Comparison and evaluation of methods for including nitrate in the total nitrogen determination of soils

A semimicro modification of the Olsen method was developed to include nitratenitrogen in the total soil nitrogen determination by the standard semimicro‐Kjeldahl method. The modified method was found to include nitrate‐nitrogen effectively from standard nitrate solutions and soils. It was significantly superior to the commonly used salicylic acid method, which was found to give incomplete recovery of nitrate‐nitrogen. In addition, changes in a number of experimental conditions such as amounts of water present and concentrated sulphuric acid used did not affect the results obtained by the modified Olsen method. In comparison with the recently recommended Raney catalysts for nitrate‐nitrogen inclusion in the total nitrogen determination of fertilisers, preliminary results using standard nitrate solutions and following their additions to the soil indicate that reduced iron was comparable. Results obtained using glucose as a reducing agent in the standard semimicro‐Kjeldahl method suggest that some nitratenitrogen was included by soil organic matter in the normal total soil nitrogen determination.