Chapter 3

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. • 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.

3. 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.

4. “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.

5. 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

6. 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__________________________________________

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

8. 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+.

9. 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

10. 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.

11. 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

12. 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 = [H+][A-]/[HA] (A- conjugate base of the acid) Kb = base-dissociation constant Kb = [OH-][A+]/[OHA] (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

13. 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 Solid AgCl is added to pure water at 25C. Some of the solid remains undissolved at the bottom of the flask. Mixture stirred for 2 days to ensure an equilibrium is reached. Ag+ conc. Determined to be 1.34x10-5M. What is Ksp for AgCl? 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

14. 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.

15. 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

16. 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.

17. 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? Solubility diagram Laboratory exercise, applying P to decrease Al toxicity?

18. 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. Plants split H2O Mangroves Mangroves 2

19. 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.

20. Nutrient Availability

21. 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

22. 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.

23. 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?

24. 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.

25. 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.

26. Lime Requirements Why do we now lime to 6.0? (and not anything above 6.0)

27. 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

28. Amount of potential acidity that needs to be neutralized

29. 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

30. 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.

31. 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

32. 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

33. 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.

35. 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

36. 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.

37. 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)

38. Important considerations to improve success of liming • Soil Testing. • Reliable soil test (representative) • soil pH may be variable in the area (year to year?) (within year?) • Amount of Lime. The buffer index from a soil test serves as a good guide for determining how much lime should be added, • When non-legumes are grown successively in the same field, it is only necessary to apply enough lime to eliminate current and future Al and Mn toxicities. • Lime recommendations for continuous wheat production in Oklahoma are to apply only ¼ the amount required to raise the pH to 6.8. • This recommendation will raise the pH above 5.5 and keep it below 6.5 to minimize the incidence of root-rot diseases. • Occasionally the buffer index for sandy, low organic matter soils will be so high that no lime is recommended. • In these cases a minimum of 0.5 ton ECCE/acre for non-legumes and 1.0 ton ECCE for legumes is recommended to assure the acidity will be corrected and the application is economical. • When lime recommendations are extremely large the amount should be split into an initial application of 5 ton/acre (230lb/1000 ft2) followed by the remainder applied a year later.

39. Considerations • Incorporation and Timing. • Lime must be physically mixed with the soil. • Pastures, perennial plantings, or no-till productions, may require three to five years before the lime causes a noticeable change in soil pH. • Important to lime fields before they are planted to a perennial crop or managed as no-till. • Systems where alfalfa is rotated with a non-legume annuals like corn or wheat, the field should be limed a year before the alfalfa is planted to take advantage of tillage operations related to corn or wheat production and allow more time for lime to react in the soil. • When lime is incorporated well, and there is good soil moisture, it may still take a year or more before noticeable change in soil pH occurs. • Tillage Depth. Lime recommendations are usually made assuming a six-inch tillage depth. • Sandy soils are usually cultivated to eight or ten inches and a proportional increase in the lime rate should be made. • For crops with a shallow root system, such as some vegetables, it may be important to reduce the lime rate to match a shallower depth of incorporation.

40. How can acid soils be managed without liming • Liming Alternative. • Acid tolerant varieties or different plant species. • Karl and Custer are not acid tolerant whereas the variety 2163 is acid tolerant. • Rye more acid tolerant than wheat. • The Al and Mn toxicity that prevent normal seedling root development in wheat can be alleviated by adding phosphate fertilizer in a band with the seed at planting. • Phosphate reacts strongly with Al to form insoluble aluminum phosphate, thus removing Al+++ from solution and the exchange complex. • Rate of 60 lb P2O5/acre is required to obtain normal fall pasture but only 30 P2O5/acre is needed if wheat is managed for grain only. • If P is not deficient, the cost of applying the P for two or three years will usually equal the cost of an application of lime that would have lasted five to eight years. • These alternatives allow normal or near normal production but do not cause a change in soil pH. • Eventually the soil must be limed for long-term production.

41. What are saline soils • Classified as saline when they contain a high enough concentration of soluble salts to interfere with normal growth and development of salt-sensitive plants. • Soluble salts are compounds, like common table salt (NaCl), where ions that make up the salt are weakly bound and have a strong attraction for water. • These ions hold water quite tightly, salty water • (higher boiling point) • (lower freezing point) • Salt is added to water used in food preparation to raise the boiling point and hasten the process. • Salt spread on icy sidewalks and roads to melt ice that would otherwise remain solid at temperatures below freezing. • Soluble salts in soils: soil water is held tightly enough by the ions that plants cannot use it (apparent moisture stress) • Saline soils characteristically remain moist longer than the rest of the field • Occupy poorly drained areas of the landscape • White surface layer of salt after they become dry. • Occur in semi-arid, temperate regions • Saline soils are uncommon in the moisture extremes of deserts and tropical rain forests.

42. Saline Soils • Saturating a soil sample with water (a paste condition) for about four hours, • Extracting the water (and dissolved salts) • Measuring its ability to conduct electricity. • Ions in water allow electricity to pass through it • More ions present the easier electricity is conducted • Conductivity is expressed in mhos/cm. • Conductivity of water is usually very low and expressed as mmhos/cm or micromhos/cm. • Soils are classified as saline when the extract of a saturated paste has an electrical conductivity (EC) equal to or in excess of 4,000 micromhos/cm. • Concentration of soluble salts, expressed as ppm, is roughly equal to 0.65 times the conductivity expressed in micromhos/cm. • Soil with an EC of 4,000 micromhos/cm will contain about 2600 ppm soluble salts in the saturated soil solution. • Saline soils Reclamation • leaching soluble salts out of the soil. • create good surface and internal drainage. • incorporating large amounts of organic matter (create large pores in the surface soil) • Good quality irrigation water can be used to hasten the process. • Deep tillage should be avoided once the organic matter is incorporated • Salt tolerant species like bermudagrass or barley should be planted to provide a vegetative cover • ** Practices to reduce surface evaporation and encourage water movement downward ???

43. What is a Sodic Soil • Abnormally high levels of exchangeable sodium (Na+). • When enough Na+ is adsorbed, clay particles repel each other. • Occurs when the exchangeable Na+ percentage (ESP) is equal to or exceeds 15 • Soil pH of sodic soils will often be above 8. • Dispersed colloids become oriented as water moves into soil and eventually they plug soil pores. • Poor internal drainage resulting in dry subsoil and a moist or wet surface layer. Crops fail because of excess surface water (“drown out”) or for lack of water (dry subsoil) even though there may have been adequate rainfall or irrigation. • Reclaimed by improving surface and internal drainage and incorporating gypsum (CaSO4) in the surface. • Gypsum dissolves to supply a high concentration of Ca++ in soil solution that replaces exchangeable Na+, freeing it to be washed out of the soil • Ca++ helps bind colloids into aggregates and restore soil permeability. Reclamation of sodic soils is similar to that of saline soils except that gypsum must be added to sodic soils.

44. What are Saline-Sodic Soils Contain salts in excess of 4,000 micromhos/cm and exchangeable Na+ in excess of 15 % Have all the features of the saline soil, and if reclamation procedures are used that do not include gypsum, they will become sodic soils when the salts are leached out. Many salt affected soils are saline-sodic because a primary soluble ion is Na+. Reclamation takes several (2 or more) years, dependent upon the time required to get about two pore volumes of good quality water to pass through the soil. Most soils are about 50 % pore space and so a “pore volume-depth” for a four foot profile would be about two feet and two pore volumes about four feet. Sandy soils in high rainfall regions may be reclaimed quite rapidly while clayey soils in semi-arid regions may take many years if rainfall is the only source of leaching water.

45. How Soluble is the Earth’s Crust • The extent to which the earth’s crust dissolves over time depends upon solubility of rocks and mineral, abundance of elements in the rocks and minerals, and rainfall. • Naturally occurring compounds containing either Na or Cl tend to be very soluble and, with time, end up in the oceans and seas of the world.

46. Turf • Does soil acidity increase in turf situations via the application of N • No continual removal of bases like in wheat and corn, thus soil acidity is diminished