Effect of Water Pretreatment on Total Nitrogen Analysis of Soils by the Kjeldahl Method

Effective methods are needed for determining total N when studying the fate of N fertilizer applied to soil. This study was concerned with the pretreatment of samples of a Vertisol (calcareous Typic Pellusterts) with water, 20 and 2 mL for macro- and semimicro-Kjeldahl methods, respectively, prior to digestion. The need for such a modification with certain clay soils was originally recommended in 1925, but its requirement in conjunction with modern Kjeldahl methods is rarely reported. The macro- and semimicro-Kjeldahl methods used involved digestion of soil with a H2SO4-K2SO4-CuSO4 5H2O-Se mixture. Both techniques yielded digestion residues with dark-colored granules and low recoveries of N with soil samples ground to pass through a sieve with 0.8-mm openings (20 mesh), unless the soil was initially treated with water. For instance, N recovery from a surface Vertisol was increased by 29% as a result of a 30-min water soaking prior to a macro-Kjeldahl digestion. Extension of the digestion period up to 5 h after initial clearing did not eliminate the need for the water pretreatment. The requirement for the water pretreatment for semimicro-Kjeldahl methods was greatly reduced when the soil was ground to pass through a sieve with 0.16-mm openings (100 mesh). Increase in soil N due to water pretreatment was reduced from 35 to 6% by grinding the coarser soil to such fineness. A salicylic acid pretreatment, involving water addition after the nitration reaction, permitted satisfactory digestion. The water pretreatment effect appeared to be associated with incomplete digestion of organic N, not with the recovery of fixed NH4+, and was found with both surface and subsoils. Water pretreatment of samples from several horizons of an Alfisol (Udic Rhodustalfs) had little effect on recovery of soil N. A water pretreatment is probably not needed to obtain maximum recoveries of N if the nonmodified Kjeldahl method results in a postdigestion soil residue without dense, darkish granules. However, the water soaking was found to reduce bumping during digestion of many soils and is recommended for routine us