Marginality Principle - Principles of Sustainable Soil Management in Agroecosystems - page 33

management zones and profitable zones in a field under a corn-soybean rotation
were associated with poorly drained level soils, while low-yielding zones were
associated with eroded or more sloping soils. Over the past decades, the impact of
landscape-dependent factors causing differences in water availability for growing
crops and impacts on yield have been well documented and demonstrate the negative
impact of soil degradation on soil water holding capacity (Spomer and Prest 1982;
Jones et al. 1989; Wood et al. 1991; McConkey et al. 1997; McGee et al. 1997; Timlin
et al. 1998).
Crop productivity is related to the ability of the crop to extract adequate water and
nutrients from the soil profile. Limitations of rooting depth or inadequate soil water
or nutrients will reduce growth and yield. Marginal soil areas within a field will
vary depending upon the growing season because of the availability of soil water.
Reducing the water holding capacity will have direct effects on crop water stress,
and with the increasing uncertainty in precipitation timing and amounts coupled
with increased atmospheric demand for water, the resulting effect will be an ampli-
fication of the negative effects of marginal soils on crop productivity.
2.5.3 e nvironmental Q uality
Environmental quality is affected as a result of runoff or leaching of nutrients, sedi-
ment, or agricultural chemicals from agricultural fields or landscapes. Rainfall inten-
sity exceeding the infiltration rate of the soil and the soil having limited capacity to
store water, causing leaching through the soil carrying soluble nutrients, are key
sources of reduction in environmental quality. It is difficult to separate environmental
quality impacts from climate impacts because of the linkage between environmental
quality and rainfall events. Under future climate scenarios, Gutowski et al. (2007)
suggested that high-intensity precipitation events would constitute a larger fraction
of total precipitation for all regions and seasons. Across the United States, precipi-
tation patterns have shown regional variability over the past 50 years (Karl et al.
2009). The upper Northwest and Southeast United States have experienced declines
in annual precipitation, while throughout the remainder of the United States, there
has been variation even within individual states (Karl et al. 2009). Projections for
North America for the period from 2040 to 2050 begin to show trends in the seasonal
patterns of precipitation, with more established patterns by the end of the twenty-first
century of increased spring precipitation and reduced summer precipitation (Karl et
al. 2009). The projected decrease in summer precipitation across the United States
into Canada has implications for agriculture since marginal soils with low water
holding capacity are already exhibiting decreases in productivity. Increases in winter
and spring precipitation for the upper portion of the United States and Canada would
have potential environmental impacts because of the increased likelihood of surface
runoff. Changes in precipitation events have already occurred and are expected to
continue to increase throughout the remainder of this century (Kunkel et al. 1999).
The consequences of increases in rainfall intensity show that runoff and sediment
movement from agricultural landscapes will increase (Nearing 2001). Increases in
surface runoff lead to potential increases in sediment transport carrying herbicides
and P from the surface. As one example of the potential impacts of changing climate,