Transcript Soil pH - University of Minnesota

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 = 100
charges involved = 2
eqwt. = 50
meq = .05 grams
Or 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
÷ CEC x 100
- meq H ÷ CEC x100
% Base Saturation - meq bases
% Hydrogen Saturation
Example: Ap Soil Horizon
Cations-- H+
Ca++ Mg++ K+
9.4
14
3
0.5
0.1
CEC = 27 meq/100g (sum of cations)
Na+
% base sat = 17.6 ÷ 27 x 100 = 65%
% hydrogen sat = 9.4÷27 x100 = 35%
pH vs. Base Saturationan 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)









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
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
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 7.5due 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.