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.