Marginality Principle - Principles of Sustainable Soil Management in Agroecosystems - page 31

rates (Hatfield 2012). An example of this variation is shown in Figure 2.4. Yield vari-
ation was related to the seasonal and cumulative patterns of soil water use from dif-
ferent soils within these production fields (Hatfield et al. 2007; Hatfield and Prueger
2011). Differences in cumulative water use were as large as 300 mm during the
growing season of a corn crop and as much as 200 mm of transpiration (Hatfield and
Prueger 2011). The primary factor causing these differences was soil water holding
capacity driven by the SOM content of the different soils within the field. Within-
field variations of soil properties have been found to suppress soil water holding
capacity and diminish yield. These factors include soils with high sand and low clay
content (James and Godwin 2003; Sandras et al. 2003; Jiang and Thelen 2004) and
soils with excessive clay content and/or an impervious high argillic horizon (Kitchen
et al. 1999, 2003). Differences across the landscape can be seen in the soil water
holding capacity on eroded side slopes of an eroded Alfisol and can be 50%-60%
less than summit and foot slope landscape positions (Jiang et al. 2008). Over the
course of the growing season, separation in the ET among soils within the field was
most pronounced during the grain-filling period, and the shortage of available soil
water, except in the years with more than adequate rainfall during this period of
the growing season, led to a reduction in water use efficiency (WUE) (Hatfield and
Prueger 2011). Increasing variation in rainfall associated with climate change will
change the seasonal patterns of rainfall and the soil water balance (Equation 2.2).
Yield patterns within fields were dependent upon the total rainfall during the grow-
ing season, with the upper slope soils producing the largest yields in the years with
above-normal rainfall and the lowest yields in years with less than normal rainfall
(Jaynes and Colvin 1997; James and Godwin 2003). Sadler et al. (2000a,b) found
a relationship between soil map units and grain yield from a field scale study on
drought-stressed corn; however, these relationships did not explain the yield vari-
ation within the field. They suggested that improved understanding of yield variation
would require more attention to within-season observations of crop water stress (soil

FIGURE 2.4 Yield variation across a corn production field in central Iowa.