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.