Marginality Principle - Principles of Sustainable Soil Management in Agroecosystems - page 32

water holding capacity, ET, and precipitation) and higher resolution of soil charac-
terization, for example, soil texture, rooting depth, and compaction. The effect of
soil topography was found to be a dominant factor by Kaspar et al. (2003) after they
evaluated corn yields across a field in central Iowa. They observed in growing sea-
sons with less than normal rainfall that there was a negative relationship relative to
elevation, slope, and curvature, while in years with above-normal rainfall, there was
a positive relationship with these terrain attributes. Kumhalova et al. (2011) found
that yield and crop nutrient concentrations of small grains were spatially related to
topography. The relationship between water flow accumulation and grain yield was
strongest in the dry years and weak for the wetter years, similar to observations by
Kitchen et al. (1999) and Kaspar et al. (2003). Observations by Hatfield and Prueger
(2011) on water use differences within fields help explain differences observed in the
other studies on yield variation within fields.
Soil water is a critical factor in plant growth, and any limitation in the soil to sup-
plying optimum amounts of water at critical growth stages induces a limitation to
plant growth, further reinforcing the concept of marginality in soils. Bouman (2007)
addressed this problem in the form of an analysis of crop production systems and
increasing crop water productivity with the overall goal of increasing food produc-
tion and saving water. His approach was based on four principles, which can apply
to maximizing the productivity of marginal soils. These principles were as follows:
“1) increase transpirational crop water productivity; 2) increase the storage size for
water in time and space; 3) increase the proportion of non-irrigation water inflows to
the storage pool; and 4) decrease the non-transpirational water outflows of the stor-
age pool” (Bouman 2007). His principle number two is a definition of a marginal
soil and is related to yield variation as shown in the previous discussion. Since soil
water is related to observed yield variation across fields in central Iowa, management
practices oriented toward increasing crop productivity per unit amount of water will
have to be based on increasing the size of the water storage pool in the soil profile.
In other soils (e.g., claypan soils in central Missouri), where infiltration of water into
the soil is a limitation, emphasis will be on increasing the movement of water into the
storage pools in order to increase water availability. No single solution to increasing
soil water availability in soils is evident; however, understanding the cause of this
marginal response provides an opportunity for modification. Machado et al. (2000)
showed that corn yield variation across a field was due to a combination of biotic
and abiotic factors, and additional soil nitrate-N levels affected yields only when
adequate soil water was available. Once soil water availability is quantified in spatial
and temporal dimensions, the development of management practices to reduce the
impact of these marginal soils can be implemented.
The value of soil water availability on crop yields is reinforced when yield data
are transformed into monetary values. In Missouri, Massey et al. (2008) evaluated
10 years of site-specific yield data for corn, soybean, and grain sorghum ( Sorghum
bicolor L.) across a 36.5-ha field with claypan soils to quantify changes in profit and
loss response. While some areas within the field were profitable almost every year,
other areas of the field showed negative profit most years. Areas of the field with
monetary loss were associated with field areas with significant loss of more than
half of the original topsoil depth. Brock et al. (2005) observed that high-yielding