Agriculture Reference
In-Depth Information
How Does Soil Form?
Even where grapevines appear to be growing in rock, we know that initially a soil,
no matter how meager, was present because when a rock is first exposed to weath-
ering, a soil begins to form. Figure 1.1 shows a very early stage of soil formation
on granite rock.
Soil is distinguished from weathered fragments of rock by the presence of
living organisms and organic matter. An assortment of inorganic minerals inher-
ited from the parent rock forms the structural framework—solid material and
spaces—within which lives a diversity of organisms. Life in the soil is a struggle,
but the cycle of growth, death, and decay is essential for healthy soil function
in cultivated fields, forests, grasslands, and vineyards. Organisms ranging in size
from bacteria, invisible to the human eye, to fungi, insects, earthworms, and bur-
rowing animals, feed on the residues of plants and other organisms and become
themselves food for subsequent generations (see “The Soil Biomass,” chapter 5).
Provided the developing soil is not unduly disturbed, the processes of soil forma-
tion result in a growing depth of soil that often exhibits a characteristic profile.
Box 1.1 presents a general description of a soil profile.
Although complex in terms of the internal processes of mineral weathering,
percolating rainwater, exchange of gases, and the cycle of life, the formation of
soil in the example of figure 1.1 tells us little about why soils are so variable in the
Figure 1.1
A thin mantle of soil forming on granite rock with early-colonizing plants.
The scale is 10 cm.
