Precipitation and Solution of Calcium Carbonate in Irrigation Operations

PRECIPITATION AND SOLUTION OF CALCIUM CARBONATE IN IRRIGATION OPERATIONS THE EXTENT TO WHICH CaCO3 precipitates from irrigation water or dissolves from soil during irrigation operations influences salinity control and the accumulation or replacement of exchangeable sodium in soil. Precipitation of CaCO3 from irrigation water decreases the salt burden of irrigation systems but increases the Na/(Ca + Mg) ratio of the soil solution and the soil exchange complex. The solution of soil CaCO3 by irrigation water, on the other hand, increases the salt burden and decreases the Na/(Ca + Mg) ratios. Eaton has proposed and denned two calculated values for evaluating the sodium hazard of irrigation waters as follows: Possible Na percentage = (Na X 100)/(Ca + Mg + Na — CO3 — HCO3), and Residual Na2CO3 content = (C03 + HC03) (Ca + Mg) Calculation of both values involves the assumption that CO3 and HCO3 are quantitatively precipitated in the soil to the limit of Ca and Mg present in the water. Doneen has extended the ideas of Eaton to the evaluation of the salinity hazard of irrigation waters and proposed the concept of "effective salinity" wherein the actual salinity is adjusted downward for carbonate precipitation by amounts based on Baton's assumption. Adjustment of irrigation water composition to take into account subsequent precipitation of carbonates in the soil is a desirable refinement for calculating leaching require1 Contribution from the U. S. Salinity Laboratory, Soil and Water Conservation Research Division, ARS, USDA, Riverside, California in cooperation with the 17 Western States and Hawaii. Received Dec. 18, 1963. Approved 'Sept. 23, 1964. 3 Eaton, F. M. 1950. Significance of carbonates in irrigation waters. Soil Sci. 69:123-133. 3 Doneen, L. C. 1954. Salination of soils by salts in irrigation water. Trans. Amer. Geophys. U. 35:943-950. ment and salt balance, and for evaluating the Na hazard of waters. The available evidence' 5 indicates, however, that the extent of carbonate precipitation from irrigation waters is highly variable, depending upon water composition and leaching percentage, and in no case approaches quantitativeness. Moreover, some waters, instead of losing carbonates by precipitation, increase in carbonate concentration as they move through the soil. The purpose of this note is to report information gained from salt-balance studies relative to the extent that carbonate is precipitated from or dissolved by irrigation waters. Over a period of years the former Rubidoux Laboratory and subsequently the U. S. Salinity Laboratory, in cooperation with other agencies, has conducted salt-balance studies in several irrigated areas of Western United States having predominately calcareous soils. For both inflow to the irrigated areas (diversions) and outflow from the areas (drainage), data are available on volume of water and on salt burden determined as individual ions. It is recognized that in most cases the outflow water includes waste irrigation water that has not passed through soil, and that this influences the validity of the data for the study at hand. Moreover, changes in the quantity of dissolved COg + HCO3 stored in the soil will influence the validity of the outflow/inflow ratio of carbonates as a measure of CaCO3 precipitation. This influence should be small, however, as only limited amounts of dissolved carbonates can accumulate in soil in the presence of dissolved and exchangeable calcium. Thus, in contrast to Cl and SO4, essentially all carbonates applied in irrigation water either precipitate in the soil or pass out of the soil in drainage water. By similar Wilcox, L. V., Blair, George Y., and Bower, C. A. 195f Effect of bicarbonate on suitability of water for irrigation. Soil Sci. 77:259-266. "Pratt, P. F., Branson, R. L., and Chapman, H. D. 1960. Effect of crop, fertilizer, and leaching on carbonate precipitation and sodium accumulation in soil irrigated with water containing bicarbonate. Trans. Intern. Soil Sci. Soc. Cong. 7th, Madison. 2:185—192.