soil reaction soil reaction is one of the most important soil characteristics soil reaction...
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Soil Reaction
Soil reaction is one of the most important soil characteristics
Soil Reaction Expression In units of activity (concentration) of
hydronium ions (H3O+) ions – cH3O+
[mmol (meq) / 100g of soil ]
In pH units = -log(cH3O+)
Types of Soil Reaction
Actual Soil Reaction → Active Reaction pHH2O
Potential Soil Reaction → exchangeable soil reaction pHKCl, E.A.
→ Hydrolytic acidity
Ha Ability of soil to change pH of hydrolytically
fissile salt solutions
Soil Reaction
Methodology of pHKCl a E.A. determination (exchangeable soil reaction)
Weight 40g of soil + 100ml 1M KCl Let the soil with KCl shake for 45 min. Measure the pHKCl value by the pH-meter
pHKCl Evaluation
Strongly Acid < 4.5
Acid 4.5 – 5.5
Weakly Acid 5.5 – 6.5
Neutral 6.5 – 7.2
Alkaline >7.2
Soil Reaction
Methodology of pHH2O determination Weight 10g of soil + 20ml overcooked distilled water Mix the soil and water for 5 min using the glass straw Measure pHH2O value by the pH-meter
Strongly Acid < 4.9
Acid 4.9 – 5.9
Weakly Acid 5.9 – 6.9
Neutral 6.9 – 7.1
Slightly alkaline 7.1 – 8.0
Alkaline 8.0 – 9.4
Strongly Alkaline >9.4
Soil Reaction
Soil Reaction
Acid sulphate soils
Forest Soils
Cultivated soils of temperate zoneCalcareous soils
Solonchaks
0
The specifics of the measurement of soil reaction
The measurement takes place in soil suspension (not in soil solution)
H3O+
Soil Particles
SOLUTION SUSPENSION
Soil Reaction
Types of Soil Reaction
Actual Soil Reaction → Active ReactionConcentration of H3O+ ions in soil suspension
H3O+
Soil Particles
Water
pHH2O
Soil Reaction
Types of Soil Reaction
Potential Soil Reaction → exchangeable soil reactionAbility of soil to change pH of neutral salt solutions
H3O+
Soil Particles
KCl solution
K+
pHKCl
E.A.
Soil Reaction
Types of Soil Reaction
Actual Soil Reaction → Active Reaction pHH2O
Potential Soil Reaction → exchangeable soil reaction pHKCl, E.A.
→ Hydrolytic acidity
Ha Ability of soil to change pH of hydrolytically
fissile salt solutions
Soil Reaction
Soil Reaction Measurement Colorimetrically (Color indicators) Titration Potentiometrically (Specific and
comparative glass calomel electrode)
Soil Reaction
Influences on Soil Reaction Measurement The degree of dissociation - (how many groups
capable of dissociation are present on the surface of solid particles)
Kind of Acidity → free H3O+
→ Reserve acidity - hydrolysis of Al3+
Al3+ + H2O → AlOH2+ + H+
AlOH2+ + H2O → Al(OH)2+ + H+
Al(OH)2+ + H2O → Al(OH)3
+ + H+
Soil Reaction
Influences on Soil Reaction Measurement The degree of dissociation Kind of Acidity → free H3O+
→ hydrolysis of Al3+
Ratio soil : water - (the wider soil : water ratio, the higher measurement error)
Salt content - (salt displaces H+ from the soil sorption complex)
CO2 content - (the higher CO2 content, the lower soil pH)
Soil Reaction