7: Salinity Management
- Page ID
- 44303
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)- 7.1: Introduction
- All soils and irrigation waters contain salt. In humid areas—soils, surface waters, and groundwaters—are normally low in salinity. Salt concentrations are minimal because rainfall typically exceeds crop water requirements, which results in dilution of any salts in the soil. The excess water normally percolates through the soil flushing salts below the crop root zone.
- 7.2: Origin of Salt in Soils
- 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.
- 7.3: Measurement of Salinity
- Ideally, soil salinity should be measured at the soil water content found in the field. This is not easily done although several methods are now available that operate at field water contents. The most common method of determining soil salinity is by extracting a solution from a soil sample that has been saturated. The procedure begins by taking a soil sample in the field.
- 7.4: Crop Salt Tolerance
- The salt tolerance of a plant is defined as the plant’s capacity to endure the effects of salt. Crop salt tolerance is not an exact value because it depends on many factors. Although salt tolerance cannot be stated in absolute terms, relative crop response to known salt concentrations under typical conditions can be predicted. For a more complete reference on crop salt tolerance see Maas and Hoffman (1977).
- 7.5: Sodicity
- 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...
- 7.6: Toxicity
- Many trace elements, such as cadmium and lithium, are also toxic to plants at very low concentrations. Suggested maximum concentrations for many trace elements are given by Pratt (1973). Fortunately, most irrigation supplies contain insignificant concentrations of these potentially toxic trace elements and are generally not a problem.
- 7.7: Leaching
- Salinity in the crop root zone can be controlled if the quality of the irrigation water is satisfactory and the flow of water through the soil is sufficient. Leaching, the net downward movement of soil water and solutes, is the key to successful irrigation where salts are a hazard. As the salinity of the irrigation water increases or if more salt sensitive crops are grown, leaching must be increased to maintain high crop yields.
- 7.8: Reclamation
- Reclamation of salt-affected soils is frequently required when semiarid or arid lands are first brought into agricultural production; when saline groundwater persists near the soil surface; or when irrigation and rainfall have failed to meet the leaching requirement. The only proven method of reclaiming salt-affected soils is the leaching of accumulated salts down below the crop root zone.
- 7.9: Salinity and the Environment
- Irrigation always degrades water quality and can cause a salinity hazard. Without proper management, the land can become waterlogged and salinized. Regardless of management, drainage water from irrigated lands carries salt that requires disposal. Questions arise as to whether salination is inevitable and if the environment is jeopardized.
- 7.10: Summary
- In regions where rainfall is not adequate to leach salts from the soil, water must be managed to avoid crop losses from excess salinity. Crops differ by nearly a factor of 10 in their sensitivity to salinity. With appropriate management to provide drainage and ensure downward movement of soil water through the crop root zone, crop productivity can be maintained even if salinity is a hazard.
- 7.11: Questions
- Why do large imbalances occur in the distribution of salts in soils? For a clay loam soil how much water must be applied before any significant reclamation will occur if a soil depth of 3 ft is to be reclaimed by continuously ponding water on the soil surface?
- 7.12: References
- Corwin, D. L., & Grattan, S. R. (2018). Are existing irrigation salinity leaching requirement guidelines overly conservative or obsolete? J. Irrig. Drain. Eng., 144(8), 02518001. Hanson, B., & May, D. (2011). Drip irrigation salinity management for row crops. Agriculture and Natural Resources (ANR) Publ. 8447. University of California, Davis. Hoffman, G. J.