The polyphosphate to soil ratio that yielded stable suspensions required for particle-size analysis of calcareous saline soils depended mainly on the amounts of soluble and exchangeable cations, especially Ca and Mg. Without removing soluble salts and organic matter, 5 g of polyphosphate per 50 g soil per liter of suspension provided stable suspensions for most saline calcareous soils in Imperial Valley. When the exchangeable plus soluble Ca and Mg exceeded about 20 me in 50 g soil, then 5 g polyphosphate were not adequate to provide stable suspensions, and excess salts had to be removed. It was possible in some of these latter soils to obtain stable suspension without removing salts by either increasing the dispersant to soil ratio or the pH or both.The reactions of polyphosphate with solid CaCO3involve mainly surface interactions and hydrolysis of carbonate without any appreciable change in soluble constituents. The polyphosphates do not appreciably change the particle-size distribution of CaCO3in soils.The reactions of Ca and Mg ions with polyphosphate involve their coordination with the polyphosphate anion to form soluble complexes with reduction in pH. Depending on pH and solute concentration, Ca++ ions in excess of 5 to 7 me/g polyphosphate initiate breakdown of the soluble complexes and precipitation of Ca-phosphates. No precipitation of Mg complexes occurs under similar conditions.