Slide 5a.   Calcium is added in sodic soils to prevent accumulation of sodium on exchange sites in the soil profile.  Increased soil sodium levels eventually cause direct negative plant responses, perhaps equally or even more important in many soils are the indirect plant effects caused by deterioration of soil physical properties in high sodium systems.  Sufficient application of calcium can reduce or prevent surface sealing in sodium-dominated systems.  Calcium is also needed in PAM-treated systems where anionic PAMs are used for erosion control.  Anionic PAMs are used in systems where PAM might be lost to riparian waters, because anionic PAMs pose no threat at low concentrations to aquatic species.  Calcium ions in water form positive ionic bridges between the negative charge sites of anionic PAM molecules and the anionic charge sites of soil minerals.  In this photo A 40 ton gypsum "dissolvinator" set to run at 200 kilograms per hectare of fine industrial grade gypsum, feeding into a supply channel for irrigating cotton.  David Moor (left) examines the dissolvinator with the Colly Farms irrigation manager.  The advantages of automation and fine control with this system must be weighed against the higher dissolvinator system cost (and user skill) and more costly high grade gypsum needed for use in this system.  See the next slide (5b) for another approach to this problem.