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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ACID, SALINE, AND SODIC SOILS
atmosphere carbonic acid
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
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
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
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
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.
Laboratory exercise, applying P to decrease Al toxicity?
Plants split H2O
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.
Buffer index of 6.2
pH scale of 14? Why?
Why do we now lime to 6.0? (and not anything above 6.0)
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
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
CaO + H2O ==== Ca(OH)2
Lime from Water Treatment
History of Water Treatment
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.