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.
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