SOIL CHEMISTRY

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SOIL CHEMISTRY
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SOIL pH

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

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SOIL pH

• Technically
• expressed as the negative logarithm of the
  hydrogen ion (H) concentration
• www.johnkyrk.com/pH.html

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SOIL pH

• At pH 7
• - neutral pH
• - indicates H concentration of . . .
• 10-7 mol/L

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SOIL pH

• Since the product of normalities of H and OH-
  ions is always 10-14, OH- is also 10-7 mol/L
• mathematically . . .
• (10-7)(10-7) 10-14

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pH scale Relative H and OH- ions
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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

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SOIL pH

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

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SOIL pH

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

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SOIL pH

• AMOUNT OF PRECIPITATION
• HUMID AREAS (heavy rainfall) ? ACID SOILS
• Ca, Mg, and Na are leached downward and
  replaced by H ions

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Percolation of Acidic Water
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Precipitation vs. Evaporation
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SOIL pH

• TYPE OF VEGETATION
• Vegetation affects pH over period of years
• grasslands higher pH
• forests lower pH

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SOIL pH

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

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SOIL pH

• Nutrients UNAVAILABLE at higher pH
• Cu Copper
• Fe Iron
• Mn Manganese
• Zn Zinc

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SOIL pH

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

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SOIL pH

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

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SOIL pH

• AZALEA
• BLUEBERRY
• RHODODENDRON
• ACID SOILS

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SOIL pH

• Mildly Acid Soils
• Wheat
• Corn
• Soybean
• Apple
• Tomato

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SOIL pH

• Neutral to Slightly Acid Soils
• Alfalfa
• Lettuce
• Muskmelon

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pH Range of Crops
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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

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How Much Lime?

• Four Factors
• 1) Present pH
• 2) Desired pH
• 3) Cation Exchange Capacity of the soil
• 4) Liming material to be used

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SOIL pH

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

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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

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SOIL pH

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

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pH reactions

• CO2 H2O ? H2CO3 ? HCO3- H
• CaCO3 2H2O ? Ca² H2CO3 2OH-
• Al³ 2H2O ? Al(OH)2 2H
• NH4 2O2 ? NO3 - H2O 2H
• 2S 3O2 2H2O ? 2 H2SO4 energy

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Soil Salinity

• Saline Soils
• - have high levels of soluble salts (except Na)
• - pH is 8.5 or less
• - SAR lt 13
• Sodic Soils
• - high in sodium
• - exchangeable sodium gt 15
• - pH 8.5-10

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Soil Salinity

• Saline-Sodic Soils
• - high levels of soluble salts and sodium
• - SAR gt 13
• - pH lt 8.5

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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