Soil Acidity and Nutrients Section D Soil Fertility and Plant Nutrition

Soils Soils behave as weak acids or bases Some soils behave like a buffered weak acid: are acid, but resist increases in pH have a “buffering capacity” Some soils behave like a buffered weak base are basic, but resist decreases in pH

Titration Curve - Acid Soil Alkaline “ pH “Buffering” Acid Base added

Soils Many soils behave like weak acids because: production of CO2 organic acids in SOM, root exudates EXCHANGEABLE CATIONS Exchangeable cations are the most important influence on soil pH. The relative proportion of acid or basic cations on the cation exchange sites determine the soil pH in almost all cases.

Titration Curve - Alkaline Soil “ pH “Buffering” Acid Acid added

Soils Some soils behave like weak bases because: presence of CaCO3 it is a weak base that will buffer soil solutions against decreases in pH

Formation of Soil Acidity “Young soil” Time “Old soil” Al3+ dissolves from minerals Ca2+, Mg2+, K+ are leached from soil Clay minerals with exchangeable Ca2+, Mg2+, K+ and Al 3+, H+ Clay minerals with exchangeable Ca2+, Mg2+, K+

Exchangeable Cations Ca2+, Mg2+, Na+, and K+ do not produce H+ in soils, and so are called the “basic cations” Al3+ and Fe3+ react to produce H+ in soils, and so are called the “acid cations” The “base saturation” is the percentage of the CEC occupied by the basic cations. It is highly related to soil pH.

“Base Saturation Percentage” 8 Low CEC Soil Soil pH High CEC Soil 2 100 Percent base saturation

Why is Al3+ called “acidic”? Al3+ ions in solution are surrounded by water molecules (octahedral coordination). The high positive charge of Al3+ causes it to pull electrons from O on H2O. This makes H2O more acidic. This is called “Al hydrolysis” Fe3+ behaves similarly.

3+ + H + [Al(H 2 O) 5 OH] 2+ Al(H O) 2 6 3+ Al H O 2

2+ 2+ + H + [Al(H 2 O)4(OH) ] [Al(H O) OH] 2 5 3+ Al H O 2

+ [Al(H O) OH] + H + [Al(H 2 O) 3 (OH) ] 2 5 3+ Al H O 2

Titration Curve—Acid Soil Alkaline “ The amount of buffering capacity will determine the lime requirement. Soils with higher CEC will have a higher L.R. pH Buffering due to Al hydrolysis Acid Lime added

Soil Acidity Al3+ Ca2+ H+ Clay Solution

Al 3+ H+

Sulfide Oxidation Mine tailings (spoils) may become acidic because of the oxidation of pyrite (FeS2): FeS2 + H2O + 7/2O2  FeSO4 + H2SO4

Al Toxicity Below a pH of ______, Al3+ may be present in concentrations that are toxic to plants. Al solubility is inversely proportional to pH. Severely inhibits root growth, interferes with P uptake. Al toxicity is the #1 problem in acid soils. Mn and Fe may be toxic as well, because they become more __________ under acid conditions. 5.5 soluble

Plants and pH Soil pH Ranges Plant preference for pH Strongly Acidic <4 5.5 6.5 7.5 8.5 Strongly Acidic Moderately Slightly Neutral Alkaline Highly Plant preference for pH Most crop plants and plants of temperate regions Most tropical plants. Tea, Coffee, azaleas Alfalfa Halophytes

Plants and pH Differing plant tolerance to acidity is mostly due to: Different nutrient requirements Ability to immobilize Al3+ in the rhizosphere or to detoxify Al3+ within the plant.

Correcting Soil Acidity Remember the two “components” of soil acidity: __________ acidity, which is ____ in the soil solution, and ____________ acidity which is _______ on the soil clay cation exchange sites. Both of these have to be neutralized to correct soil acidity. To do this, ________ is added to acid soils. active H+ reserve Al3+,H+ lime

Al 3+ H+

Liming Acid Soils Lime Sources: CaO (quicklime), Ca(OH)2 (hydrated lime), CaCO3 (ag lime), CaMg(CO3)2 (dolomitic lime) The amount of lime to add to a soil (lime requirement) depends on: pH Cation exchange capacity L.R. is usually determined by a soil test

Lime Rates

Reactions of Lime in Soils Lime reacts to neutralize solution acidity: CaCO3 + 2H+  H2O + CO2 + Ca 2+ Lime also removes exchangeable Al3+ from clay cation exchange sites (X is a cation exchange site) 3CaCO3 + 2AlX + 3 H2O  3CaX + 2Al(OH)3 + 3CO2