Soils 205-90

Lecture 11- Alteration of pH and Salt affected soils

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1.  Alteration of pH

A. Intensifying Soil Acidity

1. Acid-loving plants

· Rhododendrons, blueberries, pine,... (figure 9.19)

2. Acid-forming organic materials - pine needles, bark, peat, ...

3. Ferrous sulfate - FeSO₄

· both acid and Fe

Fe⁺² + 2 H₂O ¬¾® Fe(OH)₂ + 2 H⁺

Fe ⁺² ¾® Fe⁺³ ¬(+ 3 H₂O)® Fe(OH)₃ + 3 H⁺

· also, Al₂(SO₄)₃

4. Elemental S - S^o

2 S^o + 3 O₂ + 2 H₂O ¾® 4 H⁺ + 2 SO₄^-2

 

B. Liming Materials

1. Carbonate forms

(a) "limestone" deposits and industrial byproducts

(b) calcite = (CaCO₃) = calcium carbonate

and

dolomite = CaMg(CO₃)₂

(c) dolomitic limestone maintains Ca:Mg balance

2. Oxide and Hydroxide forms

(a) oxides formed by heating limestones

CaCO₃ ¾(heat)® CaO + CO₂

calcite

gas

burned lime or quicklime

(b) add water to oxides to form hydroxides

CaO + H₂O ¾¾® Ca(OH)₂

hydrated lime

(c) more reactive = more rapid pH change

 

C. Lime Reactions in Soil

1. Neutralize acidity

2H-X + CaCO₃ ¾¾® Ca-X + CO₂ + H₂O

acid ¾¾¾¾¾¾¾® base and gas

2. Base Saturation Percentage increases

3. Soil pH increases

4. Al solubility decreases

Al⁺³ + 3 OH^- ¾¾¾® Al(OH)₃

soluble	insoluble
(toxic)	(not toxic)

increasing pH ¾®

D. Composition Guarantees

1. Since mixtures Þ how express contents?

ü elemental Ca & Mg
ü total carbonates (CaCO₃ & MgCO₃)
ü oxides (CaO & MgO)
ü CaO equivalents
Ü CaCO₃ equivalents

2. Conversion between forms

® use ratio of molecular weights

mol. wt. of desired form
------------------------------
mol. wt. of known form

examples:

1. What is the calcium carbonate equivalent of adding 10 tons per acre of MgO?

(10 T MgO/A)(100 CaCO₃/40 MgO)

= 25 T/A CaCO₃ equivalent

2. How many grams of an ash containing 35 % Ca would you have to add to a 100 kg container of soil to meet a lime requirement of 55 grams calcium carbonate equivalent?

(55 g CaCO3/100 kg soil)(40 Ca/100 CaCO3)(100 g ash/35 g Ca)

= 63 g ash/100 kg soil

E. Lime requirement

If BSP is 20 % and CEC is 14 cmol_c/kg, ?? lime to raise BSP to 80 %?

(80 - 20/100)( 14 cmol_c/kg soil)(mol_c/100 cmol_c)(mol CaCO₃/2 mol_c)
(100 g CaCO₃/mol CaCO₃)

= 4.2 g CaCO₃/kg soil

(kg soil/1000 g soil)(454 g soil/lb soil)(lb CaCO₃/454 g CaCO₃)
( 4 x 10⁶ lb soil/A ft)( T CaCO₃/2000 lb CaCO₃)

= 8.4 T CaCO₃/A ft

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