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

Abstract

Abstract An increase in cation exchange capacity (CEC) of soils normally reduces gaseous ammonia (NH 3 ) loss. Calcium additions with urea, however, modify the influence of soil CEC on NH 3 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 NH 3 loss than would exist without soluble Ca. A knowledge of this relationship is necessary to better manipulate this technology for the reduction of NH 3 loss. This research examined the effect of CEC on NH 3 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 NH 3 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, NH 3 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‐NH 3 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 NH 3 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 NH 3 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 NH 3 losses except at 110 kg N/ha.