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Soil Cation Exchange Capacity Effects on Ammonia Loss from Surface-Applied Urea in the Presence of Soluble Calcium¹

ABSTRACT

An increase in cation exchange capacity (CEC) of soils normally reduces gaseous ammonia (NH₃) loss. Calcium additions with urea, however, modify the influence of soil CEC on NH₃ loss. The cation exchange sites are essentually saturated with Ca, leaving the NH⁺₄ in the soil solution. Cation exchange capacity, therefore, will have a different influence on NH₃ loss than would exist without soluble Ca. A knowledge of this relationship is necessary to better manipulate this technology for the reduction of NH₃ loss.

This research examined the effect of CEC on NH₃ loss in the presence of high concentrations of soluble Ca. Laboratory and greenhouse trials were conducted on three soils with CEC values of 0, 14, and 58 meq/100 g. Calcium was added with urea at rates from none to an equivalent amount. Nitrogen loss was measured directly as evolved NH₃ and indirectly as N absorbed by sudangrass.

Ammonia losses from surface-applied urea decreased as soil CEC increased both without Ca and at a Ca/N ratio of 0.25. However, as the Ca/N ratio increased above 0.25, NH₃ loss was greater from a Harkey soil [coarse, silty, mixed (calcareous), thermic Typic Torrifluvents] with moderate CEC as compared to a sand.

Greenhouse data confirmed laboratory results and the CEC-NH₃ loss reversal was observed at the Ca/N ratio of 0.30. Plant N recovery was 98% from Gila s [coarse, loamy, mixed (calcareous), thermic Typic Torrifluvents] and 70% from Harkey sicl.

Increasing rates of urea in the absence of added Ca resulted in comparable NH₃ losses with sand and Harkey sicl. Increasingly higher percent N losses with increasing N application rates were experienced with Houston Black clay. However, the addition of Ca in excess of Ca/N ratio of 0.25 generally resulted in reduced NH₃ loss as the N application rate increased. The use of fixed Ca/N ratios with increasing urea rates resulted in progressively greater percentage reductions of NH₃ losses except at 110 kg N/ha.

Key Words: N fertilizers • surface fertilization

¹ Contribution from the 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 A&M Univ., Agric. Res. Center, 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 October 13, 1980. Accepted for publication October 23, 1980.