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Chapter 3

Chapter 3. ACID, SALINE, AND SODIC SOILS. Why study acid, saline, and sodic soils?. Acid, saline, and sodic soils have unique chemical and physical properties that influence how plants grow.

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Chapter 3

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  1. Chapter 3 ACID, SALINE, AND SODIC SOILS

  2. Why study acid, saline, and sodic soils? • Acid, saline, and sodic soils have unique chemical and physical properties that influence how plants grow. • Since availability of nutrient ions is determined by their chemistry, it is important to understand how nutrient availability will be influenced by the special chemical properties of these soils. • What are acid soils? • Acid soils, technically defined, are soils that have a pH less than 7.0, since by convention pH of 7.0 is neutral, above 7.0 is basic (or alkaline) and below 7.0 is acidic. From the standpoint of plant growth, soil management is usually not affected until the pH is less than about 6.2 for legumes and 5.5 for non-legumes. • Understanding the concept of pH is fundamental to understanding and managing acid soils. Since pH is defined as the –log H+ activity, a pH change of one unit (e.g. from pH of 6.0 to pH of 5.0) represents a 10-fold increase in acidity. • .

  3. What is an acid soil? basic = high base saturation Ca++H+ Mg++ K+ K+ Ca++ H+ acid = low base saturation H+ H+ H+ Ca++ K+ Mg++ H+ H+ 80% base saturation 50% base saturation

  4. Why study acid soils? 46% of the Oklahoma tested samples had a pH of <6.0 (PPI 2005). 2007 Wheat Fields: Canadian 5.4 Garfield 5.4 Grant 5.4 Kay 5.7 Median Soil pH Values of OK Counties (all Ag. soils)

  5. Acid Soils • What causes soil acidity? • Acid soils are a natural phenomenon related to soil parent material and rainfall conditions under which the soil developed. • Soils developed from limestone parent material, for example will often be neutral or alkaline in their pH (e.g. pH > 7). • Granitic parent material, on the other hand, will favor development of an acid soil

  6. Acid Soils • Under high rainfall conditions (> 30 inches/year) parent material that is permeable, such as sandstone, will likely become acidic because there is sufficient leaching over geological time (tens and hundreds of thousands of years) to remove even basic materials like limestone. • Rainfall, by nature is slightly acidic because water and carbon dioxide form carbonic acid in the atmosphere (i.e. “acid rain” is normal). Thus, as basic materials are leached out of the parent material, H+ may remain to cause the soil to be acidic. • CO2 + H2O = H+ + HCO3- atmosphere carbonic acid • Two other factors, that contribute to soil acidity, are the removal of basic cations and use of N fertilizers associated with intensive crop production.

  7. Removal of Base Cations • As base cations decrease and soil pH drops, Al+3 saturation increases • Nutrient removal from fields (hay or stubble) • Aluminum in solution goes through stages of hydrolysis and produces acid H+ • Al3+ H20 = Al(OH)2+ + H+ • Al(OH)2+ + H20 = Al(OH)2+ + H+ • Al(OH)2+ + H20 = Al(OH)30 + H+ H+ Ca2+ Ca2+ Mg2+ K+ Ca2+ Ca2+ Mg2+ K+ Ca2+ - - - - - - - K+ Mg2+ H+ K+ H+ Ca2+ Ca2+ Ca2+ Ca2+ Mg2+ H+ Mg2+ K+ Al3+ Al3+ Al3+ Al3+ Al(OH)x insoluble Al3+

  8. The production of acidity during nitrification Nitrification NH4+ NO3- + 2H+ Nitrification of ammonia or ammonium forming fertilizers is a source of acidity in agricultural soils

  9. “Basic” and “Acidic” Cations • The term “basic cations” is used to designate cations that, when combined with hydroxide (OH-) form a compound that would dissolve in water and create an alkaline solution • The cations Na+, K+, Ca 2+ , and Mg 2+ are good examples. In contrast, the hydroxides of Al 3+ and Fe 3+ are so insoluble the ions would not be present in solution unless the solution were acidified to dissolve them. • Al 3+ and Fe 3+ , are usually referred to as acidic ions for this reason. Plants generally absorb nutrient cations in excess of nutrient anions. In this process, electrical neutrality or ion-charge balance may be maintained by simultaneous absorption of OH- or the exudation of H+ by the plant root. • In either case the result is a contribution of acidity to the soil. • Plant uptake of basic cations in excess of anions in a natural, non-agricultural environment contribute little to soil acidity because plants die and recycle the cations in-place. • Intensive agriculture accelerates the acidification because the bases are generally removed from the field with harvest and are not recycled.

  10. Urea NH3 + H2O = NH4 + OH- NH4+O2 = NO3+ 2H Root NH4 0-2 inches NO3 Net H+ addition NO3 H+ OH- NO3 2-6 inches Net OH- addition OH- Assumption: For each NH4 or NO3 take up by the plant 1 H+ or OH-1 will be exchanged. This is not the case.

  11. Plant Uptake and Exchange NO3- OH- NH4+ H+

  12. Intensive agriculture relies heavily on the use of ammoniacal sources of N. These fertilizer materials undergo biological oxidation to NO3- according to the overall general reaction NH4+ + 2O2 NO3- + 2H+ + H2O which produces two protons for every mole of N oxidized

  13. mZE 11H 42He E- elementm – massz - atomic number (# of protons in the nucleus) All hydrogen atoms have one proton__________________________________________11H 21H 31H__________________________________________ stable stable radioactive deuterium tritiummass = 1 mass=2 mass=3no neutron 1 neutron 2 neutrons1 proton 1 proton 1 proton1 electron 1 electron 1 electron__________________________________________126C 136C 146C__________________________________________stable stable radioactivemass=12 mass=13 mass=146 neutrons 7 neutrons 8 neutrons6 protons 6 protons 6 protons6 electrons 6 electrons 6 electrons__________________________________________

  14. What is the nature of soil acidity and soil buffer capacity? Soils behave as a system made up of the salt from a weak acid and strong base. Clay and soil organic matter, provide surfaces for adsorption of cations Clays have a net negative charge resulting from isomorphic substitution of divalent for trivalent ions (Mg 2+ for Al 3+ ) and trivalent for tetravalent ions (Al 3+ for Si 4+ ) within the mineral structure. Soil organic matter contributes to the net negative charge of soils because of dissociated H+ from exposed carboxyl and phenol groups. The cation exchange capacity (CEC) of organic matter is pH dependent, whereas most of the CEC from clays is not. A small contribution to soil CEC is from unsatisfied charges at broken edges of clays. The strength with which cations are adsorbed to cation exchange sites is directly proportional to the product of the charges involved and inversely proportional to the square of the distance between charges (Coulomb’s law). Consequently, the lyotropic series describing the adsorption of cations on clay particles in soils is generally considered being Al 3+ H+ > Ca 2+  Mg 2+ > K+ NH4+ > Na+.

  15. The similarity in strength of adsorption for Al+++ and H+ is because H+, although only 1/3 the charge strength of Al+++, is much smaller in diameter, allowing it to get closer to the internal negative charge of clays than is possible for the larger Al+++. • The electrostatic adsorption of cations on clay and organic matter surfaces creates a reservoir of these ions for the soil solution. The adsorbed ions are in equilibrium with like ions in the soil solution

  16. Soil pH • Relative amounts of each ion adsorbed and in solution varies depending upon their relative concentrations in the soil solution and how strongly the ion is adsorbed (lyotropic series). • Amount of H+ in the soil solution is 1/100th the amount adsorbed on cation exchange sites • We might expect the amount of Ca 2+ and K+ to be present in the soil solution at about 1/50th and 1/10th their amount adsorbed on cation exchange sites • When soil pH is determined, only the H+ in the soil solution is measured. • Soil pH referred to as “active” acidity, whereas the H+ adsorbed on exchange sites is called “potential” or “reserve” acidity. • The buffer capacity of soils, that is, their ability to resist change in pH when a small amount of acid or base is added, is a function of their exchangeable acidic and basic cations. • Soils with low CEC (e.g. sandy, low organic matter) have weak buffer capacity, while soils with high CEC (e.g. clayey, high organic matter) have strong buffer capacity.

  17. Effect of soil acidity on plants • Plant species vary in their response to acidic soil conditions. Those which have evolved and are cultivated in humid regions (e.g., fescue, blueberries, and azalea) tolerate acidic soils better than other species (e.g., bermudagrass and wheat) grown in arid and semiarid climates. • The chemical environment that plants must tolerate, or can benefit from, may be inferred from the relationship of percent base saturation and pH Soil pH

  18. pH and pOH • pH = -log [H+] • pOH = - log [OH-] • pH + pOH = -log Kw = 14 Kw = ion-product constant for water Kw = [H+][OH-] = 1 x 10-14 Ka = acid-dissociation constant Ka = [HA] + H2O [H3O+][A-] (A- conjugate base of the acid) Kb = base-dissociation constant Kb = [A-] + H2O [OH-][A+] (A+ conjugate acid of the base) Ka * Kb = Kw Ksp = solubility-product constant -degree to which a solid is soluble in water -equilibrium constant for the equilibrium between an ionic solid and its saturated solution

  19. Solubility • Solubility of a substance:quantity that dissolves to form a saturated solution (g of solute/L) • Solubility product: Equilibrium constant for the equilibrium between an ionic solid and its saturated solution AgCl  Ag+ + Cl- Ksp = [Ag+][Cl-] At equilibrium, conc of Ag+ = 1.34 x 10-5 conc of Cl- = 1.34 x 10-5 Ksp = (1.34 x 10-5)(1.34 x 10-5) = 1.80 x 10-10

  20. The percentage base saturation identifies the proportion of the CEC that is occupied by cations like Na+, K+, NH4+, Ca 2+ , and Mg 2+ compared to the acidic cations of H+ and Al 3+ . • This relationship is responsible for the fact that deficiencies of Ca, Mg and K are rare in soils with a pH near or above neutral. • Aluminum oxides (Al(OH)3, also expressed as (Al2O3 3H2O) are of such low solubility that Al 3+ usually is not present in the soil solution or on cation exchange sites until the soil pH is less than about 5.5. • The “apparent solubility” product constant (Ksp) for Al(OH)3 in soils is about 10-30. From this, the concentration of Al+++ in the soil solution and its change with change in pH can be calculated.

  21. Aluminum Solving the above at pH of 5, OH- would be equal to 10-9 The concentration of Al+++ (10-3) is moles/liter. Since the atomic weight of Al is about 27, a mole/liter would be 27 grams/liter (g/L) and the concentration of 10-3 is equal to 0.027 g/L, or 27 ppm. 27 ppm at a pH of 5

  22. Solubility Critical to the management and growth of plants in acid soils is the knowledge that Al+++ in the soil solution increases dramatically with decrease in pH below about 5.5. When solved for a soil pH of 4.0 (OH- is equal to 10-10), we have A concentration of 1.0 mole/L is equal to 27 g/L or 27,000 ppm. While there may not be a 1000-fold increase in soil solution Al 3+ concentration when pH changes from 5.0 to 4.0, these calculations should make it clear why Al 3+ concentrations may be significant at pH 4.5, for example, and immeasurable at 5.5.

  23. Al toxicity • Soluble Al is toxic to winter wheat at concentrations of about 25 ppm. • Adverse effect of soil acidity on non-legume plants is usually a result of Al and Mn toxicity. • In winter wheat, Al toxicity inhibits or “prunes” the root system and often causes stunted growth and a purple discoloration of the lower leaves. • These symptoms are characteristic of P deficiency, and are likely a result of the plants reduced ability to extract soil P. • Al toxicity versus P deficiency?

  24. pH preferences of common crops • “pH” is not an essential plant nutrient, and plants obtain their large H requirement from H2O and not H+. • Thus, it is the chemical environment, for which pH is an index, that crops are responsive to rather than the pH itself. • Non-legumes require a soil pH above 5.5 because more acidic soils tend to have toxic levels of Mn and Al present. • Crops which grow well in soils more acidic than this can tolerate these metal ions and perhaps are ineffective in obtaining Fe from less acidic soils. • Legumes usually grow best at soil pH above 6.0 because the rhizobium involved in fixing atmospheric N2 seem to thrive in an environment rich in basic cations.

  25. Soil pH Impacts • It is more than Aluminum toxicity. • Nutrient Availability is greatly influenced by pH • Some herbicides are pH “sensitive” • Physiological impact.

  26. Soil pH 4.1 Soil pH 4.0 Soil pH 4.7 Soil pH 5.1 Soil pH 5.5 Soil pH 6.7

  27. Nutrient Availability

  28. ALS inhibitorsGroup 2 • Imidazolinones • Pursuit • Raptor/Beyond • Sulfonylureas • Maverick • Osprey • Classic • Sulfonylaminocarbonyl-triazolinones • Olympus • Everest • Sulfonanilides • PowerFlex • FirstRate • Python

  29. SUs are more persistent at higher soil pH Glean (chlorsulfuron) Soil pH 7.5 Half-life ≈ 10 weeks Herbicide concentration Soil ph 5.6 Half-life ≈ 2 weeks Frederickson and Shea, Weed Sci. 34:328-332

  30. IMIs are more persistent at lower soil pH Pursuit (imazethapyr) Soil ph 4.6 Soil ph 5.6 Soil ph 6.5 Loux and Reese, Weed Tech. 7:452-458

  31. Effect of soil pH on herbicides • PSII inhibitors—atrazine, Sencor • More persistent at high soil pH

  32. Atrazine is more persistent at higher soil pH 10% control after 2 months No weed control Weed growth 90% control after 2 months Complete weed control Hiltbold and Buchanan, Weed Sci. 25:515-520

  33. How is soil acidity neutralized Most effective way to neutralize soil acidity is by incorporation of aglime. Neutralization of acid soil using aglime (CaCO3) resulting in increasing exchangeable Ca and formation of water and carbon dioxide.

  34. Lime • Aglime is effective because it is the salt of a relatively strong base (calcium hydroxide) and a weak acid (carbonic acid), and is therefore basic • Ca(OH)2 + H2CO3 === CaCO3 + H2O carbonic acid

  35. Lime needed to neutralize soil acidity • Exchangeable acidity must be neutralized in order to change soil pH because it represents most (99 %) of the soil acidity. Since the amount of exchangeable acidity in the soil, at a given pH, depends on the soil CEC, the amount of lime required is a function of clay content, organic matter content, and soil pH. • Lime requirements can be determined directly in a laboratory by quantitatively adding small amounts of a solution of known strength base (e.g. 0.1 normal NaOH), to a known amount of the acid soil mixed with water.

  36. pH and Lime • By measuring pH as the base is added, the amount of base required to obtain any pH can be estimated Buffer index of 6.2 pH scale of 14? Why?

  37. Lime • Direct determination of lime requirement is very time consuming and is not usually done in the routine determination of lime requirement by soil testing laboratories. • Direct determination identifies the amount of base, such as CaCO3, that must be applied if all the acidity is able to react with the base that is added • In practice, this is virtually impossible because of size differences between clay and organic matter colloids (very small) and the finely ground (relatively large) lime particles. • Field studies (calibration) can be conducted to develop the relationship between amounts of aglime identified by direct laboratory titration and crop response.

  38. Lime Requirements • Most soil testing laboratories use an indirect method of determining aglime requirement. • Involves adding a known quantity of a lime-like chemical solution (i.e., buffer solution of pH 7.2) to an acid soil and water mixture. • After equilibrium has been obtained (about two hours) the pH is measured. • This pH is often called the “buffer pH” or “buffer index”. The buffer index, by itself, does not identify how much lime must be added to neutralize an acid soil. • Field studies relating lime additions to soil pH are required to calibrate the buffer index, just as they would be in a direct titration approach.

  39. Lime Requirements

  40. Buffering Capacity • Buffer capacity is a function of CEC (e.g. clay and soil organic matter content). • Amount of lime required to neutralize acidity in a sandy soil (e.g. Meno fine sandy loam) and a fine textured soil (e.g. Pond Creek silt loam) will be quite different even when they have the same soil pH

  41. Amount of potential acidity that needs to be neutralized

  42. How often should lime be applied The answer to this question will depend on how intensively the soil is managed and how large is the soil buffer capacity. For example, the amount of basic cations removed in a 30-bushel wheat crop in grain and straw is shown to be about the same as that removed by a ton of good quality alfalfa hay

  43. Soil will become acidic faster, and require liming more often, if both grain and straw are harvested. • If two fields are yielding at the same level, it might be expected that a sandy soil would need to be limed at lower rates, but more frequently, than a fine textured soil.

  44. Common liming materials • Aglime. Any material that will react with, and neutralize, soil acidity may be considered for use to “lime” an acid soil. The most common liming material is “aglime”, a material that is primarily composed of calcium carbonate, mined from geological deposits at or near the earth’s surface. • Some deposits are high in magnesium carbonate and are called dolomitic limestone. Dolomitic limestone is also a good source of Mg for deep, sandy, acid soils where this nutrient may also be deficient. The mined limestone is usually crushed and sieved to obtain material of a small enough particle size to be effective for aglime. • Quick lime. Mined limestone may be processed to improve its purity and neutralizing strength. The term “lime” was initially used as a name for CaO, which may also be called unslaked lime, burned lime, or quick lime. It may be obtained by heating (burning) calcium carbonate to drive off carbon dioxide. CaCO3 + heat ==== CaO + CO2 Often used for stabilizing sewage sludge. When added to the mixture of sewage solids and water, it quickly reacts to raise the pH above 11

  45. Liming Materials • Hydrated lime. Hydrated lime, which may also be called slaked lime or builders lime, is produced by reacting quick lime with water. CaO + H2O ==== Ca(OH)2

  46. Special Formulations • Liquid lime • Formulated by mixing finely ground limestone with water and a small amount of clay. • Clay is added to help keep the lime particles suspended in the water during application. • Since the solubility of CaCO3 is low, most of the lime is present in solid form and will react like an application of solid lime. The ECCE of the formulation will be much less (depends on how much water was added) than that of the lime used in the mixture, even when the dry lime had a high ECCE. • Typically the dry lime has an ECCE of nearly 100 % and the liquid lime is about 50 % because about ½ of it is water. • Pelleted lime • Pelleted lime is created by compressing, or otherwise forming pellets out of finely ground, good quality CaCO3. • Neutralizing effectiveness of liming materials depends upon being able to maximize their surface contact with soil colloids. • The advantage of liquid lime and pelleted lime compared to conventional aglime is to minimize dust. The disadvantage is they are usually much more expensive, on a cost per ton of ECCE, than conventional aglime.

  47. Industrial by-products. • Kiln dust from cement manufacturing plants, • Fly-ash from coal burning power plants, • Residual lime from metropolitan water treatment plants. • Effectiveness of these materials will depend on particle size and neutralizing strength of the material. Lime from Water Treatment History of Water Treatment

  48. How are the neutralizing values of liming materials compared • Effective Calcium Carbonate Equivalent. • Effectiveness of the aglime identified as effective calcium carbonate equivalent, or ECCE. • Expression of the “active ingredient” of the material for neutralizing soil acidity. • ECCE of liming materials is expressed as a percentage of the material and takes into account the particle size and neutralizing strength of the material • Chemical Equivalence. • Equivalence of compounds relative to their acid neutralizing strength provides insight to their differences in neutralizing strength. • Accomplished by calculating the equivalent weight of a liming material and comparing it to the equivalent weight of CaCO3. • Only possible if the materials are pure chemically. This consideration is of interest, for example, when comparing the effectiveness of dolomitic lime (rich in MgCO3) to that of normal aglime (primarily CaCO3). The equivalent weight of each material is calculated, using the definition: • An equivalent weight is the mass of a substance that will react with one gram of H+, or one mole (6 x 1023) of charge.

  49. Equivalent weights • Equivalent weights are the chemists way of converting “apples and oranges” (etc.), all to apples. • Atomic (or molecular) weight of an ionic species, divided by its charge is equal to its equivalent weight. • For both CaCO3 and MgCO3 the charge of ions involved is two, and one mole of the carbonate ion will neutralize two grams of H+, or two moles of charge. • The molecular weight of CaCO3 is 100 and MgCO3 is 84. • Equivalent weights are ½ their molecular weights, or • CaCO3: 100/2 = 50 • MgCO3 84/2 = 42 • It only requires 42 g of MgCO3 to accomplish the same neutralizing as 50 g of CaCO3, the MgCO3 is 50/42 or 1.19 times more effective than CaCO3. • Applying the same comparison to CaO (eq. wt. 28) and Ca(OH)2 (eq. wt. 37) it is clear that these materials would be required at much lower rates than CaCO3 (eq. wt. 50)

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