Chapter%209 - Soil Reaction Refers to the degree of soil...

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Soil Reaction Refers to the degree of soil acidity or alkalinity. Soil reaction is important because it affects nutrient availability, microbial activity and plant growth. It is measured in units of pH, which is approximately defined as pH = -log [H + ] Most plant species perform best in pH range 5.5 to 6.5 or 7.0 but some prefer extremes. For example azaleas and blueberries grow best under more acidic conditions. In contrast alfalfa does best under somewhat alkaline conditions. Sources of H + and OH - Strongly acid soils pH < 5 Adsorbed H + and Al 3+ dominate of the CEC. Equilibrium with these cations in solution accounts for low solution pH. Moderately acidic soils 5 pH < 6.5 In this pH range, there is a lower fraction of adsorbed acidic species and higher percentage base saturation. Adsorbed and solution Al is more highly hydrolyzed. Exchangeable H + and Al(OH) x (3-x)+ are responsible for the moderate soil acidity. Neutral to alkaline soils pH 6.5 The CEC is dominated by basic cations. Remaining H + and Al(OH) x (3-x)+ is associated with pH-dependent exchange sites may be considered bound. Exchange equilibria very strongly favor adsorption of these species at these type sites so their concentration in solution is low. In the relative absence of acidic cations, pH may rise above 7. Carbonate and bicarbonate salts are present at alkaline pHs. Hydrolysis of these anions generates OH - . For example, CaCO 3 W Ca 2+ + CO 3 2- CO 3 2- + 2H 2 O W H 2 CO 3 + 2OH - Classification of Soil Acidity Active Exchangeable Residual Total Active acidity refers to H + in solution. Exchangeable refers to adsorbed H + and Al(OH) x (3-x)+ that is subject to displacement by addition of an excess of extracting cation such as K + . Residual is adsorbed H + and Al(OH) x (3-x)+ that is unextractable by unbuffered salts. Total acidity includes
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