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

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

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 23/1=23 39/1=39 40/2=20 24/2 = 12

MEQ wt .023 .039 .02 .012

Sample calculation for equivalent weight for lime or CaCO3

CaCO3 - formula wt. = 40 + 12 + 48 = 100charges involved = 2eqwt. = 50meq = .05 gramsOr one meq of Lime = .05grams

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

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 = > 30 (depends on kind of clay)

A high CEC value (>25)

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 (<5) is a good indication that a soil is sandy with little or no organic matter that cannot hold many cations.

http://www.spectrumanalytic.com/support/library/ff/CEC_BpH_and_percent_sat.htm

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 = 9.4÷27 x100 = 35%

pH vs. Base Saturation-

an approximate relationship

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

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 pH=6.5

5. overcomes acidifying effects of fertilizers

6. reduces metal toxicity to plants (solubility vs. pH)

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.

Use of Precision Agriculture: Farmer’s 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/envqual/wq0450.htm

Field -- Soil pH -- Soybean Yield

Sample % BS Problem Calculate the amount of CaCO3 which must be added to an

acre furrow slice of this soil to raise the soil’s 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

DYAD

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

SOIL = CEC of 27meq/100g and BS = 32%

90-32=58%change in base or .58x27=15.66 me of H+ to neutralize

15.66x1000lbs=15660/2000lbs/ton= 7.8tons

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.

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

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.