Abstract
Ammonia loss from surface-applied urea occurs because urea hydrolysis increases the pH of the placement site microenvironment. Addition of Ca-salts with urea will control or reduce the microsite pH, thus reducing NH3 losses. The degree of Ca-saturation of the cation exchange sites may influence the ratio of calcium:urea required to control ammonia loss. A laboratory study was conducted to determine if adsorbed Ca or CaCO3 additions (acid soils only) had a measureable impact on Ca control of NH3 loss from surface applied urea at various Ca:urea ratios.
With urea alone applied to the soil surface varying the adsorbed Ca content of the treatment soil did not influence NH3 loss. The addition of CaCl2 with urea on the same pretreated soils generally resulted in NH3 losses reflecting the initial pH of the soil. The Ca-saturated acid soils and those acid soils receiving CaCO3 had higher NH3 losses than untreated soils in the presence of urea with soluble CaCl2. It was noted that increasing the calcium:urea ratios progressively depressed the NH3 loss from all soils. Increasing the percent Na-saturation of the calcareous Harkey soil to 25 and 50% (ESP) reduced Ca control of NH3 loss due to Ca being exchanged for Na on the cation exchange sites.
Inclusion of CaCl2 with the urea mixture on the surface of the pretreated acid soils resulted in stepwise differences in NH3 loss concuring with the increases in pretreatment soil pH values (differing exchangeable Ca content). Other parameters that influence the amount of NH3 loss, such as acidic buffer capacity and CEC, appeared more important than anticipated for control of NH3 loss with the calcium:urea mixture. On Ca enriched soils the calcium:urea mixture was only slightly less effective in its ability to control NH3 losses than on untreated soils.
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Contribution from the Texas Agric. Exp. Sta., Texas A&M University System, College Station, TX 77843, USA
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Fenn, L.B. Effects of initial soil calcium content on ammonia losses from surface-applied urea and calcium-urea. Fertilizer Research 16, 207–216 (1988). https://doi.org/10.1007/BF01051371
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DOI: https://doi.org/10.1007/BF01051371