Agriculture Reference
In-Depth Information
Time
The climate of the earth has changed over geological time. The most recent large
changes were associated with the alternating glacial and interglacial phases of the
Pleistocene. These phases were accompanied by a rise and fall in sea level, by ero-
sion and deposition, and by corresponding adjustments in Earth's crust, all of
which produced radical changes in the distribution of parent material, the vege-
tation, and the shape of the landscape. On a shorter time scale of only a few thou-
sand years, changes can occur in the biotic factor of soil formation, as shown by
the succession of plant species on a weathering rock surface (fig. 1.1). The relief
factor also changes through changes in slope form and the distribution of ground-
water. During an even shorter period of time, parent material can change as a re-
sult of volcanic eruptions.
The Concept of Steady State.
When the rate of change of a soil property with
time is negligibly small, the soil is said to be in steady state with respect to that
property. For example, for a soil forming on a gentle slope, the rate of natural
erosion may just counteract the rate of rock weathering so that the soil appears to
be in steady state. However, because pedogenic processes do not all operate at the
same rate, soil properties may not attain steady state at the same time. Soil for-
mation is not linear with time. In soils of temperate regions, the accumulation of
C, N, and organic P follows a “law of diminishing returns,” whereas clay accu-
mulation follows a sigmoidal trend (fig. 1.12).
Because colonization of weathering parent material by plants and animals is
an integral part of soil formation, steady state should be considered in the con-
text of the soil-plant ecosystem. Studies of such ecosystems on parent material ex-
posed by the last retreat of Pleistocene ice (ca. 11,000 years B.P.) suggest that a
stable combination of soil and vegetation (in the absence of human intervention)
is achieved in 1,000-10,000 years. The system is stable in the sense that any
changes occurring are immeasurably small in the period since scientific observa-
tion began (ca. 150 years). The soil in such systems is sometimes referred to as
mature
.
1.3.5
Figure 1.12
The approach to steady state
for soil N, organic P, and clay
content under temperate
conditions (White 1997).
Reproduced with permission
of Blackwell Science Ltd.
Steady state
Clay
N
Organic P
2
4
6
8
10
Time (years
×
10
-3
)
