Soil Reaction Soil reaction is one of the most important soil characteristics Soil Reaction Expression  In units of activity (concentration) of hydronium ions (H 3 O + ) ions – c H3O+ [mmol (meq) / 100g of soil ]  In pH units = -log(c H3O+ )

Types of Soil Reaction  Actual Soil Reaction → Active Reaction pH H2O  Potential Soil Reaction → exchangeable soil reactionpH KCl, E.A. → Hydrolytic acidity Ha Ability of soil to change pH of hydrolytically fissile salt solutions Soil Reaction

Methodology of pH KCl 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 pH KCl value by the pH-meter  Filter the soil suspension  Pipette 50 ml of the soil extract (exactly) into the titration flask, add 3 drops of phenolphtalein  Titration by 0.02M NaOH until a light pink color  Note the NaOH consumption (a) and calculate the E.A. value pH KCl Evaluation Strongly Acid< 4.5 Acid4.5 – 5.5 Weakly Acid5.5 – 6.5 Neutral6.5 – 7.2 Alkaline>7.2 Soil Reaction E. A. Evaluation (mmol.100g -1 ) Very Strong> 1.14 Strong 0.57 – 1.14 Medium0.4 – Moderate0.23 – 0.4 Weak< 0.23

E. A. Calculation E. A. = a x f x M x 5 x 1.75 [meq / 100g of soil ] aConsumption of NaOH for titration fFactor of NaOH MNaOH molarities 5Conversion to 100g of soil 1.75Constant of the incomplete displacement Soil Reaction

Calculation of Lime Requirement LR = E.A. x E x ρ d x A x Td x [t] E.A.Exchangeable acidity [meq / 100g] EEquivalent of corrective (CaCO 3 – 50; CaO 28) ρ d Bulk density [1,500 kg / m 3 ] AArea of liming [10,000m 2 ] → t/ha TdTopsoil depth [0.2 m] Conversion to tons Soil Reaction

Methodology of pH H2O determination  Weight 10g of soil + 20ml overcooked distilled water  Mix the soil and water for 5 min using the glass straw  Measure pH H2O value by the pH-meter Strongly Acid< 4.9 Acid4.9 – 5.9 Weakly Acid 5.9 – 6.9 Neutral6.9 – 7.1 Slightly alkaline 7.1 – 8.0 Alkaline8.0 – 9.4 Strongly Alkaline >9.4 Soil Reaction

Acid sulphate soils Forest Soils Cultivated soils of temperate zone Calcareous soils Solonchaks 0

The specifics of the measurement of soil reaction  The measurement takes place in soil suspension (not in soil solution) H3O+H3O+ Soil Particles SOLUTION SUSPENSION Soil Reaction

Types of Soil Reaction  Actual Soil Reaction → Active Reaction Concentration of H 3 O + ions in soil suspension H3O+H3O+ Soil Particles Water pH H2O Soil Reaction

Types of Soil Reaction  Potential Soil Reaction → exchangeable soil reaction Ability of soil to change pH of neutral salt solutions H3O+H3O+ Soil Particles KCl solution K+K+ pH KCl E.A. Soil Reaction

Types of Soil Reaction  Actual Soil Reaction → Active Reaction pH H2O  Potential Soil Reaction → exchangeable soil reactionpH KCl, 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 H 3 O + → Reserve acidity - hydrolysis of Al 3+ Al 3+ + H 2 O → AlOH 2+ + H + AlOH 2+ + H 2 O → Al(OH) H + Al(OH) H 2 O → Al(OH) H + Soil Reaction

Influences on Soil Reaction Measurement  The degree of dissociation  Kind of Acidity → free H 3 O + → hydrolysis of Al 3+  Ratio soil : water - (the wider soil : water ratio, the higher measurement error)  Salt content - (salt displaces H + from the soil sorption complex)  CO 2 content - (the higher CO 2 content, the lower soil pH) Soil Reaction