Principles of Soil (0604101) 7- Soil Chemical Properties: Soil pH
Prof. Jawad Al-Bakri
[email protected]
Department of Land, Water and Environment Faculty of Agriculture The University of Jordan
Contents of soil pH Topic 1. 2. 3. 4. 5. 6. 7.
What is pH? Soil pH Classification of soil pH Importance of soil pH Measuring soil pH Sources and factors controlling soil pH Managing soil pH 7.1. Managing pH of calcareous and basic soils 7.2. Managing pH of acidic soils
1. What is pH? • pH is defined as the decimal logarithm of the reciprocal of the hydrogen ion activity, in a solution. pH = -log (H+) • pH is a measure of the acidity or basicity of an aqueous solution. • The higher the concentration of H+ in solution, the lower the pH value, the higher is the acidity. • pH ranges from 0 to 14. • pH < 7 means acidic solution or media, pH > means basic solution or media, while pH of 7 is a neutral solution or media. • In arid areas, the term calcareous soils is used to describe basic soils with high carbonate content.
Acid Base Chemistry • Acid-base chemistry is an important part of everyday life. • Hydrogen ions are protons - in water they react (H20) to form hydronium ions, such as H3O+ • Acids can react with metals and other materials. In dilute concentrations, acids are responsible for the sour taste of lemons, limes, vinegar and other substances. • Bases are also very reactive and produce OH- (Hydroxide). The strong base NaOH is used in many household cleaning agents such as oven cleaner and drain clog-remover.
H3O+ depends on the Strength of acid and Initial concentration Of acid
2. Soil pH • Soil pH is a measure of hydrogen ions in the active zone only, not in the reserve zone • Hydrogen associated with the cation exchange (reserve zone) is not measured by the pH meter.
Active zone
Reserve zone
solid
Soil solution
solid
Cation exchange sites
3. Classification of soil pH The United States Department of Agriculture classifies soil pH ranges as follows: Denomination Ultra acid Extreme acid Very strong acid Strong acid Moderate acid Slight acid Neutral Slightly alkaline Moderately alkaline Strongly alkaline Very strongly alkaline
pH range < 3.5 3.5–4.4 4.5–5.0 5.1–5.5 5.6–6.0 6.1–6.5 6.6–7.3 7.4–7.8 7.9–8.4 8.5–9.0 > 9.0
4. Importance of soil pH • Soil pH is the single most important chemical property of the soil (like soil texture is to the physical properties)
1. Knowing the pH of the soil will quickly allow you to determine if the soil is suitable for plant growth and what nutrients will be most limiting. 2. Soil pH is related to CEC and base saturation (next slide) 3. Soil pH has a strong relationship with nutrient availability. 4. Different plants live in different pH ranges.
How is CEC affected by soil pH? OH
OH
OH Broken edge
• CEC increases with increasing pH in pH dependent minerals.
• CEC decreases with decreasing pH in pH dependent minerals
Soil pH and base saturation
Soil pH
% Base Saturation
3.9
0
5.3
25
6.2
50
7.1
75
7.5
90
8.0
100
How is nutrient availability affected by soil pH? 1. The solubility of many plant nutrients decreases with increasing pH, except Mo. 2. The behavior of each element differs from another, however the best range is 6 to 7.5
• High solubility • High plant availability
• Low solubility • Low plant availability
• Notice from the figure that the solubility (availability) of many plant nutrients decreases with increase in pH • This includes the micronutrients: Cu, Zn, Mn, and Fe • Deficiency symptoms of these elements appear on plants grown in soils with high pH • Some elements increase in availability and solubility with increase in pH:; such as Molybdenum
5. Measuring Soil pH • Soil pH is commonly determined by pH meter. • The soil is mixed with water and left for 30 minutes. The sample is filtered and he extract pH is measured.
6. Sources and factors controlling soil pH
• Some soils are high in pH, some are acidic, and others are neutral. The mechanisms and factors that control the pH of a soil are: 6.1 Base Saturation 6.2 Hydrolysis of Calcium Carbonate 6.3 Hydrolysis of Sodium Carbonate 6.4 Hydrolysis of Aluminum The source of these compounds is weathering
6.1 Base Saturation The higher the base saturation, the higher the soil pH
• Notice that base saturation is higher in the soil on the left. This is because there are more cations in the reserve zone on the left. • Because the soil on the left has higher base saturation, it is expected to have higher pH.
6.2 Hydrolysis of calcium carbonate CaCO3 + H2O
Ca2+
+
HCO3-
+ OH-
Hydrolysis of calcium carbonate produces OHOH- in solution increases the pH This reaction can produce a soil pH as high as 8.3, not higher
In calcareous soils, carbonate hydrolysis controls soil pH
6.3 Hydrolysis of sodium carbonate Na2CO3 + H2O
Na+
+
HCO3-
+ OH-
Hydrolysis of sodium Carbonate produces OHThis reaction can produce a soil pH higher than 8.3
Many time the soil pH is higher than 10 These soils are alkaline if the sodium is exchangeable.
Why does the hydrolysis of CaCO3 produce pH 8.3 while hydrolysis of Na2CO3 produces very high pH?
CaCO3
Na2CO3
Slightly soluble produces little OH-
Very soluble Produces more OH-
Lower pH than Na2CO3
Higher pH than CaCO3
Produces calcareous soils
Produces alkaline soils
6.4 Hydrolysis of Aluminum Aluminum compounds becomes soluble at pH below 5. How? As Base Saturation decreases, the concentration of H+ on the exchange sites increases H+ ions bind to the exchange sites of clay minerals
This makes the clay mineral unstable and Al3+ is released into the soil solution Al3+ + H2O
Al(OH)2+ + H+
Aluminum hydrolysis reduces the soil pH to less than 5.5. At this pH, Al3+ is dominating the soil system.
7. Managing soil pH • The best range of soil pH is the slightly acidic to neutral (6.5-7.5). The pH 6.8 is seen as the magic number. • High pH will reduce availability of many nutrients. • The very low pH ( NO3- + H2O + 2 H+ 2. Adding Organic matter (O.M. decomposition)
Organic acids ionized : R-COOH---> R-COO- + H+ OM will increase respiration of bacteria which will also modify pH: CO2 + H2O ----> H2CO3 = H+ HCO3-
7.1. Managing pH of calcareous and basic soils 3. Adding Sulfur and sulfuric acid 3.1. Adding sulfur to soil
S (sulfur)+ O₂ + CO₂ + 2H₂O → H₂SO₄ + CH₂O. 3.2. Adding sulfuric acid with irrigation water
The synthesized sulfuric acid dissolves calcium carbonate and makes gypsum. H₂SO₄ (sulfuric acid) + CaCO₃ (calcium carbonate) → H₂O + CaSO₄ (gypsum)
Gypsum ionizes into Ca²⁺ and sulfate (SO₄²⁻).
Adding sulfur and sulfuric acid to reduce pH around plant roots
7.2. Managing pH of acidic soils
• Acidic soils with pH
