Soil Renewal and Sustainability - Principles of Sustainable Soil Management in Agroecosystems - page 481

largely influenced by the chemistry of the parent material making up the plant root-
ing medium (soil), climate, topography, time of weathering, and even the native
plants growing in this soil material. The unique combination of nutrient availability,
physical characteristics of the developing soil, and climate leads to a condition that
typically favors growth of selected plants or plant types over others. Plant character-
istics, in turn, impact soil development through root and shoot contributions to soil
organic matter, soil structure development, soil-plant-water dynamics, and nutri-
ent cycling. Soil (and climate conditions) favoring highly productive plant systems,
prairies, for example, develop with high soil organic matter contents and favorable
soil structure because of massive plant root systems; because of relatively fertile par-
ent material, uptake of nutrients and incorporation in the growing plant tissue, and
efficient nutrient cycling, these soils also tend to develop and maintain highly fertile
conditions if not disturbed . In contrast, less favorable climate and/or soil materials
that have poor physical and/or nutrient conditions (hereafter termed fertility) support
only plants that can tolerate such conditions. These plants typically have low total
production and/or have low nutrient content; as such, these plants typically have low
or little food/feed value.
The synergistic relationship between soil and plants develops very slowly relative
to a human life span. In fact, most of the world's highly productive soils have devel-
oped over thousands of years. When this relationship is disrupted or changed, the
soil development process changes. Not only do development processes change when
agriculture husbandry replaces natural ecosystem processes, soil itself is often heav-
ily manipulated, a process normally unfavorable for sustained productivity. With
these changes occurring, and often leading to soil erosion and alterations of the soil
profile, soil development rates become very important, as once a soil's productive
capacity is reduced, or even lost, new development is key to regaining productivity
levels.
17.6 SOIL RENEWAL RATES
To understand soil renewal rates, the ecosystem, of which soils are a part, must be
understood to the best of our ability. For the past 60+ years, the qualitative method
of understanding soil formation has been based on the soil forming factors proposed
by Jenny (1941): climate, vegetation, relief, parent material, and time. Most of these
factors were first proposed by Dokuchaev (1883) but were developed into a state
factor model by Jenny. This model has been used to provide a broad conceptual
framework for understanding soil formation, but Jenny's original assumption of the
independence of the factors is not generally accepted by soil scientists. However,
these factors set the stage for the processes of additions, removals, transfers, and
transformations outlined by Simonson (1959) to occur. These models have been used
to explain soil properties and horizon differentiation. Subsequently, man was added
as a sixth soil forming factor (Jenny 1980). An objective of this section is to review
rates of soil formation and renewal as they relate to erosion rates, soil loss tolerance,
and sustainability.
Soils are classified and evaluated based on horizons present and their degree of
differentiation. In the US classification system, soil taxonomy, a distinction is made