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Understanding pH

Understanding pH. pH = - log (H + ion concentration). ?. pH = 7. neutral. As pH decreases…. As pH increases…. Brady and Weil, 2002. Optimum pH ranges have been identified for many crops. Native species also have pH preferences. http://asecular.com/forests/graphics.jpg.

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Understanding pH

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  1. Understanding pH

  2. pH = - log (H+ ion concentration) ? pH = 7 neutral As pH decreases… As pH increases… Brady and Weil, 2002

  3. Optimum pH ranges have been identified for many crops

  4. Native species also have pH preferences http://asecular.com/forests/graphics.jpg

  5. Understanding Soil pH Neutral Brady and Weil (2002)

  6. Do plants actually care about soil pH? The acid infertility complex Problems related to nutrient availability and metal toxicity in acid soils

  7. For most soils and crops, total nutrient availability is optimized between pH 5.5 and 7. Nutrient availability varies with pH

  8. Molybdenum is more available at high pHs. most http://www.farmtested.com/research_pp.html

  9. Understanding aluminum toxicity Toxic forms of Al are bioavailable at low pHs Aluminum toxicity is minimal above pH 5.5 http://www2.ctahr.hawaii.edu/tpss/research_extension/rxsoil/alroot.gif

  10. Crop varieties differ in their sensitivity to Al toxicity Brady and Weil, 2002

  11. Reserve vs. active acidity Soil pH is primarily a measure of active acidity Reserve acidity Active acidity Brady and Weil, 2002

  12. Understanding reserve acidity Very little lime is needed to neutralize the active acidity in soils Reserve acidity resupplies the active acidity BIG pH change Small pH change Reserve acidity Active acidity Reserve acidity Active acidity Low CEC soil High CEC soil

  13. Each charge depicted on this diagram represents 1 centimol of charge per kg of soil K+ - - Ca+2 What is the CEC of this soil ? - - Mg+2 - - H+ Humus H20 H20 H20 Exchangeable acidity exchangeable ions soil solution H20 H20 H20 - Clay - - Al+3 - + 3H2O  Al(OH)3 + 3H+ + - + - K+ SO4-2 What is the “base” saturation ? Ca+2

  14. Each charge depicted on this diagram represents 1 centimol of charge per kg of soil K+ Many people refer to non-acid cations as base cations - - Ca+2 - - Mg+2 - - H+ Humus H20 H20 H20 Exchangeable acidity exchangeable cations soil solution H20 H20 H20 - Clay - - Al+3 + 3H2O ↔ Al(OH)3 + 3H+ - + - + - K+ SO4-2 What is the “base” saturation of this soil? Ca+2

  15. Is pH related to base saturation ? It is probably more accurate to say that pH is related to acid saturation 100 80 60 40 20 0 Acid Saturation, %

  16. Soil acidity increases when H+ producing processes exceed H+ consuming processes. H+ consuming processes H+ producing processes

  17. Many processes add H+ ions to soils 1) Carbonic acid forms when carbon dioxide dissolves in water. H+ ions are released when carbonic acid dissociates: H2CO3 -> H+ + HCO3- 2) Organic acids form during the decomposition of organic matter. H+ ions are released when these organic acids dissociate. 3) Sulfuric and nitric acids form during the oxidation of reduced forms of N and S (e.g., NH4+ from fertilizer, elemental S). 4) Sulfuric and nitric acids form when sulfur oxides and nitric oxides (released into the atmosphere by automobile emissions, industry smoke stacks, volcanoes, forest fires) dissolve in precipitation. H2SO4 and HNO3 are strong acids and fullydissociate in water. 5) Roots release H+ to balance internal charge when cation uptake exceeds anion uptake.

  18. Many processes consume H+ ions in soils • 1) Weathering of most minerals (e.g., silicates, carbonates…) • 2) Decomposition of organic anions • 3) Reduction of oxidized forms of N, S and Fe. • 4) Roots release OH- or HCO3- to balance internal charge when anion uptake exceeds cation uptake. • 5) Inner sphere adsorption of anions (especially sulfate)

  19. Sources of pH buffering Carbonates Lime (CaCO3) ↓ fountain of soil youth? Young soil Old soil Chadwick and Chorover ( 2001)

  20. Acid inputs promote leaching of non-acid cations Brady and Weil, 2002

  21. S and N oxides cause acid precipitation Brady and Weil, 2002

  22. plant uptake mineralization nitrification

  23. Alfalfa field with dead strip where lime was not applied How much lime should be applied ?

  24. Lime requirements should be guided by soil testing

  25. Pocket pH meters can be very useful but require regular calibration !!!

  26. Salt pH vs. water pH Why do labs in arid regions use a salt solution? Brady and Weil, 2002

  27. Sources of variation in soil pH measurements 1. The nature and type of inorganic and organic constituents that contribute to soil acidity. 2. The soil to solution ratio used in measuring pH. 3. The salt content of the diluting solution used to achieve the desired soil to solution ratio. 4. The carbon dioxide content of the soil and solution. 5. Errors associated with standardization of the equipment used to measure pH.

  28. The amount of lime needed to bring about a 1 unit change in pH varies widely between soils

  29. “Illinois method” of determining lime requirement How do you know which line to use ? http://iah.aces.uiuc.edu/pdf/Agronomy_HB/11chapter.pdf

  30. Choosing the right line Line A: Dark colored silty clays and silty clay loams (CEC > 24) Line B: Light and medium colored silty clays and silty clay loams, dark colored silts and clay loams (CEC 15-24) Line C: Light and medium colored silt and clay loams, dark and medium colored loams, dark colored sandy loams (CEC 8-15) Line D: Light colored loams, light and medium colored sandy loams and all sands (CEC < 8) Line E: Mucks and peat (organic soils). Light colored (< 2.5% OM) Medium colored (2.5-4.5% OM) Dark colored (4.5% OM)

  31. Lime requirements determined using the “Illinois method” assume the following: A. A 9-inch tillage depth. If tillage is less than 9 inches, reduce the amount of limestone; if more than 9 inches, increase the lime rate proportionately. In no-till systems, use a 3-inch depth for calculations (one-third the amount suggested for soil moldboard-plowed 9 inches deep). B. Typical fineness of limestone. Ten percent of the particles are greater than 8-mesh; 30 percent pass an 8-mesh and are held on 30-mesh; 30 percent pass a 30-mesh and are held on 60-mesh; and 30 percent pass a 60-mesh. C. A calcium carbonate equivalent (total neutralizing power) of 90 percent. The rate of application may be adjusted according to the deviation from 90. Rates of lime should be adjusted if any of these assumptions are not accurate

  32. It takes time for lime to react in soil

  33. Soil pH and lime requirement can vary widely within fields

  34. Both past management and inherent soil properties affect soil pH and lime requirement

  35. Insufficient lime is applied in IL to neutralize the acidity from N fertilizers http://iah.aces.uiuc.edu/pdf/Agronomy_HB/11chapter.pdf

  36. Have you ever seen a stream look like this ?

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