Gardening Without Irrigation: or without much, anyway | Page 8

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