Marginality Principle - Principles of Sustainable Soil Management in Agroecosystems - page 30

years) was to reduce the depth of the topsoil in the upper hillslope positions by at
least 20 cm.

2.5 SCALES OF VARIATION
One of the critical questions scientists must address is how to assign a value to mar-
ginality in soils. If we maintain that marginality represents a loss of soil function,
then any change from optimal function will have a negative impact. This concept
and its implications can be illustrated by examining soil conditions in the context of
water availability, crop productivity, and environmental quality, which determine the
value of a degraded soil.

2.5.1 W ater a vailaBility
Water availability within soils is a function of the texture, SOM content, and depth. As
the texture changes from sand to clay, there is an increase in the AWC, and these values
can be found in any US county-level soil survey. There can be as much as a threefold dif-
ference in soil water holding capacity as the texture decreases from sand to clay soils. As
Hudson (1994) pointed out, there is an increase in the available soil water with increas-
ing organic matter content. Finally, the depth of the soil has a large influence since total
available water depends upon the cumulative water holding capacity of the different soil
layers. Water is central to the proper functioning of all biological activity, for example,
plants, microbes, insects, and soil animals, and without water, these life forms would
cease to exist. Water availability is complicated by the seasonality of precipitation and
ET. If we consider that winter months have low ET rates, there is less water use from the
soil compared to the summer, when ET is higher, causing the apparent effect of variable
precipitation to be less noticeable on water availability.
Water availability impacts biological activity in the soil, and degraded soils will
have a reduced biological activity. A method to evaluate water availability is to
examine the soil water balance, as shown in Equation 2.2:

SW i = SW i −1 + P i − E i − R i − L i + I i (2.2)

where SW i is the soil water content on day i , SW i -1 is the soil water content on the
previous day, P i is the precipitation on day i , E i is the evaporation on day i , R i is the
runoff on day i , L i is the leaching from the bottom of the soil layer on day i , and I i
is the irrigation water applied on day i . This simple soil water balance model shows
the impact of a degraded soil on the ability to maintain soil water content without
increasing leaching or runoff during many precipitation events. This aspect of a
marginal soil allows for further development of the subsequent discussion of crop
productivity and environmental quality.

2.5.2 c roP P roDuctivity
Water availability has a greater source of variability in corn ( Zea mays L.) yields
across different production fields in the Midwest United States than N application