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 100units = cmol c /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 (cmol c /kg), calculate the ESP Na + = 2; Ca +2 = 4; Mg +2 = 3; H + = 1; Al +3 = 1 CEC = sum of all exchangeable cations: CEC = =11 cmol c /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 SAR = [Na + ] [Ca +2 + Mg +2 ] ½ units = mmol c /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 units mmol/L, mmol c /L, meq/L, mg/L Use ion valence to go from mol/L to mol c /L  Na + has one charge, so 1 mol/L = 1 mol c /L  Ca +2 has two charges, so 1 mol/L = 2 mol c /L 1 eq = 1 mol c or 1 meq = 1 mmol c  1 cmol c = 10 mmol c 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 mmol c x 1 mmol c /meq x 40.1 mg/mmol = 816 mg/L or ppm Ca