Organic Matter and Sustainability - Sustainable Management of Soil Organic Matter - page 3

quality). Although all components must be satisfied to meet the goal of
sustainable land use, it is recognized that sustainability has a time scale and
that sustainability for any specific land use is relative rather than indefinite
(Smyth and Dumanski, 1995), and in most cases can only be evaluated
from long-term experimentation (Reeves, 1997). Furthermore, the time
scale may differ for each component. Even though loss of sustainability in
any one component would classify the land use system as 'unsustainable',
in reality a tension emerges as short-term economic viability can be
sustained by inputs to the system even when the natural resource base
has exceeded its sustainability time scale. This latter scenario has serious
implications for soil organic matter and the sustainability of intensive farm-
ing systems, as most economic models utilized to describe sustainability in
agriculture often disregard or marginalize the natural resource component
(Farshad and Zinck, 1993).
Sustainability can also be approached using ecological models.
Ecosystem components (e.g. plant biomass, consumer and decomposer
organisms, and soil) are connected by ecosystem processes (e.g. production
and decomposition), that in turn are regulated by driving variables such as
climate, soil type (e.g. texture and mineralogy) and landscape. In intensive
agroecosystems, the main driving variable, or regulator, is the farmer or
society. In contrast to natural ecosystems, agroecosystems are 'open' and
complex with major exports of primary production (i.e. harvest), and inputs
of nutrients and energy, and have additional economical and sociological
components with their own driving variables (e.g. demands of society
for agricultural goods). In a sense, agroecosystems, in contrast to natural
ecosystems, are 'super-systems' where management decisions are often
dominated and controlled by the socioeconomic components. The
dominance of the socioeconomic components may justify continuous
outputs from a system, even when the ecological components of the system
are failing, or excessive inputs (e.g. fertilizer) above that of the natural
storage capacity of the system lead to pollution of adjacent ecosystems.
Similarly to the observations stated above for sustainable land manage-
ment, ecology and economics can be in conflict, leading to a decrease
in soil organic matter and consequently an adverse affect on organic
matter-dependent soil functions.
Concept of Soil Quality
Soil quality involves placing value on a soil in relation to a specific use
(Larson and Pierce, 1994; Carter et al ., 1997). The seminal idea involves
categorizing the 'fitness of a soil for a specific use', but this basic concept is
now enlarged to address those soil functions that allow the soil to accept,
store and recycle water, nutrients and energy. The evaluation of soil quality