Colorimetric Determinations of Exchangeable Ammonium, Urea, Nitrate, and Nitrite in a Single Soil Extract1,2

Interest in N transformations in soils has increased greatly in recent years, particularly with increased use of urea‐containing fertilizers. However, only one procedure for determination of exchangeable ammonium, nitrate, nitrite, and urea in a single soil extract has been proposed and it requires the use of both distillation and colorimetric equipment. It would be useful to have a set of colorimetric procedures for the determination of these N forms in a single extract. Previous research indicated sodium sulfate, as an extractant, would be least likely to produce unwanted interferences in these procedures. Thus, research was initiated to determine (i) if Na 2 SO 4 , could be used as an extractant for exchangeable NH 4 + , NO 3 ‐ , NO 2 ‐ and urea and (ii) necessary modifications to make desirable colorimetric procedures usable in a Na 2 SO 4 extraction system. A procedure for the colorimetric determinations of exchangeable NH 4 + , urea, NO 3 ‐ , and NO 2 ‐ in soil using a single soil extract is described. The soil sample is extracted with Na 2 SO 4 containing phenylmercuric acetate for 30 min and then leached with four 25 ml portions of extractant and made to volume. The extract is analyzed for NH 4 + by a modified Nessler reagent, urea by either diacetyl monoxime and thiosemicarbazide or p‐dimethyl aminobenzaldehyde depending on concentration, NO 3 ‐ by chromotropic acid and NO 2 ‐ by 4‐aminobenzenesulfonic acid and 1‐aminonaphthalene hydrochloride. The procedure gave quantitative recovery of NH 4 + ‐N, urea‐N, NO 3 ‐ N, and NO 2 ‐ ‐N added simultaneously to five different soils.