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Understanding Soil Acidity

Understanding Soil Acidity. Neutral. Brady and Weil (2002). pH = - log (H + ion concentration). pH = 7. neutral. As pH increases…. As pH decreases…. Brady and Weil, 2002. Optimum pH ranges have been identified for many crops.

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Understanding Soil Acidity

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  1. Understanding Soil Acidity Neutral Brady and Weil (2002)

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

  3. Optimum pH ranges have been identified for many crops

  4. Collective term for the challenges faced by crops growing in acid soils The acid infertility complex

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

  6. Molybdenum becomes more available as pH goes up ! most ^ http://www.farmtested.com/research_pp.html

  7. 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

  8. Multiple forms of soil acidity Soil pH is primarily a measure of active acidity Reserve acidity Active acidity Brady and Weil, 2002

  9. Understanding reserve acidity Very little lime is needed to neutralize the active acidity in soils Reserve acidity resupplies the active acidity ΔpH ΔpH Reserve acidity Active acidity Reserve acidity Active acidity Low CEC soil High CEC soil

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

  11. 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, %

  12. pH dependent charge The dominant clay minerals in IL have mostly permanent charge

  13. The charge on humic substances (and low activity clays) is very pH dependent pH dependent charge H+ ions dissociate when the soil pH increases and reassociate when the pH drops. Brady and Weil (2002)

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

  15. 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 → HCO3- + H+ 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). NH4+ + O2 → NO3- + 2H+ S0 + O2 → SO4-2 + 2H+ 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. VERY IMPORTANT PART OF SOIL FORMATION

  16. 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) which displaces hydroxyl (OH-) groups

  17. Acidity What is liberated and what is left behind when plant biomass is burned ? Oxides of C, N and S Elements that have traditionally been called “bases” Oxides of Ca, Mg and K Alkalinity

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

  19. Sources of pH buffering in soils Carbonates Chadwick and Chorover ( 2001)

  20. K+ H+ The pH of a plant’s rhizosphere changes as the plant regulates its internal charge balance. NO3- ?

  21. Which plant received nitrate ? Which plant received ammonium ? http://departments.agri.huji.ac.il/plantscience/topics_irrigation/uzifert/4thmeet.htm

  22. Acid inputs promote leaching of non-acid cations Why does leaching of these anions cause soil acidification ? Brady and Weil, 2002

  23. Complete N cycle (no net acidification) released into the soil 1H+consumed Nitrification is an acidifying process, right?? 1H+consumed NH3 The 2 H+ produced during nitrification are balanced by 2 H+ consumed during the formation of NH4+ and the uptake of NO3- by plants

  24. Excellent but focused on Australian soils

  25. Standard values for the quantity of lime needed to neutralize the acidity generated by specific N fertilizers Assumes: 1) all ammonium-N is converted to nitrate-N and 2) half of the nitrate is leached.

  26. Harvest of crop biomass removes alkalinity from agricultural fields http://www.ianrpubs.unl.edu/epublic/pages/publicationD.jsp?publicationId=111

  27. Scenario Corn/soybean rotation 200 bushels, 50 bushels All P supplied as DAP N applied as DAP and AA Acidity from N fertilizer 3.6 x 52 lbs of N in DAP required to supply P removed in harvest 1.8 x 150 lbs of N in AA Acidity from grain harvest 25 x 180 lbs of N harvested/100 25 x 200 lbs of N harvested/100 ~ 190 lbs of lime ~ 270 lbs of lime ~ 45 lbs of lime ~ 50 lbs of lime Projected lime requirement ~ 0.3 tons/rotation

  28. Alfalfa field with dead strip where lime was not applied How should lime rates be determined?

  29. Lime rates should be guided by soil testing

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

  31. Sources of variation in soil pH measurements 1. The soil to solution ratio used when measuring pH. 2. The salt content of the diluting solution used to achieve the desired soil to solution ratio. 3. The carbon dioxide content of the soil and solution. 4. Errors associated with standardization of the instrument used to measure pH.

  32. Why measure soil pH using a salt solution ? Water pH > Salt pH Brady and Weil, 2002

  33. Soil pH depends on method used to measure it !! As a result, the method of measurement should be reported whenever soil pH data is discussed.

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

  35. When a soil is limed, Ca+2 from the lime displaces exchangeable acidity from the soil colloids. The active acidity that is generated reacts with the carbonate ions from the lime, producing water and carbon dioxide. H+ Ca+2 soil colloid + CaCO3 soil colloid + H2O + CO2 H+

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

  37. 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)

  38. Not all limestone is the same ! Pure calcium carbonate has a calcium carbonate equivalency (CCE) of 100 and is the standard against which all liming materials are compared. A ton of material with a CCE of 90 % can neutralize 10% less acid than a ton of pure calcium carbonate. Liming materials that are finely ground, have more surface area in contact with the soil solution than coarser ground materials and thus will neutralize soil acidity more rapidly. Fineness of grind is rated according to the percentage of material that will pass through 8-, 30-, and 60-mesh screens.

  39. http://www.agr.state.il.us/news/pub/2007LimeBook.pdf

  40. Page from the 2008 IL Lime book Multiply by these factors

  41. Adjusting for differences in lime particle size distribution

  42. 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

  43. It takes time for lime to react in soil

  44. pH measurements on the fly Soil pH and lime requirement can vary widely within fields

  45. Both past management and inherent soil properties affect soil pH and lime requirement Why is variable rate lime more likely to pay than variable rate N, P or K?

  46. Insufficient lime is applied to neutralize total acid inputs to IL soils South eastern IL has few quarries and the greatest lime deficit http://iah.aces.uiuc.edu/pdf/Agronomy_HB/11chapter.pdf

  47. Barak P, Jobe BO, Krueger AR, Peterson LA, Laird DA 1997. Effects of long-term soil acidification due to nitrogen fertilizer inputs in Wisconsin. PLANT AND SOIL. 197(1): 61-69Abstract:Agroecosystems are domesticated ecosystems intermediate between natural ecosystems and fabricated ecosystems, and occupy nearly one-third of the land areas of the earth. Chemical perturbations as a result of human activity are particularly likely in agroecosystems because of the intensity of that activity, which include nutrient inputs intended to supplement native nutrient pools and to support greater biomass production and removal. At a long-term fertility trial in South-Central Wisconsin, USA, application of ammoniacal N fertilizer resulted in significant increases in exchangeable acidity accompanied by decreases in cation exchange capacity (CEC), base saturation, and exchangeable Ca2+ and Mg2+ . Plant analysis shows that a considerable portion of the alkalinity generated by assimilation of N (and to a lesser extent by S) is sequestered in the above-ground plant parts as organic anions and is not returned to the soil if harvested. Elemental analysis of soil clays indicates a loss of 16% of the CEC. The reversibility of this change is doubtful if the changes are due to weathering of soil minerals.

  48. Summary of common soil fertility problems that rarely occur in soils with pHs between 5.5 and 7

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