Mulching

Gardening under a permanent thick mulch of crude organic matter is
recommended by Ruth Stout (see the listing for her book in More
Reading) and her disciples as a surefire way to drought-proof
gardens while eliminating virtually any need for tillage, weeding,
and fertilizing. I have attempted the method in both Southern
California and western Oregon--with disastrous results in both
locations. What follows in this section is addressed to gardeners
who have already read glowing reports about mulching.
Permanent mulching with vegetation actually does not reduce
summertime moisture loss any better than mulching with dry soil,
sometimes called "dust mulching." True, while the surface layer
stays moist, water will steadily be wicked up by capillarity and be
evaporated from the soil's surface. If frequent light sprinkling
keeps the surface perpetually moist, subsoil moisture loss can occur
all summer, so unmulched soil could eventually become desiccated
many feet deep. However, capillary movement only happens when soil
is damp. Once even a thin layer of soil has become quite dry it
almost completely prevents any further movement. West of the
Cascades, this happens all by itself in late spring. One hot, sunny
day follows another, and soon the earth's surface seems parched.
Unfortunately, by the time a dusty layer forms, quite a bit of soil
water may have risen from the depths and been lost. The gardener can
significantly reduce spring moisture loss by frequently hoeing weeds
until the top inch or two of earth is dry and powdery. This effort
will probably be necessary in any case, because weeds will germinate
prolifically until the surface layer is sufficiently desiccated. On
the off chance it should rain hard during summer, it is very wise to
again hoe a few times to rapidly restore the dust mulch. If hand
cultivation seems very hard work, I suggest you learn to sharpen
your hoe.
A mulch of dry hay, grass clippings, leaves, and the like will also
retard rapid surface evaporation. Gardeners think mulching prevents
moisture loss better than bare earth because under mulch the soil
stays damp right to the surface. However, dig down 4 to 6 inches
under a dust mulch and the earth is just as damp as under hay. And,
soil moisture studies have proved that overall moisture loss using
vegetation mulch slightly exceeds loss under a dust mulch.
West of the Cascades, the question of which method is superior is a
bit complex, with pros and cons on both sides. Without a long winter
freeze to set populations back, permanent thick mulch quickly breeds
so many slugs, earwhigs, and sowbugs that it cannot be maintained
for more than one year before vegetable gardening becomes very
difficult. Laying down a fairly thin mulch in June after the soil
has warmed up well, raking up what remains of the mulch early the
next spring, and composting it prevents destructive insect
population levels from developing while simultaneously reducing
surface compaction by winter rains and beneficially enhancing the
survival and multiplication of earthworms. But a thin mulch also
enhances the summer germination of weed seeds without being thick
enough to suppress their emergence. And any mulch, even a thin one,
makes hoeing virtually impossible, while hand weeding through mulch
is tedious.
Mulch has some unqualified pluses in hotter climates. Most of the
organic matter in soil and consequently most of the available
nitrogen is found in the surface few inches. Levels of other mineral
nutrients are usually two or three times as high in the topsoil as
well. However, if the surface few inches of soil becomes completely
desiccated, no root activity will occur there and the plants are
forced to feed deeper, in soil far less fertile. Keeping the topsoil
damp does greatly improve the growth of some shallow-feeding species
such as lettuce and radishes. But with our climate's cool nights,
most vegetables need the soil as warm as possible, and the cooling
effect of mulch can be as much a hindrance as a help. I've tried
mulching quite a few species while dry gardening and found little or
no improvement in plant growth with most of them. Probably, the
enhancement of nutrition compensates for the harm from lowering soil
temperature. Fertigation is better all around.