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A Comparison of Calcium Carbonate Precipitation and pH Depression on Calcium-Reduced Ammonia Loss from Surface-Applied Urea¹

ABSTRACT

The role of calcium (Ca) in reducing ammonia (NH₃) loss from soil by way of calcium carbonate (CaCO₃) precipitation and soil pH depression was investigated by applying increasing rates of urea with fixed ratios of Ca/N in the laboratory and greenhouse. Soil pH, Ca precipitation, and the rate of hydrolysis of added urea were related to reduction of NH₃ loss.

Calcium/nitrogen ratios of 0.25 were equally effective in reducing NH₃ loss independent of N application rates. Higher ratios of Ca/N at at 55 and 110 kg N/ha application rates did not further depress NH₃ loss. At application rates of 550 and 1,100 kg N/ha, Ca levels above a Ca/N ratio of 0.25 resulted in additional large depressions of NH₃ loss. This additional effect was due to a depression in soil pH from increased Ca activity and a dramatically reduced rate of urea hydrolysis. The chemical reaction of Ca with urea was complete at a Ca/N ratio of 0.25. Additional Ca, if added with a high enough rate of urea (550 and 1,100 kg N/ha) remained soluble and reduced the soil pH and was responsible for one half of the total reduction in NH₃ loss.

Greenhouse data showed that the addition of Ca with urea increased plant recovery of fertilizer N more than predicted from laboratory data. Nitrogen rates as low as 55 kg/ha, applied in a surface band or as large granules, resulted in improved plant recovery of added N. Broadcast applications at 165 kg N/ha also resulted in good fertilizer N recovery at Ca/N ratios of 0.25 and 0.50.

¹ Contribution from Texas Agric. Exp. Stn., Texas A&M Univ., College Station, TX 77843.

² The First and Third Authors are Associate Professor and Laboratory Technician, respectively, Texas Agric. Exp. Stn., located at Texas A&M Agric. Res. Center at El Paso, 1380 A&M Circle, El Paso, TX 79927. The Second Author is Professor, Texas A&M Univ. Agric. Res. And Ext. Center, P. O. Box 10607, Corpus Christi, TX 78406.

Received for publication November 5, 1979. Accepted for publication May 5, 1980.