b.) Addition of hydrogen

i.) From the decomposition of organic matter
ii.) Roots take up basic cations and exchange them with H
iii.) Acid forming fertilizers

2NH4+ (ammonium) + 4O2

2NO3- (nitrate) + 4H+ (acidic hydrogen) + 2H2O

Estimates of 1.8 to 3.6 lb of CaCO₃ is required to neutralize the acidity generated from 1 lb of NH₄⁺ nitrogen:

Acidity generated from common N sources
Nitrogen Source	lb CaCO3/lb N needed to neutralize the acidity
Anhydrous ammonia	1.8
Urea	1.8
Ammonium nitrate	1.8
Ammonium sulfate	5.4
Monoammonium phosphate	5.4
Diammonium phosphate	3.6

Note:  Acidity is reduced by the volatilization and denitrification of nitrogen. For example, under flooded soil conditions when the residual nitrates in the soil are denitrified, the soil pH value will increase. Nitrate fertilizers containing a basic cation such as Ca, K, or Na will also increase the soil pH value.

c.) Reaction of H with clay

The hydrogen will cause clay particles to break down, further releasing more toxic aluminum.

2.) Forms of Soil Acidity

a.) Definitions:

i.) Active acidity

Refers only to H⁺ and not Al³⁺ in the soil solution

ii.) Exchangeable acidity

• Includes exchangeable Al³⁺
  
• Includes exchangeable H. Usually there is a small amount in acid mineral soils but it is more abundant in organic soils
  
• It is extracted with a neutral unbuffered salt solution, such as KCl, CaCl₂ or NaCl
  

iii.) Non-exchangeable or residual acidity

This is comprised of weak acids not replaced by neutral unbuffered salt solution and H⁺ which bonds with OH^-. This is the type of acidity caused by organic matter and bound Al. Bound Al occurs in soils primarily as Al polymers (long chain compounds) and is denoted as Al (OH)_x^x+

1) Al (OH)xx+ + OH- Al (OH)xx+

2) COOH + OH- COO- + H2O

b.) Cation Exchange Site Saturation

i). Exchangeable Aluminum and Basic Cations

Soil chemists have defined exchangeable cations in acid soils as those cations extracted with a neutral unbuffered salt solution. The sum of these cations is termed the effective cation exchange capacity. A neutral unbuffered salt solution will extract only cations that are held at active exchange sites at a particular pH of the soil. The exchangeable acidity extracted from soils with a neutral unbuffered salt is Al.

Acid mineral soils at pH 5.0 have their active exchange sites occupied by Al and at pH 6.0 these sites are countered by exchangeable bases. The relative cation saturations have an important effect on the cation composition of the soil solution and in turn on plant growth.

Cation composition of the exchange sites as related to the soil pH value.
pH	Al	Ca + Mg + K	CEC
	meq/100g
4.5	0.91	0.20	1.11
5.4	0.34	0.91	1.25
5.9	0.10	1.60	1.70

ii.) Exchangeable Manganese

The exchangeable Mn content of soils is related to soil pH. Highest levels of exchangeable Mn are present when the pH is 5.0 and lower. Exchangeable Mn decreases sharply when soils are limed to pH 6.0.

c.) Soil Solution Composition

i.) Aluminum

The concentration of Al in the soil solution is related to pH of the soil, the Al saturation of effective cation exchange capacity, and the salt concentration of the system. At a pH of 5.5 the concentration of Al in the soil solution is quite low. However, as the pH drops from 5.0 to 4.5, the Al concentration increases markedly.

The Al concentration of the soil solution is related to the Al saturation of the effective CEC of the soil. The concentration of Al in the soil solution is low until the exchangeable Al saturation exceeds 60% and then increases rapidly. When the Al saturation is greater than 60%, the soil solution concentration of Al is greater than 1 ppm and may be as high as 5 or 6 ppm.

ii.) Manganese

Water-soluble Mn content of acid soils is closely related to the soil pH, being high below pH 5.0 but decreasing rapidly as the pH value increases to 6.0.

iii.) Calcium

The predominant cation in the soil solution of most acid soils is Ca. Concentrations of soil solution Ca are increased considerably when acid soils are limed.

d.) Formation of exchangeable Al

Clays with hydrogen ion on the exchange complex are not stable. The aluminosilicate structure decomposes to form Al saturated clays.

Aluminum and basic cation content of different soils. Notice the difference in aluminum saturation even though the pH values are approximately the same.

Soi1	pH	Al	Ca + Mg +K	%Al saturation
		------------ meq/100g ------------
Norfolk	4.5	0.91	0.20	82
Lynchburg	4.6	1.96	0.46	81
Portsmouth	4.7	4.18	3.63	54

Return to Soil Acidity & Liming Training Schedule

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