Effect of Calcium Carbonate and Ionic Strength on Boron Adsorption in Calcareous Soils

Boron is an essential element for plant growth. Soluble and adsorbed boron are short- and long-time sources of B for plants. Adsorbed B controls equilibrium concentration and its deficiency and toxicity in soils. Considering the importance of adsorption and desorption reactions in B behavior in soils, boron adsorption isotherms were measured in five soils differed in calcium carbonate equivalent (CCE 0-80%). Experimental data were described with linear and non-linear forms of the Langmuir, Freundlich, Toth, and Langmuir-Freundlich equations. Analysis of the model descriptions revealed that the Langmuir equation was the best equation for prediction of the exerimental data. The latter data and the predictions by the Langmuir equation showed that boron was weakly adsorbed in soils when compared with other oxyanions. Boron adsorption maximum () was calculated between 1 to 3 µmol /g soil. Correlating boron adsorption maximum with soil properties revealed that the amount of adsorbed B was not a simple function of the soil properties, rather, it reflected the effect of different soil properties such as pH, texture, carbonates and oxides contents. Among soil properties, active CCE showed a direct relationship with boron adsorption maximum. Removing calcium carbonates from a soil sample (18% CCE and pH 7.4) reduced boron adsorption maximum by 35%. In addition, the effect of three levels of ionic strength i.e. 0.01, 0.1 and 0.5 M NaCl, was measured in a soil sample. The results revealed that B adsorption maximum increased by 24 and 71% with increasing ionic strength from 0.01 to 0.1 and 0.5 M. This could be due to an increase in pH-dependent surface charges and/or a decrease in the thickness of the double layer.