soil the
crop might yield a respectable amount without needing any irrigation
at all.
West of the Cascades we expect a rainless summer; the surprise comes
that rare rainy year when the soil stays moist and we gather
bucketfuls of chanterelle mushrooms in early October. Though the
majority of maritime Northwest gardeners do not enjoy deep, open,
moisture-retentive soils, all except those with the shallowest soil
can increase their use of the free moisture nature provides and
lengthen the time between irrigations. The next chapter discusses
making the most of whatever soil depth you have. Most of our
region's gardens can yield abundantly without any rain at all if
only we reduce competition for available soil moisture, judiciously
fertigate some vegetable species, and practice a few other
water-wise tricks.
_Would lowering plant density as much as this book suggests equally
lower the yield of the plot? Surprisingly, the amount harvested does
not drop proportionately. In most cases having a plant density
one-eighth of that recommended by intensive gardening advocates will
result in a yield about half as great as on closely planted raised
beds._
Internet Readers: In the print copy of this book are color pictures
of my own "irrigationless" garden. Looking at them about here in the
book would add reality to these ideas.