7.2: Origin of Salt in Soils
- Page ID
- 44429
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Salt-affected soils are part of the geochemical processes that have continued since ancient geologic time. Soluble salts originate from the weathering of primary minerals in rocks forming the continents. The types of soluble salts depend on the composition of the weathered rock. Normally, salts move from sites of weathering into the groundwater system, eventually moving into streams and then into oceans. The present-day location of salts is dominated by the amount of water that has passed through each point of the hydrologic system. If rainfall is high, as in humid climates, most salts have been transported into oceans or to deep groundwater systems. In arid environments where rainfall is limited, salts are frequently still present in the soil.
Salts accumulate in landscapes having certain relief and geologic conditions. Salt moves with water; thus, saline conditions are linked to lowlands or depressions where water naturally drains and accumulates. Often this situation is associated with restricted internal drainage of the soil, which is conducive to high water table conditions. Salts frequently accumulate in these low areas. Low-lying lands may be relatively small areas in fields or they may be as large as the Great Basin of Utah and Nevada. Drainage water collects at some terminus in closed basins and evaporates. Water in these terminals increases in salt content and, eventually, may lose biological value and become less attractive for recreation.
In addition to weathering, secondary deposits are a major source of saline soils. Throughout geologic history, large portions of the continents have been covered by saline seas. Marine sediments deposited during extended periods of inundation serve as parent material for large areas now devoted to agriculture. These secondary deposits include shales, sandstones, mudstones, and conglomerates. Saline marine shales, for example, are notorious sources of salt. A prime example is the Mancos shale formation that occurs extensively in the upper Colorado River Basin of Colorado, Wyoming, and Utah.
When new lands are developed and brought under irrigation, soils that are prone to salt accumulation are frequently very saline. Before crop production is economically feasible, these salt-affected soils must be reclaimed. The reclamation process, whether it be for saline, sodic, or toxic soils, requires copious amounts of nonsaline water to flush the salts from the intended crop root zone. Frequently, man-made drainage systems are required to augment natural drainage to remove the extra water applied to flush salts from the soil.
Once irrigated lands are in production, the primary source of salt is the irrigation water. The salt introduced into the crop root zone by irrigation is additive to any salt already present in the soil.
An irrigation source contains 500 mg of dissolved salt per liter of water. How much salt is applied to a 50-acre corn field if 15 inches of irrigation water are applied?
Given: Salt concentration of irrigation water (C)
= 500 mg/L = 500 ppm
Depth of irrigation water (da) = 15 in
Field size (A) = 50 ac
Find: Amount of salt applied (W)
Solution
C1 = 0.226 C da (Eq. 5.18)
C1 = 0.226 (500 ppm)(15 in)
C1 =1695 lb / ac
\(W= 1695 \text{ lb/ac } ( 50\text{ ac}) \left(\dfrac{1\text{ ton}}{2,000\text{ lb}}\right)\)
W = 42 tons