Rotary tilling
only blends amendments into the top 6 or 7 inches of soil. Rigorous
double digging by actually trenching out 12 inches and then spading
up the next foot theoretically allows one to mix in significant
amounts of organic matter to nearly 24 inches. But plants can use
water from far deeper than that. Let's realistically consider how
much soil moisture reserves might be increased by double digging and
incorporating large quantities of organic matter.
A healthy topsoil organic matter level in our climate is about 4
percent. This rapidly declines to less than 0.5 percent in the
subsoil. Suppose inches-thick layers of compost were spread and, by
double digging, the organic matter content of a very sandy soil were
amended to 10 percent down to 2 feet. If that soil contained little
clay, its water-holding ability in the top 2 feet could be doubled.
Referring to the chart "Available Moisture" in Chapter 2, we see
that sandy soil can release up to 1 inch of water per foot. By dint
of massive amendment we might add 1 inch of available moisture per
foot of soil to the reserve. That's 2 extra inches of water, enough
to increase the time an ordinary garden can last between heavy
irrigations by a week or 10 days.
If the soil in question were a silty clay, it would naturally make 2
1/2 inches available per foot. A massive humus amendment would
increase that to 3 1/2 inches in the top foot or two, relatively not
as much benefit as in sandy soil.
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