We have shown that the traditional Kjeldahl method applied to clay soils of the Duero Basin (Spain) is not effective; so, it is necessary to add HC1 + HF to free trapped ammonium. This fixed ammonium has been measured by two methods which give significantly different results. The level of fixed ammonium in cultivated clay soils is relatively high (it ranges from 180 to 490 ppm). Fixed ammonium is significantly correlated to clay content and total inorganic nitrogen of the soil. A proposed method of analysis for total N is given.