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