7.8.1: Saline Soils
- Page ID
- 44436
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)The amount of water that must leach through the soil profile to remove soluble salts depends primarily on the initial soil salinity level, the technique of applying water, and the soil type. Water suitable for irrigation is normally suitable for reclamation. The relationship between the fraction of the initial salt concentration (Co) remaining in the soil profile (Cf /Co) and the amount of water leached through the profile (dL) per depth of soil (dS) to be leached (dL/dS) when water is ponded continuously on the soil surface can be described by:
\(\left(\dfrac{C_f}{C_o}\right)\left(\dfrac{d_L}{d_s}\right)=K \) (7.9)
where K is a constant that differs with soil type. Equation 7.9 defines the curves in Figure 7.12 for organic (peat) soils where K = 0.45, for fine-textured (clay loam) soils where K = 0.3, and for coarse-textured (sandy loam) soils where K = 0.1. The initial offset at the top of each curve in Figure 7.12 is indicative of the amount of water that must be added to the profile before leaching commences.
Figure 7.12. Depth of leaching water per unit depth of soil required to reclaim a saline soil by continuous ponding (adapted from Hoffman, 1986).
The amount of water required for leaching soluble salts, particularly for fine- textured soils, can be reduced by intermittent applications of ponded water or by sprinkling. The differences in leaching efficiency among the leaching methods (continuous ponding versus intermittent ponding or sprinkling to prevent water ponding on the soil surface) are caused by differences in dispersion and diffusion. The concept of soil pores is useful in visualizing these differences. The amount of solution retained in small soil pores is considerable for saturated soils, as for continuous ponding, and decreases with decreasing soil water content. Consequently, the drier the soil, as with intermittent ponding or sprinkling, the larger the fraction of water flowing through fine pores and the more efficiently the leaching water displaces the saline solution. The reclamation equation for intermittent ponding and sprinkling can be written as:
\(\left(\dfrac{C_f}{C_o}\right)\left(\dfrac{d_L}{d_s}\right)=0.1 \) (7.10)
By intermittent ponding or sprinkling, the effect of soil type is minimal. One disadvantage of intermittent ponding or sprinkling is that the period of time required for reclamation may be extended beyond that required by continuous ponding.
An irrigator has a saline field and wishes to reclaim to a soil depth of 3 feet. The ECe of the soil now averages 10 dS/m and the irrigator desires the final ECe to be 2 dS/m. How much water must be continuously ponded on the soil surface if the soil is clay loam? How much water is needed if the water is applied by sprinkling without saturating the soil?
Given: Clay loam soil, K = 0.3 for continuous ponding and
K = 0.1 for sprinkling
ds = 3 ft
ECo = 10 dS/m
ECf = 2 dS/m
Find: The depth of water to apply for reclaiming the soil by continuous ponding and sprinkling.
Solution
\(\left(\dfrac{C_f}{C_o}\right)\left(\dfrac{d_L}{d_s}\right)=K \)
\(d_L=\dfrac{(K d_s C_o)}{C_f} \)
For continuous ponding:
\(d_L=\dfrac{(0.3 \times 3 \text{ft} \times 10 \text{ dS/m})}{2\text{ dS/m}} \)
\(d_L =4.5 \text{ ft} \)
For sprinkling:
\(d_L=\dfrac{(0.1 \times 3 \text{ft} \times 10 \text{ dS/m})}{2\text{ dS/m}} \)
\(d_L=1.5\text{ ft} \)