5. Buffering capacity
• Soils high in SOM and clay minerals are more
resistant to change in pH
• Sandy soils and highly weathered soils are
least buffered
• Base Saturation =  exchangeable bases
CEC
BS = (exch Ca + Mg + Na + K)
(exch Ca + Mg + Na + K + Al + H)
6. Lime Requirement
•
•
•
•
Amount of CaCO3 needed to increase the pH
of the soil to an optimum pH
Depends on soil mineralogy, % clay fraction,
% OM, cultivation practices (leaching,
fertilization, etc)
Variety of liming materials
Only practical to raise pH to ~6 (KClextractable acidity is ~0)
Lime material
•
•
•
•
•
•
CaCO3 calcic limestone
CaMg(CO3)2 Dolomite
CaO: Quick lime
CaOH calcium hydroxide
Byproducts: ground shells, cement factory
waste
Consume H+ and provide an alternative
cation for the exchange phase (Ca or Mg)
Liming to increase soil pH
• Lime characteristics




cost
purity
speed of effect (fine ground vs coarse)
ease of handling
• Lime requirement
 depends on pH, CEC and buffer capacity of the
soil
• Lime Application: small amounts split and
incorporated into the soil
To increase pH in a well-buffered soil requires much more lime than in
a sandy or weathered soil; more lime required to go from 6 to 7 than
from 4-5
http://wwwlb.aub.edu.lb/~webeco/SIM215acidsoilsandlimimg_files/image002.gif
• a major threat to agricultural productivity in
arid regions
• One-third of the world’s irrigated land is
salinized
• More than one million hectares affected
• Salts cause both osmotic effects and specific
ion toxicity
Sources of Soil Salinity
Natural causes:
• Weathering of parent material with little or no leaching
• more salinity in hot, dry regions (climate + irrigation)
• Accumulation of salts in enclosed drainage basins
• Coastal spray and inundation
• High water tables (capillary rise brings salts to the
surface)
Anthropomorphic causes of Salinity
• Irrigation
 Not just with poor quality water
 Inadequate leaching and drainage
• Acid rain (enhances weathering; salt
production)
• Application of fertilizers, manures, biosolids,
composts which are often saline
Salt-impacted
agricultural soils
Measurement of Salinity
•
•
Electrical Conductivity (EC) is an measure of the flow of electricity
through a material
Saline soils and salty water conduct more electricity than nonsaline
soils or pure water. It is the ions that pass or conduct electricity
from one ion to the next.




•
As salt concentration increases, EC increases.
Acidic or low pH solutions also exhibit high EC
Expressed in dS/m (SI units) or mmhos/cm (old unit)
dS/m = mmhos/cm
Use an EC ‘bridge’ or meter to measure how well water extracted
from soil can conduct electricity:
Dissolved ions and
two metal plates
Voltage is applied & ions move
toward oppositely charged plates
EC values for common waters (dS/m)
•
•
•
•
Deionized water: 0.0005 to 0.002
NMSU tap water: 0.5 to 1.0 (rarely this high)
Seawater: 40 to 55
Good irrigation water: < 0.7
 Rio Grande N of Las Cruces is good
 Quality decreases (EC increases) downstream
• Poor quality irrigation water: > 3
• Saturated Paste Extract EC of saline soils: ≥ 4
Relate I and EC
• Ionic strength is a parameter that estimates the
interaction between ions in solution.
• Salts are ionic solids that dissolve in water
• Empirical relationships:
 I = 0.0127 EC (in arid and semi-arid regions)
 I = 0.014 EC (in humid regions)
 Easier to calculate because you don’t need full
composition of solution
Instruments to measure EC
Conductivity meter
Electromagnetic induction
Time Domain Reflectometry
Measurement of Salinity – TDS
TDS – Total dissolved solids
 Cations + anions + anything <2 microns
 Good quality water has <500 mg/L or ppm TDS
 measure using gravimetry or EC
• Evaporate water off and accurately weigh the residue
• Problematic due to hydration and volatilization
 EC (dS/m) x 640 ≈ TDS (mg/L)
• TDS ‘meters’ are really EC meters with conversion factor
Osmotic potential (OP)
• That portion of the Total Soil Water Potential due to the
presence of solutes in soil water
• Salts reduce the water potential by inhibiting the
movement of water molecules
• OP (kPa) ≈ -0.40 x EC (dS/m)
Pure Water OP = 0
Water Diluted by a Solute (Red Spheres)
OP is negative
Water moves from regions of higher water potential
(pure water = 0) to regions of lower water potential
(saline water = -x) across a semi-permeable membrane
(e.g., plant roots)
http://www.genomestudy.com/BIO196/Lab4/osmosis.gif
Atm = -20,000 kPa
Leaf = -500 kPa
Root = -70 kPa
Soil = -50 kPa
http://www.uic.edu/classes/bios/bios100/lecturesf04am/changesinwaterpotential.jpg
Sodium Hazard –
dispersion and Na toxicity
ESP > 15%
Dispersed soil
Na saturated
ESP < 15%
Flocculated soil
Good soil structure
Ca saturated
Sodicity Measurement
• Sodium Adsorption Ratio = “SAR”
SAR =
SAR =
[Na+]
[Ca+2 + Mg+2]½
units = mmol/L
[Na+]
[(Ca+2 + Mg+2) / 2]½
units = mmolc/L
(old units = meq/L)
The concentration of cations in the soil
saturated paste extract (solution phase)
Sodicity Measurement
• Exchangeable sodium as a percent of the total CEC
= “ESP”
ESP = exchangeable Na X 100
units = cmolc/kg soil
CEC
(old units = meq/100g)
The concentration of cations on the soil exchange phase
Low ESP
High ESP
http://www.terragis.bees.unsw.edu.au/terraGIS_soil/images/ec-ncps-soil_solution-4.jpg.jpg
Nomogram for
estimating ESP to/from
SAR (Handbook 60,
U.S. Salinity Lab, 1954)
Saline
and
Sodic
Soils
Halophytes are
plants which
tolerate or even
demand sodium
chloride
concentrations in
the soil water they
absorb.
Saline Soils
• Most common salt problem and the easiest to correct
• EC > 4.0 dS/m; SAR < 13 or ESP < 15
• May be called ‘white alkali’ because of the accumulation
of salts on the surface
• Typical ions: Ca+2, Mg+2, K+, Na+; SO4-2, Cl-, HCO3-
Soil Chemistry of Saline Soils
• pH is usually 7.8 - 8.2 but can also be acidic
Soil Physical Condition
• Soil physical condition is generally good (well
aggregated with good internal fluid movement)
• Crusting may be a problem
Plant Growth Problems
• Osmotic potential contributes significantly to total
water potential; inability of plant to extract water
is the major plant growth problem on saline soils.
• Toxic ions can be a problem (Na+, Cl-, HCO3-)
• Plants differ in their tolerance to salt
Increasing NaCl concentration 
http://www.isb.vt.edu/news/2001/news01.dec.html
Chile pepper response to salinity
Sodic Soils
• Less common problem and much more expensive to
correct.
• EC < 4.0 dS/m; SAR > 13; ESP >15
• May be called black alkali because of the
accumulation of humic material (black color) on the
surface (Na causes organic matter to disperse)
• Too much Na is the problem
• Typical ions are Na+, Ca+2, Mg+2, K+ ; Cl-, SO4-2,
HCO3-, CO3-2
Soil Chemistry of Sodic Soils
• pH is usually 8.5 or greater because Na is high (Na is a
strong base-forming cation)
Soil Physical Condition
• Soil physical condition is poor (Na disperses the colloids
resulting in the loss of aggregation)
• Very slow or no fluid exchange
Plant Growth Problems
• Poor aeration and standing water
• Toxic ions (Na+, Cl-, and HCO3-) can be a problem
• Some plants may be tolerant to poor fluid exchange and
high Na
Saline-Sodic Soil
• EC > 4; SAR > 13 ESP > 15
• Combination of problems found in saline
and sodic soils
• Soil physical condition is more like a saline
soil in that drainage is normal
15 year old
pecans
south of
Las Cruces
that are
stunted by
sodium
This saline-sodic soil
near Vado is one of the
worst in the Mesilla
Valley. A heavy clay
layer keeps water from
freely draining.
The SAR of this soil is
about 25 and the EC is
about 15 dS/m.
The white salt is
mainly NaCl and
Na2SO4.
Reclamation of Saline and Sodic Soils
Saline Soils
• Leach with good water
• The leaching requirement (LR) can be used
Sodic Soils
• Exchange Na with Ca and leach. CaSO4, So, H2SO4 are used.
The H2SO4 dissolves CaCO3 in the soil to produce Ca+2 and
the So is converted into H2SO4 in the soil by microorganisms.
• Leach with good water
• Growth of plants (barley, triticale, halophytes) that can
withstand poor aeration and high levels of Na.
• Can take several years.
Problems caused by Salinity and Sodicity
• Osmotic effects: by lowering the osmotic potential
and making it difficult for plant to extract water
• Specific Ion effect: Na, Cl, H4BO4, HCO3 can be
toxic and can cause imbalances in the uptake and
utilization of other cations
• Soil structure deteriorates and aeration decreases
• Plants get stunted and exhibit small dark bluish green
leaves
Leaching Requirement:
Amount of water needed to remove excess salts from saline soils
LR = ECiw/ECdw
ECiwis EC of irrigation water
ECdw is maximum acceptable salinity of the soil solution
Example: if EC of irrigation water is 2.5 dS/m and crop can
tolerate an EC of 6 dS/m. What is LR?
LR = 2.5/6 = 0.4
If root zone needs 15 cm of water to be fully wetted, then amount
of water to be leached = 15*0.4= 6 cm
So supply 15 + 6 or 21 cm of water total to irrigate and leach