Marginality Principle - Principles of Sustainable Soil Management in Agroecosystems - page 28

Eickhout et al. (2006) stated that over the next 20 years, in order to meet food
demand, there may have to be an additional clearing of forest land for production to
offset the declining soil quality in the current land resource base. They advocated the
need to consider N dynamics in current and future food production systems, increase
our emphasis on N use efficiency, and focus on improvements in agronomic manage-
ment to offset the impacts of soil degradation.
Soil degradation will have an effect on the marginality of soils and change ade-
quate soils to poor soils and excellent soils to adequate soils. With these changes,
there will be negative effects on crop production and crop production efficiency,
rendering more risk into our ability to produce sufficient food, feed, and fiber for an
ever-increasing world population.
2.4.1 S oil m anagement P racticeS
A critical component of soil degradation is the linkage between human soil man-
agement practices and the creation of conditions with potential for increased soil
degradation. Soils in cultivated row crops are particularly vulnerable to loss of veg-
etative cover, residue removal, and tillage systems. Implementation of any tillage
practice compared to a virgin soil or sod may lead to a decrease in the physical qual-
ity attributes of the soil (Reynolds et al. 2007). Reynolds et al. (2007) observed that
converting bluegrass ( Panicum dichotomiflorum Michx.) sod to a corn ( Zea mays
L.)-soybean rotation with moldboard plow (MP) caused the surface soil physical
characteristics—for example, bulk density, macroporosity, air capacity, plant avail-
able water capacity (AWC), and saturated hydraulic conductivity—to decline within
3 to 4 years to levels similar to long-term corn-soybean with MP systems. Compared
to virgin soil and sod systems, even the no-till (NT) system with the corn-soybean
rotation showed declines in soil physical quality. Similar results were found by
Evrendilek et al. (2004) in the Taurus Mountains of the southern Mediterranean
region of Turkey. They evaluated the changes in SOC content and other physical
soil properties over a 12-year period from three adjacent ecosystems, cropland (con-
verted from grasslands in 1990), open forest, and grassland, in a Mediterranean pla-
teau. Conversion of grassland into cropland increased the bulk density by 10.5% and
soil erodibility by 46.2% and decreased SOM by 48.8%, SOC content by 43%, AWC
by 30.5%, and total porosity by 9.1% for the 0- to 20-cm soil depth. They observed
that SOC content was positively correlated with AWC, total porosity, mean weight
diameter (MWD), forest, and grassland, and negatively correlated with bulk density,
pH, soil erodibility, and cropland.
In the upper US Midwest, Olson's (2010) 20-year tillage study quantified the
amount and rates of SOC storage and retention as a result of sod conversion to NT,
chisel plow (CP), and MP tillage systems. The sloping (6%) and eroding soils in the
plot area had been in sod for 15 years prior to the establishment of the tillage treat-
ments. Although the NT and CP plots stored and retained 8.4 and 0.6 Mg C ha -1 more
SOC in the soil than MP, no SOC sequestration occurred in any of the cultivated sys-
tems. The SOC level of the plot area was higher at the start of the experiment (before
tillage) than at the end of the study, with NT plots losing a total of 6.8 Mg C ha -1 ; the
CP lost 15.1 Mg C ha -1 , and the MP lost 15.2 Mg C ha -1 . Olson (2010) concluded that