Sustainable Soil Management Is More than What and How Crops Are Grown - Principles of Sustainable Soil Management in Agroecosystems - page 340

defining the principles and practices of sustainable agriculture (Warkentin 1995). To
identify the causes of agricultural soil degradation, it is necessary to agree on signs
that clearly characterize this phenomenon and its degree. However, the “definition”
of what is considered soil degradation has been regarded as a rather relative term,
because an objective or quantitative evaluation of the evolution of soil quality and
productivity is quite a complex undertaking. Further, similarly to what has been
proposed by many authors with regard to the process of soil erosion (Verheijen et
al. 2009), the extent of soil degradation, which may be considered “acceptable” or
“tolerable” (i.e., which is not understood as such), is far from being clear.
Agricultural soil degradation is generally understood as loss in the quality or pro-
ductivity of soil as a result of human activities, leading as a consequence to less pro-
ductivity or even its abandonment for agricultural use. In the Guidelines for General
Assessment of the Status of Human-Induced Soil Degradation (Oldeman 1988), the
different forms of human-induced soil degradation are distinguished comprehen-
sively between two main categories: (1) displacement of soil material through water
and wind erosion and (2) chemical and physical deterioration, such as depletion of
soil nutrients and organic matter, salinization, acidification, and pollution, but also
compaction, sealing and crusting, truncation of the soil profile, or waterlogging.
Despite this distinction between the two categories, there is a strong relationship
between them once occurrence and degree of soil displacement are appreciated as
being a consequence of chemical and physical deterioration of the soil. In addition,
both categories of agricultural soil degradation may lead to severe off-site effects
such as sedimentation of reservoirs, harbors, or lakes; flooding; riverbed filling and
riverbank erosion; and eutrophication of water bodies.
In these earlier definitions and descriptions of agricultural soil degradation, soil is
treated mainly as a physical entity. In reality, however, a productive agricultural soil
is a living system in which biological processes carried out by soil microorganisms
and mesofauna are key elements in the creation, maintenance, and enhancement
of soil health and its productive capacity. Soil health represents the soil's physical,
chemical, hydrological, and biological status and its ability to respond to agricul-
tural production inputs and to climatic variability including extreme weather events.
For example, soil physical and chemical characteristics such as soil structure and
porosity, soil aeration, water infiltration and drainage, soil water and nutrient hold-
ing capacity, total exchange capacity, and pH are greatly influenced by soil biologi-
cal properties such as soil organic matter (SOM) turnover and the dynamics of soil
biodiversity, which has an intimate relationship with plant roots, affecting its pheno-
typic expression below and above the ground. Deterioration of soil biological health,
and consequent loss in soil productive capacity, is often not given much prominence
in agricultural soil management and degradation research or in farming system man-
agement. Thus, the role of soil microorganisms and mesofauna and the SOM they
require in order to function effectively and self-sustainably in the maintenance of soil
health and the important role they play in crop phenotypic expression and crop per-
formance are overlooked. This includes diverse kinds of symbiotic relationships that
exist between soil biodiversity and plants about which we know very little (Uphoff et
al. 2006), presumably because of difficulty in establishing, through scientific experi-
mentation, the causal relationships with productivity and ecosystem services.