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Ammonia Loss and Associated Reactions of Urea in Calcareous Soils¹

ABSTRACT

Urea in soil is a mobile compound which can be lost by leaching or ammonia (NH₃) volatilization. This research examined the contributions of water additions and chemical reactions in explaining the behavior of applied urea.

Urea [CO(NH₂)₂] initially moves undissociated into the soil with added water. Ammonia losses from surface-applied urea thus decreased with increasing amounts of applied water. Ammonia losses from urea were minimal if injected into the soil 2.5 cm or more, whereas NH₃ losses from (NH₄)₂SO₄ (AS) were still high when placed 7.5-cm deep. Application of AS produced chemically equivalent amounts of Ca and Mg whereas urea and NH₄OH precipitated Ca and Mg. The amount of Ca and Mg precipitated increased with decreasing NH₃ losses. Precipitation of divalent cations enhanced adsorption of NH₄ on soil cation exchange sites, thereby reducing upward movement of NH₄ and subsequent NH₃ losses. The presence of Ca from reaction of AS with CaCO₃ enhanced upward movement of NH₄ and increased NH₃ losses.

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

² Associate and Assistant Professors, respectively, Texas Agric. Exp. Stn., Texas A&M Res. Center, 1380 A&M Circle, El Paso, TX 79927.

Received for publication February 21, 1978. Accepted for publication September 22, 1978.

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