The roots of plants, soil animals, and most soil microorganisms need
to breathe oxygen. Like other oxygen burners, they expel carbon
dioxide. For all of them to grow well and be healthy, the earth must
remain open, allowing air to enter and leave freely. Otherwise,
carbon dioxide builds up to toxic levels. Imagine yourself being
suffocated by a plastic bag tied around your neck. It would be about
the same thing to a root trying to live in compacted soil.
A soil consisting only of rock particles tends to be airless. A
scientist would say it had a high bulk density or lacked pore space.
Only coarse sandy soil remains light and open without organic
matter. Few soils are formed only of coarse sand, most are mixtures
of sand, silt and clay. Sands are sharp-sided, relatively large rock
particles similar to table salt or refined white sugar. Irregular
edges keep sand particles separated, and allow the free movement of
air and moisture.
Silt is formed from sand that has weathered to much smaller sizes,
similar to powdered sugar or talcum powder. Through a magnifying
lens, the edges of silt particles appear rounded because weak soil
acids have actually dissolved them away. A significant amount of the
nutrient content of these decomposed rock particles has become plant
food or clay. Silt particles can compact tightly, leaving little
space for air.
As soil acids break down silts, the less-soluble portions recombine
into clay crystals. Clay particles are much smaller than silt
grains. It takes an electron microscope to see the flat, layered
structures of clay molecules. Shales and slates are rocks formed by
heating and compressing clay. Their layered fracture planes mimic
the molecules from which they were made. Pure clay is heavy, airless
and a very poor medium for plant growth.
Humusless soils that are mixtures of sand, silt, and clay can become
extremely compacted and airless because the smaller silt and clay
particles sift between the larger sand bits and densely fill all the
pore spaces. These soils can also form very hard crusts that resist
the infiltration of air, rain, or irrigation water and prevent the
emergence of seedlings. Surface crusts form exactly the same way
that concrete is finished.
Have you ever seen a finisher screed a concrete slab? First, smooth
boards and then, large trowels are run back and forth over liquid
concrete. The motion separates the tiny bits of fine sand and cement
from denser bits of gravel. The "fines" rise to the surface where
they are trowelled into a thin smooth skin. The same thing happens
when humusless soil is rained on or irrigated with sprinklers
emitting a coarse, heavy spray. The droplets beat on the soil,
mechanically separating the lighter "fines" (in this case silt and
clay) from larger, denser particles. The sand particles sink, the
fines rise and dry into a hard, impenetrable crust.
Organic matter decomposing in soil opens and loosens soil and makes
the earth far more welcoming to plant growth. Its benefits are both
direct and indirect. Decomposing organic matter mechanically acts
like springy sponges that reduce compaction. However, rotting is
rapid and soon this material and its effect is virtually gone. You
can easily create this type of temporary result by tilling a thick
dusting of peat moss into some poor soil.
A more significant and longer-lasting soil improvement is created by
microorganisms and earthworms, whose activities makes particles of
sand, silt, and clay cling strongly together and form large,
irregularly-shaped grains called "aggregates" or "crumbs" that
resist breaking apart. A well-developed crumb structure gives soil a
set of qualities farmers and gardeners delightfully refer to as
"good tilth." The difference between good and poor tilth is like
night and day to someone working the land. For example, if you
rotary till unaggregated soil into a fluffy seedbed, the first time
it is irrigated, rained on, or stepped on it slumps back down into
an airless mass and probably develops a hard crust as well. However,
a soil with good tilth will permit multiple irrigations and a fair
amount of foot traffic without compacting or crusting.
Crumbs develop as a result of two similar, interrelated processes.
Earthworms and other soil animals make stable humus crumbs as soil,
clay and decomposing organic matter pass through their digestive
systems. The casts or scats that emerge _are crumbs._ Free-living
soil microorganisms also form crumbs. As they eat organic matter
they secrete slimes and gums that firmly cement fine soil particles
together into long lasting aggregates.
I sadly observe what happens when farmers allow soil organic matter
to run down every time I drive in the country. Soil color that
should be dark changes to light because mineral particles themselves
are usually light colored or reddish; the rich black or chestnut
tone soil can get is organic matter. Puddles form when it rains hard
on perfectly flat humusless fields and may stand for hours or days,
driving out all soil air, drowning earthworms, and suffocating crop
roots. On sloping fields the water runs off rather than percolating
in. Evidence of this can be seen in muddy streams and in more severe
cases, by little rills or mini-gullies across the field caused by
fast moving water sweeping up soil particles from the crusted
surface as it leaves the field.
Later, the farmers will complain of drought or infertility and seek
to support their crops with irrigation and chemicals. Actually, if
all the water that had fallen on the field had percolated into the
earth, the crops probably would not have suffered at all even from
extended spells without rain. These same humusless fields lose a lot
more soil in the form of blowing dust clouds when tilled in a dryish
state.
The greatest part of farm soil erosion is caused by failing to
maintain necessary levels of humus. As a nation, America is losing
its best cropland at a nonsustainable rate. No civilization in
history has yet survived the loss of its prime farmland. Before
industrial technology placed thousands of times more force into the
hands of the farmer, humans still managed to make an impoverished
semi-desert out of every civilized region within 1,000-1,500 years.
This sad story is told in Carter and Dale's fascinating, but
disturbing, book called _Topsoil and Civilization _that I believe
should be read by every thoughtful person. Unless we significantly
alter our "improved" farming methods we will probably do the same to
America in another century or two.