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. And I seriously doubt that many
gardeners would be willing to thoroughly double dig to an honest 24
inches.

Trying to maintain organic matter levels above 10 percent is an
almost self-defeating process. The higher the humus level gets, the
more rapidly organic matter tends to decay. Finding or making enough
well-finished compost to cover the garden several inches deep (what
it takes to lift humus levels to 10 percent) is enough of a job.
Double digging just as much more into the second foot is even more
effort. But having to repeat that chore every year or two becomes
downright discouraging. No, either your soil naturally holds enough
moisture to permit dry gardening, or it doesn't.

Keeping the Subsoil Open with Green Manuring