Soil Science Society of America Journal · 1981 · 187 citations · 0 references
Abstract This work describes an ultraviolet (UV) spectrophotometric difference method for quantitative assay of soil extracts for NO 3 ‐ and NO 2 ‐ . The method is rapid, sensitive, and very reliable even in the presence of organic matter and other non‐nitrate species that interfere with NO 3 ‐ determinations and that are often found in soils. The method measures NO 3 ‐ by first measuring the total absorbance of the soil extract solution at 210 nm, which arises from the presence of both NO 3 ‐ and non‐nitrate species. The absorbance of the non‐nitrate species is determined after reducing NO 3 ‐ to nonabsorbing species using Raney nickel catalyst in acid medium. The absorbance difference is attributed to NO 3 ‐ alone and is proportional to its concentration. If NO 2 ‐ is present in the original soil extract, both NO 3 ‐ and NO 2 ‐ are measured separately by incorporating an additional step into the procedure, which selectively removes NO 2 ‐ with sulfamic acid. Soil NO 3 ‐ determined by this method on extracts from nine Illinois soils were highly correlated ( r 2 = 0.998) with results obtained by steam distillation. Correlation with results from a direct UV method was poorer. Recovery of standard additions of NO 3 ‐ and NO 2 ‐ were 99% and 98.5%, respectively. The minimum detectable concentrations in the soil samples are 0.45 µg NO 3 ‐ ‐N/g of soil and 0.64 µg NO 2 ‐ ‐N/g of soil, with the linear ranges extending to 100 and 140 µg/g of soil, respectively. In the absence of NO 2 ‐ , 50 samples were analyzed in 3 hours' time with coefficients of variation at the 99% confidence interval (CI) of 0.68, 1.2, and 6% for 15 to 100, 5 to 15, and 1 to 5 µg NO 3 ‐ ‐N/g of soil. When samples are also assayed for NO 2 ‐ , the time is extended to 4 hours for 50 samples with coefficients of variation at the 99% CI, of 0.71, 1.4, and 10% for 15 to 100, 5 to 15, 1 to 5 µg NO 3 ‐ ‐N/g of soil.