At the end of this section, you should know: 1. Factors which cause soil acidity. 2. The different forms of acidity, e.g., active and exchangeable. 3. The soil solution composition of acidic soils. 4. The effects of exchangeable aluminum on the soil solution composition.
IntroductionSoils of the humid regions have developed under conditions in which rainfall exceeds evapotranspiration during most of the year. Under these conditions there has been a gradual depletion of soil bases (such as Ca, Mg and K) and the development of soil acidity. The soil clays often contain coatings of Fe and hydroxy Al. These materials significantly affect the retention and availability of fertilizer cations and anions in acid soils. The capacities of these soil materials to fix P, Mo, S, and B are influenced by liming.
Acid mineral soils at pH 5.0 and lower often contain appreciable amounts of Al and Mn in the soil solution which are detrimental to plant growth. Optimum growth and efficient use of fertilizer nutrients in acid soils require the addition of lime to eliminate the toxic effects of Al, H and Mn.
1.) Origin of Soil Acidity
a.) Removal of Ca and Mg
i.) Rainfall in excess of evapotranspiration (leaching) removes Ca and Mg from the soil. Soils of humid regions usually contain little weatherable Ca and Mg minerals. ii.) Removal by crops
Calcium and magnesium composition of some crops.
|lbs/ton of crop|
i.) From the decomposition of organic matterEstimates of 1.8 to 3.6 lb of CaCO3 is required to neutralize the acidity generated from 1 lb of NH4+ nitrogen:
ii.) Roots take up basic cations and exchange them with H
iii.) Acid forming fertilizers
2NH4+ (ammonium) + 4O2 2NO3- (nitrate) + 4H+ (acidic hydrogen) + 2H2O
Acidity generated from common N sources Nitrogen Source lb CaCO3/lb N
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 clayThe hydrogen will cause clay particles to break down, further releasing more toxic aluminum.
2.) Forms of Soil Acidity
i.) Active acidityRefers 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)xx+
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 ManganeseThe 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
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.) ManganeseWater-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.) CalciumThe 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 AlClays 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
------------ 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