Geoscience Reference
In-Depth Information
EC
a
measurements of soils. Consequently, there is a reasonable understanding of what is being
measured, even though the measurement is complicated by the interaction of several soil properties
that influence the conductive pathways through the bulk soil. Another factor is that the mobiliza-
tion of EC
a
measurement equipment is comparatively easy and can be accomplished at a reason-
able cost. Tractor- and all-terrain vehicle (ATV)-mounted platforms have made intensive field-scale
measurements commonplace (Cannon et al., 1994; Carter et al., 1993; Freeland et al., 2002; Jaynes
et al., 1993; Kitchen et al., 1996; McNeill, 1992; Rhoades, 1993). Basin- and landscape-scale assess-
ments are possible with airborne electromagnetic (AEM) systems (Cook and Kilty, 1992; George
and Woodgate, 2002; George et al., 1998; Munday, 2004; Spies and Woodgate, 2004; Williams
and Baker, 1982). However, AEM applications in agriculture have been primarily used to iden-
tify geological sources of salinity, because AEM penetrates well below the root zone to depths of
tens of meters, whereas surface EMI for agricultural applications, such as the Geonics EM38* or
DUALEM-2† electrical conductivity meters, generally penetrates to depths confined mainly to the
root zone (i.e., 1.5 to 2 m). Mobilization made it possible to create maps of EC
a
variation at field
scales, making EC
a
a practical field measurement. Finally, because EC
a
is influenced by a variety of
soil properties, the spatial variability of these properties can be potentially established, providing a
wealth of spatial soil-related information.
2.2.1 M
e a s u R e M e n t
of f
s
of i l
s
a l i n i t y
w i t h
ec
a
The measurement of soil salinity has a long history prior to its measurement with EC
a
. Soil salinity
refers to the presence of major dissolved inorganic solutes in the soil aqueous phase, which consist
of soluble and readily dissolvable salts including charged species (e.g., Na
+
, K
+
, Mg
+2
, Ca
+2
, Cl
−
,
HCO
3
−
, NO
3
−
, SO
4
−2
, and CO
3
−2
), nonionic solutes, and ions that combine to form ion pairs. The
need to measure soil salinity stems from its detrimental impact on plant growth. Effects of soil
salinity are manifested in loss of stand, reduced plant growth, reduced yields, and, in severe cases,
crop failure. Salinity limits water uptake by plants by reducing the osmotic potential making it
more difficult for the plant to extract water. Salinity may also cause specific-ion toxicity or upset
the nutritional balance of plants. In addition, the salt composition of the soil water influences the
composition of cations on the exchange complex of soil particles, which influences soil permeability
and tilth.
Six methods have been developed for determining soil salinity at field scales: (1) visual crop
observations, (2) the electrical conductance of soil solution extracts or extracts at higher than nor-
mal water contents, (3) in situ measurement of ER, (4) noninvasive measurement of electrical con-
ductance with EMI, (5) in situ measurement of electrical conductance with TDR, and (6) multi- and
hyperspectral imagery.
Visual crop observation is the oldest method of determining the presence of soil salinity. It is a
quick method, but it has the disadvantage that salinity development is detected after crop damage
has occurred. For obvious reasons, the least desirable method is visual observation because crop
yields are reduced to obtain soil salinity information. However, remote imagery is increasingly
becoming a part of agriculture and represents a quantitative approach to the antiquated method of
visual observation that may offer a potential for early detection of the onset of salinity damage to
plants. Even so, multi- and hyperspectral remote imagery are still in their infancy with an inability
at the present time to differentiate osmotic from matric or other stresses, which is key to the success-
ful application of remote imagery as a tool to map salinity and water content.
* Geonics Limited, Inc., Mississaugua, Ontario, Canada. Product identification is provided solely for the benefit of the
reader and does not imply the endorsement of the USDA.
† DUALEM, Inc., Milton, Ontario, Canada. Product identification is provided solely for the benefit of the reader and does
not imply the endorsement of the USDA.
