Soil Chemical Analysis

Iron Oxide Removal from Soils and Clays by a Dithionite-Citrate System Buffered with Sodium Bicarbonate

Abstract: The oxidation potential of dithionite (Na2S2O4) increases from 0.37 V to 0.73 V with increase in pH from 6 to 9, because hydroxyl is consumed during oxidation of dithionite. At the same time the amount of iron oxide dissolved in 15 minutes falls off (from 100 percent to less than 1 percent extracted) with increase in pH from 6 to 12 owing to solubility product relationships of iron oxides. An optimum pH for maximum reaction kinetics occurs at approximately pH 7.3. A buffer is needed to hold the pH at the optimum level because 4 moles of OH are used up in reaction with each mole of Na2S2O4 oxidized. Tests show that NaHCO3 effectively serves as a buffer in this application. Crystalline hematite dissolved in amounts of several hundred milligrams in 2 min. Crystalline goethite dissolved more slowly, but dissolved during the two or three 15 min treatments normally given for iron oxide removal from soils and clays. A series of methods for the extraction of iron oxides from soils and clays was tested with soils high in free iron oxides and with nontronite and other iron-bearing clays. It was found that the bicarbonate-buffered Na2S2O4-citrate system was the most effective in removal of free iron oxides from latosolic soils, and the least destructive of iron silicate clays as indicated by least loss in cation exchange capacity after the iron oxide removal treatment. With soils the decrease was very little but with the very susceptible Woody district nontronite, the decrease was about 17 percent as contrasted to 35–80 percent with other methods.

Manual for the geochemical analyses of marine sediments and suspended particulate matter

Abstract: Accurate and precise sampling and analytical procedures are essential in environmental geochemical studies. This report provides a detailed description of the techniques and analytical procedures for sampling, grain size determinations, and for precise and accurate AAS determination of the major and trace metals in marine sediments and suspended particulate matter. In addition, it describes the procedures for the chemical partition of the metals, determination of readily oxidizable organic matter, and calcium carbonate. A separate section discusses the normalization of trace metal data.

Determination of nitrogen in soil by the Kjeldahl method

Abstract: 1 The reliability of the Kjeldahl method for the determination of nitrogen in soils has been investigated using a range of soils containing from 0·03 to 2·7% nitrogen2 The same result was obtained when soil was analysed by a variety of Kjeldahl procedures which included methods known to recover various forms of nitrogen not determined by Kjeldahl procedures commonly employed for soil analysis From this and other evidence presented it is concluded that very little, if any, of the nitrogen in the soils examined was in the form of highly refractory nitrogen compounds or of compounds containing N—N or N—O linkages3 Results by the method of determining nitrogen in soils recommended by the Association of Official Agricultural Chemists were 10–37% lower than those obtained by other methods tested Satisfactory results were obtained by this method when the period of digestion recommended was increased4 Ammonium-N fixed by clay minerals is determined by the Kjeldahl method5 Selenium and mercury are considerably more effective than copper for catalysis of Kjeldahl digestion of soil Conditions leading to loss of nitrogen using selenium are defined, and difficulties encountered using mercury are discussed6 The most important factor in Kjeldahl analysis is the temperature of digestion with sulphuric acid, which is controlled largely by the amount of potassium (or sodium) sulphate used for digestion7 The period of digestion required for Kjeldahl analysis of soil depends on the concentration of potassium sulphate in the digest When the concentration is low (eg 0·3 g/ml sulphuric acid) it is necessary to digest for several hours; when it is high (eg 1·0 g/ml sulphuric acid) short periods of digestion are adequate Catalysts greatly affect the rate of digestion when the salt concentration is low, but have little effect when the salt concentration is high8 Nitrogen is lost during Kjeldahl analysis when the temperature of digestion exceeds about 400° C9 Determinations of the amounts of sulphuric acid consumed by various mineral and organic soils during Kjeldahl digestion showed that there is little risk of loss of nitrogen under the conditions usually employed for Kjeldahl digestion of soil Acid consumption values for various soil constituents are given, from which the amounts of sulphuric acid likely to be consumed during Kjeldahl digestion of different types of soil can be calculated10 Semi-micro Kjeldahl methods of determining soil nitrogen gave the same results as macro-Kjeldahl methods11 The use of the Hoskins apparatus for the determination of ammonium is described12 It is concluded that the Kjeldahl method is satisfactory for the determination of nitrogen in soils provided a few simple precautions are observed The merits and defects of different Kjeldahl procedures are discussed

A literature review and evaluation of the. Hedley fractionation: Applications to the biogeochemical cycle of soil phosphorus in natural ecosystems

Abstract: The Hedley fractionation recognizes plant-available forms (Resin Pi, Bicarb Pi, and Bicarb Po) and refractory forms (NaOH Pi, NaOH Po, sonic Pi, sonic Po, HCl Pi, Residual P) of soil phosphorus. This updated survey of the recent literature shows that the sequential fractionation proposed by Hedley et al. can also be used to separate forms of organically bound soil phosphorus from the geochemically bound fractions. We consider that biological P includes all the extracted organic fractions (Bicarb Po, NaOH o, sonic Po) and geochemical P includes the remaining fractions (Resin Pi, Bicarb Pi, NaOH Pi, sonic Pi, HCl Pi) and the Po and Pi in the Residual fraction. Data from the Hedley fractionation suggest that the contribution of geochemical versus biological processes to soil phosphorus availability varies with pedogenesis. The pool of primary phosphate declines and the NaOH and sonicated-NaOH phosphorus fractions increase as phosphorus becomes geochemically fixed to the iron and aluminum oxides in more highly weathered soils. The sum of organic-P fractions — biological P — is an increasing proportion of total available P as a function of soil development. Therefore, the Hedley fractionation provides a valuable index of the relative importance of biological processes to soil phosphorus content across a soil weathering gradient.