Because reduced Cr has been considered to be the stable form in soils, we were surprised to find that added trivalent Cr oxidizes readily to the hexavalent form under conditions prevalent in many field soils. The key to the oxidation appears to be the presence in the soil of oxidized Mn, which serves as the electron acceptor in the reaction. The relative ability of a soil to oxidize Cr may be predicted by measuring Mn reducible by hydroquinone, or it may be determined directly by means of a quick test in which Cr(III) is added to a fresh moist soil sample.Oxidation of Cr by soils was not discovered earlier because the importance of studying fresh field soils, rather than crushed, dried, stored samples, was not appreciated. Plants were severely damaged by Cr(VI) formed from Cr(III) added to fresh soil samples. Hexavalent Cr still was present in a soil stored moist at 25°C for 5 mo.

Dried, pulverized, and sieved soil samples are prepared and stored for laboratory research convenience. Drying and increasing time of storage both tend to push soil, which is metastable, toward increased surface acidity, reduced Mn, and increased solubility and oxidizability of soil organic matter. Reformation of metastable moist soil is a slow process mediated by environmental conditions interacting with life in the soil. Similar effects undoubtedly occur in the field as soils are dried.Two sets of problems confront the researcher using dried soil samples: those associated with drying itself and those associated with remoistening. The behavior of a dried sample immediately after adding water to it is different from that of the continuously moist soil. Remoistening for a longer period is followed by a microbiological explosion. The behavior of the soil for an extended period, perhaps more than a month after rewetting, may be anomalous, or at least unpredictable. Keeping a soil moist and aerobic, though certainly inconvenient, is the most satisfactory method of storage for many research uses.

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Adsorption and reduction of added Cr(VI) were characterized in soils with contrasting pH's, organic matter contents, and chemical and mineralogical properties.Presence of soil organic matter brought about spontaneous reduction of Cr(VI) to Cr(III), even at pH's above neutrality. Reduction did not occur in soils very low in organic matter unless an energy source was added. Cow manure added to practically organic‐free Cecil B2 reduced Cr(IV) only after the pH had been lowered below 3 with HCl.The solubility of Cr(VI) in the presence of excess Al changed in a pattern remindful of orthophosphate. All of the soils, except a pH 7.8 Cca horizon material, adsorbed Cr(VI). Presence of orthophosphate prevented the adsorption of Cr(VI), presumably by competition for the adsorption sites. Consistent with this finding, KH2PO4 was found to be the best extracting agent for Cr(VI).It was concluded that behavior of Cr(VI), if it remains in soils, is similar to that of orthophosphate. However, unlike phosphate, Cr(VI) is quickly reduced by soil organic matter. Thus, Cr(VI) added to a soil will remain mobile only if its concentration exceeds both the adsorbing and the reducing capacities of the soil.

A simple and sensitive method is described that determines, colorimetrically, oxidizable organic matter in solutions from acid soils. It relies on measuring the loss of color by a Mn(III)‐pyrophosphate complex as Mn(III) becomes reduced by organic C in the presence of concentrated H2SO4. The method is applicable to 1‐mL samples containing 0.08 to 4.0 µmol of organic C and is practically free of interferences in aerobic solutions.

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Based on adsorption studies with Fe(OH)3, Cr(OH)3, and soils, 10 mM K‐phosphate buffered at pH 7.2 was found to be adequate for characterizing exchangeable Cr(VI). The quantity of Cr(VI) removed from solution by 19 A and 19 B horizon soils from the northeastern United States was partitioned into two fractions, based on the amount of soil‐Cr extracted with the phosphate buffer. The fraction removed by the buffer was termed “exchangeable;” that retained by the soil was termed “nonexchangeable.” The nonexchangeable Cr(VI) not recovered by phosphate had been either reduced to an insoluble form of Cr(III), or had been precipitated or very tightly adsorbed by the soil as anionic Cr(VI).Of 20 µmol/100 g soil (23 kg/ha) Cr(VI) added, an average of 7.2 was removed by the unlimed 38 soils (mean pH 5.4). Of this quantity, 4.8 µmol were exchangeable by phosphate. Surface soils generally removed less Cr(VI) than B horizons (5.8 vs. 8.6 µmol/100 g), and the proportion of Cr(Vi) removed in phosphate‐exchangeable form also was lower. Liming to pH 7 decreased the average quantity of Cr(VI) removed by the soils to 2.6, with exchangeable and nonexchangeable forms each accounting for about 50% of the total.Sulfate and phosphate added with Cr(VI) to two A and two B horizon soils decreased Cr(VI) removal by the soil, with phosphate having a greater effect than sulfate. Although liming decreased Cr(VI) removal by the soils, even less Cr(VI) was held in exchangeable or nonexchangeable form in limed treatments containing the competing anions.Gallic acid rapidly reduced Cr(VI) held by soil colloids in exchangeable form except in limed samples of spodic horizons and a Hydric Dystrandept Ap, all high in amorphous, organically complexed Al and Fe sesquioxides.

Organically complexed Cr(III) added to soils may remain soluble, whereas the free Cr(III) metal ion would quickly become adsorbed and/or hydrolyzed and precipitated in the absence of soluble, complexing ligands. To separate these two phenomena, a laboratory procedure was developed to simulate the disposal of Cr(III)‐containing organic wastes in soils, and to investigate the solubilities of organic complexes of this heavy metal after addition to soil. The method involves adding Cr(NO3)3 to solutions of organic ligands adjusted to a pH between 3 and 8 to form inorganic hydrolyzed species and organic complexes at each pH. After measuring soluble Cr vs. pH, moist soil is added to bring the pH of each Cr‐ligand system to 6.5–7.0, thereby precipitating nncomplexed, inorganic Cr(III), and labile Cr(III)‐organic complexes. Subtracting Cr(VI) formed from total soluble Cr in the soil suspension gives a relative measure of organically complexed Cr(III) originally present in the aqueous systems at each pH.Citric acid, diethylenetriaminepentaacetic acid (DTPA), fulvic acids, and water‐soluble organic matter from air‐dried soil kept Cr(III) in solution above pH 5.5, and prevented its immediate removal by soil.Added Cr‐citrate remained soluble for at least 1 y in limed samples of an Ultic Hapludalf Ap horizon incubated at field capacity moisture. Because organic acids similar to those studied here may be found in sewage sludge, animal manures, and industrial wastewater, these results should be useful in predicting the fate of Cr(III) added to soils amended with such wastes.