Agriculture Reference
In-Depth Information
4.2 GRAZING IMPACTS ON SOIL QUALITY
Soil structure and biological health have been directly linked to food production,
food security, and environmental quality (i.e., air and water quality, climate stabil-
ity). Unfortunately, moderate to severe degradation of soils (i.e., loss of soil biodi-
versity, poor soil structure, and loss of nutrients) has occurred in many agricultural
settings as a result of heavy human demands. The pressure to produce as much food
and fiber as possible is high, often without sufficient attention to conserving and nur-
turing resources and processes fundamental to primary production, i.e., soil, water,
carbon, and air, and the biophysical processes that control the stocks and flows of
these. Reports on the state of land suggest that soil sediment, nutrients, and organic
matter stocks are being degraded at rates far exceeding a sustainable level. This has
had enormous direct and indirect consequences on the profitability, productivity,
and quality of agroecosystems worldwide (National Research Council [NRC] 1993;
United States Department of Agriculture-Natural Resources Conservation Service
[USDA-NRCS] 1996; FAO 2011).
Scientific assessment of soil quality is essential to monitoring the sustainability of
agricultural systems. Soil quality is a complex subject, encompassing the many valu-
able services humans derive from soil and the many ways soils influence terrestrial
ecosystems. Different definitions of soil quality have been proposed, each reflecting
a different perspective on the use and value of soils:
•
Potential utility of soils in landscapes resulting from the natural combina-
tion of soil chemical, physical, and biological attributes (Johnson et al. 1992)
•
Capability of soil to produce safe and nutritious crops in a sustained manner
over the long term, and to enhance human and animal health, without impair-
ing the natural resource base or harming the environment (Parr et al. 1992)
•
Capacity of a soil to function within ecosystem boundaries to sustain bio-
logical productivity, maintain environmental quality, and promote plant
and animal health (Doran and Parkin 1994)
•
Capacity of soil to function (Karlen et al. 1997)
•
How well soil does what we want it to do (Schjønning et al. 2003)
Soil quality can be determined from a variety of soil properties or processes (i.e.,
indicators), the selection of which may be partially dependent on land use. Indicators
of soil quality will reflect important soil functions (Magdoff and Weil 2004), including
•
Producing vigorous and healthy plants
•
Cycling and retaining globally important nutrients, e.g., (i) storing nitrogen
in soil and releasing it to roots for efficient plant production and (ii) storing
carbon in soil and releasing it to the atmosphere in a dynamic balance that
stabilizes atmospheric concentration of CO
2
•
Supplying plants with water, nutrients, and plant growth-promoting compounds
•
Protecting water quality (both groundwater and surface water) from nutri-
ent and pathogenic contamination
•
Providing physical stability and support for vegetation, buildings, and roads
