7.5: Sodicity
- Page ID
- 44432
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)If sodium is the predominate cation adsorbed in the soil, the clay particles in the soil swell and soil aggregates disperse. This deterioration leads to reduced penetration of water into and through the soil. When calcium and magnesium are the predominate cations, the soil tends to have a granular structure that is easily tilled and readily permeable. Excess sodium becomes a concern when the rate of infiltration is reduced to the point that the crop cannot be adequately supplied with water or when the hydraulic conductivity of the soil profile is too low to provide adequate drainage. Sodium may also add to cropping difficulties because of crusting seed beds; temporary saturation of the surface soil; and the increased potential for disease, weeds, soil erosion, lack of oxygen, and inadequate nutrient availability (Hoffman and Shalhevet, 2007).
To assess the sodium hazard of irrigation water, the sodium absorption ratio (SAR) is normally calculated. SAR is defined as:
\(SAR = \dfrac{C_{Na}}{\sqrt{C_{Ca}+C_{Mg}}}\) (7.4)
where ion concentrations (C) are in units of moles of charge per m3 (molc/m3 ) for sodium (Na), calcium (Ca), and magnesium (Mg). Equation 7.4 is valid for soil water under steady-state conditions where the SAR of the irrigation water approximates the SAR of the soil water. The SAR for the soil water under nonsteady-state conditions needs to be adjusted. Figure 7.10 can be used to determine whether an irrigation water will lead to a sodicity problem. If the relationship between the SAR of the irrigation water and its salinity results in a point to the left in Figure 7.10, a sodicity hazard is likely to occur. If the point is between the two lines, a slight to moderate sodicity hazard is likely.
Figure 7.10. Division of waters that cause inadequate water penetration because of chemical conditions (adapted from Rhoades, 1982).
Ionic concentrations are sometimes reported in units of milliequivalents per liter (meq/L). The relationship between the two units frequently used to report ionic concentrations is:
\(\text{mol}_c/\text{m}^3=\dfrac{\text{meq/L}}{\text{valence of ion}}\) (7.5)
where the valence of the ion can be one or more. Recall from chemistry that the valence of sodium is positive one and the valence of calcium and magnesium is positive two.
Water from an irrigation well in Arizona has an electrical conductivity of 0.4 dS/m at 25°C and the concentration of sodium is 33 meq/L. The concentrations of calcium and magnesium are 24 and 8 meq/L, respectively. Determine whether this irrigation water will create a sodicity hazard.
Given: CNa = 33 meq/L
CCa = 24 meq/L
CMg = 8 meq/L
EC = 0.4 dS/m
Find: Will water cause a sodicity hazard?
Solution
\(C_{Na} = \dfrac{(33\text{ meq/L})}{1}=33 \text{ mol}_c/ \text{m}^3\)
\(C_{Ca} = \dfrac{(24\text{ meq/L})}{2}=12 \text{ mol}_c/ \text{m}^3\)
\(C_{Na} = \dfrac{(8\text{ meq/L})}{2}=4 \text{ mol}_c/ \text{m}^3\)
\(SAR=\dfrac{C_{Na}}{\sqrt{C_{Ca}+C_{Mg}}}=\dfrac{33}{\sqrt{12+4}} \)
\(SAR=8.2 (\text{mol}_c/ \text{m}^3)^{1/2}\)
From Figure 7.10, the intersection of lines extended from a SAR of 8.2 and an EC of 0.4 dS/m indicates that water penetration will probably be decreased due to excess sodium.