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Salt-affected Soils

Evaporation. Capillary rise. water table. Salt-affected Soils. 1. 通常發生於 semiarid and arid regions ∵annual precipitation< evapotranspiration 2.Natural sources of soil salinity a) Mineral weathering b) Fossil salts c) Atmospheric precipitation 3.Local salt accumulation

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Salt-affected Soils

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  1. Evaporation Capillary rise water table Salt-affected Soils 1.通常發生於semiarid and arid regions ∵annual precipitation< evapotranspiration 2.Natural sources of soil salinity a) Mineral weathering b) Fossil salts c) Atmospheric precipitation 3.Local salt accumulation Soils in low-lying areas

  2. 4.Human Activities • Salts from areas of over-irrigation accumulate in poorly drained areas. • Relatively saline irrigation water are used. • Oil-field development, waste-spreading operation, and crop fertilization

  3. soln. Area A d 1 R Resistance (ohm) A d 灌溉水質 1.Total Dissolved Solids (TDS) 大部分之灌溉用水 TDS < 1000 mg‧L-1 2.Electrical Conductivity (EC) C (conductance) (mho) = C =L L (specific conductance) (mho/cm) 1 S (Siemen)=1 mho 1 dS‧m-1=1 m mho/cm

  4. 3.Water Quality Relations (a) TDS (mg/L) ≈ EC (dS‧m-1) × 640 (b) Sum of cations or anions (m mole (+ or -) ‧L-1) ≈EC (dS‧m-1) ×10 (c) OP (bars) ≈ EC (dS‧m-1) ×(-0.36) (d) Hardness = [Ca 2+ + Mg2+ ] Alkalinity = [HCO3- + CO32- ]

  5. Sodium Hazard 1.Irrigation water (High Na content)  high ESP of soil Soil crust badly and swell or disperse greatly decreasing the hydraulic conductivity or permeability of soil to water 2.為何不以Na concentration直接來表示？ 不能表示出ESP.因為ESP之決定不僅在於Na+在 soln. 之濃度，而是在於Na+和Ca2+、Mg2+爭取exch. sites。 3.SAR(sodium adsorption ratio), U.S. Salinity Lab. Conc. Unit : m moles (+)/L 來表示relative sodium status of irrigation water and soil soln.

  6. ． Conc. Unit : m mole/L ＊ assume only Na and Ca in the system

  7. 所以 ESR (Exchangeable Sodium Ratio) = Exch. cation conc. Unit = m mole (+)/kg KG (Gapon exchange coefficient): The range of KG is commonly 0.010~0.015 (L/m mole)-1/2

  8. ∵ Ca2+ and Mg2+ behave quite similar同理 ESR=KG‧SAR If considering Na+, Ca2+, Mg2+ in the system

  9. Unit : m mole (+)/L

  10.  Increasing SAR of water Bicarbonate Hazard • High HCO3- conc.High pHiron＆other micronutrients deficiency • Residual sodium carbonate (RSC) conc. unit = m moles (+ or - )/L 可用來predict the tendency for calcium carbonate to precipitate from high-bicarbonate water. RSC > 2.5 hazardous 1.25 < RSC < 2.5 potentially hazardous RSC < 1.25 generally safe Empirical but worked reasonably well

  11. 表3.1台灣省灌溉用水水質標準 (67.7.5府建水字第59931號公告) 附註：天然之水質若超過本標準之限值，得不受本標準之限制。

  12. Characterization of Salt-affected Soils • 1.Na-status (i) ESP (Exchangeable Sodium Percent) 直接測定ESP費時麻煩，且常有誤差

  13. (ii) SAR (Sodium Adsorption Ratio)  Soil Soln. 做為 exch.-Na status 之 index Conc. 單位： m moles(+)/L

  14. Effects of salts on soils and plants 1. Main effect on plants—Osmotic Effect Semipermeable membrane Water High salt conc.

  15. Water potential Outside the membrane Inside the membrane High salt Osmotic potential 愈負  Water potential 愈負 Difficult to extract water • Osmotic adjustment Production of organic acids or update of salts 使 osmotic potential愈負  water potential 愈負 然而消耗energy，並發生葉組織細胞不斷之分裂但不增長之現象 Possible to continuously obtain water

  16. 2.Specific-ion effects Plants are sensitive to specific ions in irrigation water or soil soln. Boron, Chloride, ESP Effect 3.Nutritional imbalances e.g. bicarbonate toxicity 是由於high pH引起iron或其他micronutrients之deficiency,而不是bicarbonate直接引起之effect.

  17. Other toxic Solutes B, Li, Na and Cl B (Boron) 缺乏～毒害 之間只相差a few mg/L

  18. Leaching Requirement (LR) Ddw: Amount of drainage water Diw: Amount of irrigation water ECiw: Electrical conductivity of irrigation water ECdw: Electrical conductivity of drainage water 為了維持Salt balance (Salt entering = Salt leaching)所需之Leaching fraction (假設no salt propitiation, no salt dissolved from soil minerals, and no salt taken up by the crop)

  19. Ddw‧ECdw=Diw‧ECiw ECdw (allowable EC of the saturation extract) can be obtained from existing salt tolerance data (Table 9.2). e.g. Soybean ECdw≦ 5.5 ds‧m-1 at 25℃

  20. Reclamation 1 Saline, non-sodic soils Leaching ponding-inefficient basin-furrow method

  21. Reclamation 2 Saline-Sodic Soils ; Sodic soils a) Decrease ESP • application of gypsum (CaSO4‧2H2O) • or tillage (深耕) bring subsoil lime to the surface • 對calcareous-sodic soils 施用S b) Leaching

  22. 3 Sodic soils  High-salt water-reclamations 1st leaching with 600 m moles(+)‧L-1 salt conc. SAR 60 (High-salt water) Soil ESP=40-50 2nd leaching with 1st fourfold dilution of the high-salt water 150 m mole(+) ‧L-1 salt conc., SAR 30

  23. 3rd leaching with 2nd fourfold dilution of the high-salt water 37.5 m mole(+) ‧L-1 salt conc., SAR 15 4th leaching with 3rd fourfold dilution of the high-salt water 9.4 m mole(+) ‧L-1 salt conc., SAR 7.5 • Problem: Drainage facilities need to be adequate to remove large quantities of salty drainage water.

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