Soil - Agroecology: The Ecology of Sustainable Food Systems - page 99

8 Soil

Soil is a complex, living, changing, and dynamic compo-
nent of the agroecosystem. It is subject to alteration, and
can either be degraded or wisely managed. In much of
present-day agriculture, with the availability of an array
of mechanical and chemical technologies for rapid soil
modification, soil is all too often viewed primarily as a
growth medium, something from which to extract a har-
vest. Farmers often take the soil for granted, and pay little
attention to the complex ecological processes that take
place below the surface. The premise of this chapter, in
contrast, is that a thorough understanding of the ecology
of the soil system is a key part of designing and managing
sustainable agroecosystems in which the long-term fertil-
ity of the soil is maintained.
The word soil , in its broadest sense, refers to that
portion of the earth's crust where plants are anchored; this
includes everything from the deep soils of a river bottom-
land to a crevice in a rock with a bit of dust and plant
debris. More specifically, the soil is that weathered super-
ficial layer of the earth that is mixed with living organisms
and the products of their metabolic activities and decay
(Odum and Barrett, 2004). Soil includes material derived
from rocks and organic and inorganic substances derived
from living organisms, and the air and water occupying
the spaces between soil particles.
From an agricultural perspective, an “ideal” soil is
made up of 45% minerals, 5% organic matter, and 50%
space, with the “space” filled half with water and half with
air. It is hard to find anything that we can call a “typical”
soil, since each site or location has unique properties that
ultimately determine the final outcome of the soil forma-
tion process.

only about 0.5 to 1.5 t of topsoil per acre is formed annu-
ally in areas of corn and wheat production in the central
Midwest region of the U.S. (Daily, 1995). In contrast,
about 5 to 6 t of soil per acre are estimated to erode from
conventionally farmed land in these areas, with soil losses
often exceeding 15 to 20 t/acre in some years (NRCS,
2005).

F ORMATION OF R EGOLITH

As a whole, the layer of unconsolidated material between
the soil surface and the solid bedrock of the earth below
is called the regolith . The most basic element of the
regolith is its mineral component, made up of soil par-
ticles formed from the breakdown of the bedrock or
parent material. At any particular location, these soil
particles may have been derived from the bedrock below,
or they may have been transported from elsewhere.
Where a soil's mineral particles have been formed in
place from the bedrock below, the soil is a residual soil.
Where the mineral particles have been carried from some
other location by wind, water, gravity, or ice, the soil is
a transported soil .

Physical Weathering

The weathering of rock and rock minerals is the original
source of mineral soil particles, whether the particles
remain in a location or are moved elsewhere. The com-
bined forces of water, wind, temperature, and gravity
slowly peel and flake rock away, accompanied by the
gradual decomposition of the minerals themselves. Water
can seep into cracks and crevices in rock, and with heating
and cooling causing alternating swelling and contracting,
rock begins to fragment. In addition, the carbon dioxide
contained in the water that seeps into cracks can form
carbonic acid, pulling elements such as calcium and
magnesium from the minerals of the rock and forming
carbonates, and in the process weakening the crystalline
structure of the rock and making it more susceptible to
further physical weathering. Finer particles mix with larger
particles, promoted by the physical movement created by
the combined forces of gravity, temperature change, and
alternating wetting and drying. Even the abrasive forces
of rocks against each other during this movement can form
smaller particles. Eventually the unconsolidated regolith
takes form.

PROCESSES OF SOIL FORMATION
AND DEVELOPMENT

Biological processes combine with physical and chemical
processes in each particular climatic region and location
to form soil. Once formed, soil changes and develops due
to these and other biological, physical, and chemical pro-
cesses. With variations in slope, climate, and type of
vegetative cover, many different soils can form in close
juxtaposition with one another, even though the parent
material may be fairly similar.
Natural processes of soil formation and development
take considerable time. For example, it is estimated that

99