Abstract
The effect of salinity and SAR on Gapon selectivity coefficient of two Vertisols , collected from Duhok Governorate in northern Iraq(Summel and Zakho) classified taxonomically as (fine clay-smectitic-calcareous-cracked soils), were studied in a laboratory experiment. Twenty solutions , including five TEC levels (5,10, 20, 50 and 100 mmolc.l-1 and four SAR levels 2.5, 5.0 ,10 and 25( mmol.l)0.5 were synthesized to equilibrate the soil samples using chloride salts of sodium and calcium, magnesium . SAR of equilibrium solution decreased compared to equilibrating solution especially at high SAR and low electrolyte solutions . At low salt concentrations , the dissolution of CaCO3 and Ca bearing minerals causes a decrease in SAR of the equilibrium solution but their effect becomes less at high electrolyte concentrations . Exchangeable sodium increased with increase in SAR and TEC . Irrecpective of TEC , NaX increased by about 2.8-fold and 0.8-fold and irrecpective of SAR , the increase was about 1.3-fold and 2.1-fold for Summel and Zakho sites respectively. An increase in ESP was observed when correction for anion exclusion was calculated especially at higher TEC level. All relationships between ESR and SAR were significant for each salinity level and when the treatments were handled as a composite data . The values of Gapon constant were in the range of (0.0057- 0.0259) and (0.0155- 0.0425)( mmol.l)-0.5 and Correction for anion exclusion caused an increase by about 18% and 11% for Summel and Zakho sites respectively compared to not corrected samples .
