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

Abstract

AbstractThe 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.