Reduction of nitrate prior to Kjeldahl digestion

A new method is described for reducing nitrate prior to Kjeldahl digestion of soil and crop samples. Zinc is used to reduce CrIII to CrII, which in turn reduces NO3—N. Zinc powder is mixed with the sample, a solution of CrK(SO4)2 in H2SO4 added and the mixture allowed to stand at room temperature for 2h. The usual Kjeldahl reagents (K2SO4, CuSO4 and H2SO4) are then added and the digestion completed in a digestion block at 360°C. Nitrate is quantitatively converted to NH4—N, whether on its own, in plant material or in soil.