Soil and Water Salinity
• Dissolved salts decrease the osmotic potential of soil
water (which lowers the Total Soil Water Potential)
• a decrease in soil solution potential increases the
amount of energy which plants must expend to
extract water from the soil (water flows from high to
low potential)
• As a result, respiration is increased and the growth
and yield of most plants decline progressively as
osmotic potential decreases.
• Although most plants respond to salinity as a function
of the total osmotic potential of soil water, some
plants are susceptible to specific ion toxicity
Saline and
Sodic Soils:
review the
classification
scheme for the
various soils
Electrical Conductivity – ability to conduct
an electrical current through a material
• Saline soils and salty water conduct
electricity better than nonsaline soils or
pure water.
-- more dissolved ions in water = higher
electrical conductivity.
-- measure resistance to current and take reciprocal
-- dS/m (SI units) or mmhos/cm (old units)
Measurement of Salinity – TDS and EC
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
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
Note change on HW 3, #8: calculate ESP using
measured cations in your data (instead of SAR); then
you can estimate SAR using Figure 9.22 nomogram
Example for HW 3, # 8:
• Using the measured cations in your soil
(cmolc/kg), calculate the ESP
Na+ = 2; Ca+2 = 4; Mg+2 = 3; H+ = 1; Al+3 = 1
CEC = sum of all exchangeable cations:
CEC = 2 + 4 + 3 + 1 + 1 =11 cmolc/kg
ESP = (Na/CEC) x 100
ESP = (2/11) x 100 = 18%
Fig 9.22
page 301
ESP = 18 so SAR ≈ 15 from fig below
or SAR ≈ 16 using USSL nomogram
Nomogram for
estimating ESP to/from
SAR (more accurate than
textbook figure)
(Handbook 60, U.S.
Salinity Lab, 1954)
Sodicity Measurement
• Sodium Adsorption Ratio = “SAR”
SAR =
SAR =
[Na+]
[Ca+2 + Mg+2]½
units = mmol/L
[Na+]
[Ca+2 + Mg+2]½
2
units = mmolc/L
(old units = meq/L)
The concentration of cations in the soil solution
Converting between various units
mmol/L, mmolc/L, meq/L, mg/L
• Use ion valence to go from mol/L to molc/L
 Na+ has one charge, so 1 mol/L = 1 molc/L
 Ca+2 has two charges, so 1 mol/L = 2 molc/L
• 1 eq = 1 molc or 1 meq = 1 mmolc
 1 cmolc = 10 mmolc
• Use atomic weights to go from moles to mass
Na = 23 g/mol (or mg/mmol)
Calculating solid concentration based on
solution concentrations
• Need to know how much solution (“extracting” or
equilibrium volume)
 Common to use 100 mL; or 1:5 soil:water ratios
 When using sat’d paste extract, need to know SP
• Need to know how much soil (preferably on a dryweight basis)
 Common to use 5 to 10 g soil
• 1 kg = 1000 g
• 1 L = 1000 mL
• 1 ppm = 1 mg/L (solution) or 1 mg/kg (solid)
Example
• How many ppm (mg/L) of Ca+2 are in the
sample given in class? (Ca = 40.7 meq/L)
40.7 meq/L x 1 mmol/2 mmolc x 1 mmolc/meq x
40.1 mg/mmol = 816 mg/L or ppm Ca