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Extraction and colorimetric methods were evaluated for determining total P in soils. A procedure involving simultaneous digestion of 60 soil samples with perchloric acid and determination of extracted orthophosphate with an ascorbic acid method was evaluated. Analysis of diverse soils indicated that the proposed and conventional HClO4 digestion procedures yielded essentially the same total P values; however, both HClO4 methods underestimated total P by 1–6% when compared to Na2CO3 fusion. A comparison of colorimetric orthophosphate procedures indicated the method of Murphy and Riley (1962) was suitable for determination of orthophosphate following either HClO4 digestion or Na2CO3 fusion. Determination of total P in soils by tube HClO4 digestion and estimation of extracted P by the method of Murphy and Riley (1962) enables rapid and precise estimation of total P in a wide range of soils.

Progress of the Dumas and Kjeldahl procedures over the past century is reviewed. Many recent papers claim that various modifications of the standard Kjeldahl method enhance accuracy, precision, or speed, and reduce cost of analysis. Furthermore, several authors advocate use of Pyrex tubes heated in an aluminum block for digestion instead of traditional Kjeldahl flasks. A review of current semimicro-Kjeldahl methodology suggests that proceeding under the following conditions gives satisfactory results: sample size, 50–200 mg for plant material, 100–500 mg for soil; digestion acid, concentrated H2SO4; sample size ratio (mL/g) of 16:1 for soils and 22:1 for plant materials; digestion salt, minimum 0.33 g K2SO4 added/mL H2SO4; catalysts, add CuSO4 5H2O at rate of 10% (w/w) of K2SO4 added, HgO at 5% (w/w) of K2SO4, or add Se to K2SO4–CuSO4 5H2O mixtures at rate of 1% (w/w) K2SO4; digestion time, 1 and 3 h past clearing for plant materials and soils, respectively; pretreatments, use salicylic acid or reduced iron to recover nitrate from sample. Use of Pyrex tubes heated in an aluminum block appears suitable for digestions. Our results indicate that the 2 commercial tube digestion systems tested produce satisfactory recovery of total N from soils and plant materials. Ammonium in Kjeldahl digests may be quantitatively determined by distillationtitration, ammonia electrode, or colorimetric techniques.