Lecture 12 b Soil Cation Exchange Capacity

1
Lecture 12 b Soil Cation Exchange Capacity

• In most soils, 99 of soil cations can be found
  attached to micelles (clay particles organic
  matter) and 1 can be found in solution.
• Cations in the soil (mainly Ca, Mg, K and
  Na) maintain an equilibrium between adsorption
  to the negative sites and solution in the soil
  water.
• This equilibrium produces exchanges -- when one
  cation detaches from a site (leaving it free),
  another cation attaches to it.
• Therefore the negatively charged sites are called
  cation exchange sites.
• The total number of sites is the Cation Exchange
  Capacity or CEC

2
Cation Exchange Capacity

• 1) the number of cation adsorption sites per unit
  weight of soil or
• 2) the sum total of exchangeable cations that a
  soil can adsorb.
• CEC is expressed in milliequivalents (meq) per
  100 g of oven dry soil.
• Equivalent weight molecular or atomic wt (g)
• valence or
  charges per formula

3
Milliequivalent (MEQ)

• 1 meq wt. of CEC has 6.02 x 10 20 adsorption
  sites
• MEQ of Common Cations
• Element Na K Ca Mg
• Valence 1 1 2 2
• Eq. Wt 23/123 39/139 40/220 24/2
  12
• MEQ wt .023 .039 .02 .012

4
Sample calculation for equivalent weight for lime
or CaCO3
CaCO3 - formula wt. 40 12 48
100 charges involved 2 eqwt. 50 meq .05
grams Or one meq of Lime .05grams
5
Calculation of CEC with clay and OM

• Assume Avg CEC for OM 200 meq/100g
• Assume Avg CEC for clay 50 meq/100g
• CEC ( OM x 200) ( Clay x 50)
• From soil data soil with 2 OM and 10 Clay
• 200 x .02 50 x .1 4 5 9 meq/100 g

6
Predicting CEC

• 1) sum of cations remove all cations and total
  the amount
• 2) NH4 saturation soil is saturated with NH4
  - the NH4 is replaced by Ca and the NH4
  removed is measured.
• 3) Estimation based on texture
• Sand 0-3 meq/100 g LS to SL 3-10
• Loam 10 - 15 Clay Loam 15-30
• Clay gt 30 (depends on kind of clay)

7

• A high CEC value (gt25) is a good indicator that a
  soil has a high clay and/organic matter content
  and can hold a lot of cations.
• Soil with a low CEC value (lt5) is a good
  indication that a soil is sandy with little or no
  organic matter that cannot hold many cations.

8
http//www.spectrumanalytic.com/support/library/ff
/CEC_BpH_and_percent_sat.htm
9
Base Saturation vs pH

• Base Saturation - meq bases CEC x 100
• Hydrogen Saturation - meq H CEC x100
• Example Ap Soil Horizon
• Cations-- H Ca Mg K Na
• 9.4 14 3 0.5
  0.1
• CEC 27 meq/100g (sum of cations)
• base sat 17.6 27 x 100 65
• hydrogen sat 9.427 x100 35

10
pH vs. Base Saturation-an approximate
relationship
11
Buffering Capacity

• The ability of soil to resist change in pH.
• The amount of H in the soil solution is small
  compared with the H, Al 3 adsorbed on the
  soil colloids (reserve)
• Neutralization (by the addition of bases) of the
  solution H (H is removed from the system)
  results in a rapid replacement of H from the
  exchangeable H on the soil colloid.
• CaCO3 when added to soil will neutralize H.
  CaCO3 Lime
  (dolomitic MgCO3 CaCO3

12
Why apply lime ?

• 1. helps nutrients become available to plants
  (solubility vs. pH)
• 2. improves soil structure
• 3. provides nutrients for plant growth -Ca Mg
• 4. promotes growth of beneficial microorganisms-
  they like pH6.5
• 5. overcomes acidifying effects of fertilizers
• 6. reduces metal toxicity to plants (solubility
  vs. pH)

13

• Use of Precision Agriculture
• Farmers fields have variable yields across the
  landscape.
• Variations can be traced to management
  practices, soil properties and/or environmental
  characteristics.
• Soil characteristics that affect yields include
  texture, pH, structure, moisture, organic matter,
  nutrient status and landscape position.
• Environmental characteristics include weather,
  weeds, insects and disease.
  Source http//extension.missouri.edu/explore/env
  qual/wq0450.htm

• Aerial photograph, soil pH and 3-year average
  grain yields for central Missouri farm
• The higher grain yields that appear spatially
  related to the high pH area may be caused by
  favorable soil conditions related to pH.
• Correlation between yield and a soil parameter is
  not certain proof that pH is the cause of higher
  yields.
• Past management of this portion of the field may
  have been the more important factor resulting in
  higher yields because the area of high yield is
  substantially smaller than the area of high pH.

Field -- Soil pH --
Soybean Yield
14
Sample BS Problem

• Calculate the amount of CaCO3 which must be added
  to an acre furrow slice of this soil to raise the
  soils base saturation to 90
• SOIL CEC of 17meq/100g and BS 32
• (hint takes 1000 lbs CaCO3/acre to neutralize 1
  meq of H/100 g 90 - 32 58 change in BS
• 0.58 x 17 meq/100g 9.86 meq/100g of H to
  neutralize
• or 9.86/100 X 1000 lbs CaCo3/100g 9860 lbs
• OR
• 9.86 meq x .05g/meq .493g/100g and
• .493/100g is to X / 2,000,000lbs or X 9860 lbs.
• Divided by 2000 lb/ton 4.9 tons

15
DYAD

• Calculate the tons of CaCO3 which must be added
  to an acre furrow slice of this soil to raise the
  soils base saturation to 90 if the CEC is now
  27 instead of 17

16

• SOIL CEC of 27meq/100g and BS 32
• 90-3258change in base
• or .58x2715.66 me of H to neutralize
• 15.66x1000lbs15660/2000lbs/ton 7.8tons

17

• In the Southeast US, if fertilizer and lime is
  applied to raise the base saturation of a
  kaolinitic soil to 85 percent as commonly done in
  the Midwest, the resulting pH would be between
  7.1 and 7.5- due to low CEC from Kaolinite
• Soil pH values in that range would result in a
  major problem with zinc and manganese deficiency.
  
• Thus, soils are only limed to 60-70 BS.

Ap Bt1 Bt2 Bt3 BC
Tifton soils formed in loamy sediments of marine
origin. Cotton, peanuts,soybeans, and corn are
the principal crops grown on these soils in
Georgia
18

• CEC and Soil Testing
• Because the CEC of a soil is relatively constant
  unless large amounts of organic matter are added,
  it is not measured or reported with a routine
  soil test.
• Ca Mg Ratio and Soil Testing
• Some soil testing labs will report ideal calcium
  to magnesium ratios for plant growth.
• However, most plants tolerate a very wide range
  of soil calcium to magnesium ratios.
• Adjusting the ratios of calcium and magnesium on
  the exchange complex by adding gypsum (calcium
  sulfate) or Epsom salts (magnesium sulfate) has
  not been shown to significantly benefit plant
  growth.
• Gypsum is primarily used as a soil amendment to
  improve water penetration and increase the level
  of calcium in the soil.

19
The End