SOIL CHEMISTRY

SOIL pH A measure of the degree to which the soil is Acidic or Basic; also known as... Soil Reaction

SOIL pH Technically: expressed as the negative logarithm of the hydrogen ion (H+) concentration

SOIL pH At pH 7: - neutral pH - indicates H+ concentration of mol/L

SOIL pH Since the product of normalities of H + and OH - ions is always , OH - is also mol/L mathematically... (10 -7 )(10 -7 ) =

pH scale: Relative H+ and OH- ions

SOIL pH therefore... at neutral pH (pH 7 on a scale from 0 to 14) amount of H + ions = amount of OH - ions solution is neither acidic nor basic

SOIL pH Most agricultural soils pH range from 4 to 9... common productive agriculture soils fall between pH 5 to 8

SOIL pH 2 factors affecting pH are: 1) Amount of precipitation 2) Type of vegetation

SOIL pH AMOUNT OF PRECIPITATION – HUMID AREAS (heavy rainfall)  ACID SOILS Ca+, Mg+, and Na+ are leached downward and replaced by H+ ions

Percolation of Acidic Water

Precipitation vs. Evaporation

SOIL pH TYPE OF VEGETATION – Vegetation affects pH over period of years: grasslands – higher pH forests – lower pH

SOIL pH The most critical effects of soil pH are indirect... the availability (unavailability) of certain nutrients is greatly influenced by soil pH

SOIL pH Nutrients UNAVAILABLE at higher pH: Cu – Copper Fe – Iron Mn – Manganese Zn – Zinc

SOIL pH additionally... nitrogen-fixing bacteria associated with legumes are seriously hindered in acid soils Critical with beans and peas

SOIL pH ADJUSTING SOIL pH - Crops vary considerably regarding soil pH preferences

SOIL pH AZALEA BLUEBERRY RHODODENDRON ACID SOILS

Mildly Acid Soils: Wheat Corn Soybean Apple Tomato

SOIL pH Neutral to Slightly Acid Soils: Alfalfa Lettuce Muskmelon

pH Range of Crops

SOIL pH RAISING Soil pH: ADD LIME Factors to consider before applying - soil type - how much to raise the pH - fineness of lime - type of lime

How Much Lime? Four Factors: 1) Present pH 2)Desired pH 3) Cation Exchange Capacity of the soil 4) Liming material to be used

SOIL pH AMOUNT OF LIME NEEDED TO RAISE pH: Sandy Loam < Clay Loam WHY? Cation Exchange Capacity (CEC) of clay particles

Buffering Effect of Cation Exchange Capacity on Liming: pH measures only the Active Acidity in soil solution When lime is added, the H+ ions neutralized are quickly replaced from the Reserve Acidity

SOIL pH LOWERING Soil pH: Add Sulfur - May be necessary for acid-loving crops - May also benefit potato production by reducing potato scab

pH reactions CO 2 + H 2 O → H 2 CO 3 → HCO H + CaCO 3 + 2H 2 O → Ca² + + H 2 CO 3 + 2OH - Al³ + + 2H 2 O → Al(OH) H + NH O 2 → NO H 2 O + 2H + 2S + 3O 2 + 2H 2 O → 2 H 2 SO 4 + energy

Soil Salinity Saline Soils - have high levels of soluble salts (except Na) - pH is 8.5 or less - SAR < 13 Sodic Soils - high in sodium - exchangeable sodium > 15 - pH

Soil Salinity Saline-Sodic Soils - high levels of soluble salts and sodium - SAR > 13 - pH < 8.5

Reclaiming Salted Soils Saline Soils are the most easily reclaimed - flood to leach out salts Sodic Soils cannot usually be reclaimed - first must treat with Gypsum to remove sodium - calcium replaces sodium - sodium sulfate leaches out