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 ESPNa+ = 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 100ESP = (2/11) x 100 = 18%

Fig 9.22page 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

SAR = [Na+] [Ca+2 + Mg+2]½ units = mmolc/L

2 (old units = meq/L)

The concentration of cations in the soil solution

• Sodium Adsorption Ratio = “SAR”

SAR = [Na+] units = mmol/L [Ca+2 + Mg+2]½

Converting between various unitsmmol/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 dry-weight 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