Using Humus to Increase Soil Moisture

Maintaining topsoil humus content in the 4 to 5 percent range is
vital to plant health, vital to growing more nutritious food, and
essential to bringing the soil into that state of easy workability
and cooperation known as good tilth. Humus is a spongy substance
capable of holding several times more available moisture than clay.
There are also new synthetic, long-lasting soil amendments that hold
and release even more moisture than humus. Garden books frequently
recommend tilling in extraordinarily large amounts of organic matter
to increase a soil's water-holding capacity in the top few inches.
Humus can improve many aspects of soil but will not reduce a
garden's overall need for irrigation, because it is simply not
practical to maintain sufficient humus deeply enough. 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. 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.