The selectivity of the Tamm and Mehra-Jackson reagents to iron (hydr)oxides was verified chemically by comparing the efficiencies of the two methods of iron extraction: parallel (commonly adopted) and sequential, i.e., by calculating the difference DELTA Fe = [Fe_(dit) - (Fe_(ox(paral)) + Fe_(dit(seq)))]. The expected equality of the extracted iron (i.e., AFe ~ 0) is rarely observed. A positive balance reaching AFe = (0.5-0.8)% predominates upon iron extraction from forest soils of the Cis-Ural region. This is probably due to the stable Fe(II)-oxalate forming in the course of the Tamm extraction, which is incompletely dissolved by the dithionite-citrate-bicar-bonate (DCB). On the contrary, a negative balance with AFe reaching -0.4% predominates in the steppe soils of the Stavropol region. This may be caused by Fe(II) minerals (pyrite and siderite) acting as catalyzers in Tamm's extract, while being weakly soluble in the DCB. To follow the additivity principle for the Tamm and Mehra-Jackson extracts, we suggest a modification of the Schwertmann criterion K_(Sch) = (Fe_(ox) : Fe_(dit(paral))), which sometimes exceeds 1, to K_(Sch)~m = Fe_(ox) : (Fe_(ox) + Fe_(dit(seq))), which is always less than 1. The values of the modified Schwertmann criterion better agree with the color of the Cis-Ural forest soils expressed in the CIE-L*a*b* optical system as compared with the initial Schwertmann criterion. In the steppe soils, the use of the modified criterion makes it possible to replace the illogical values of K_(Sch) = 1.11-1.37 by the quite acceptable K_(Sch)~m = 0.68-0.83.
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机译：Tamm和Mehra-Jackson试剂对铁氧化铁的选择性通过比较两种铁提取方法的效率进行了化学验证：平行（通常采用）和顺序提取铁，即通过计算差值Delta Fe = [Fe_ （dit）-（Fe_（ox（paral））+ Fe_（dit（seq）））]。很少观察到所提取铁的预期相等性（即AFe〜0）。从顺式-乌拉尔地区森林土壤中提取铁时，达到AFe =（0.5-0.8）％的正平衡为主。这可能是由于在Tamm萃取过程中形成了稳定的草酸Fe（II）-草酸，其被连二亚硫酸盐-柠檬酸-bicar-bonate（DCB）完全溶解。相反，在斯塔夫罗波尔地区的草原土壤中，AFe达到-0.4％的负平衡占主导地位。这可能是由于Fe（II）矿物（黄铁矿和菱铁矿）在Tamm提取物中起催化剂的作用，而微溶于DCB。为了遵循Tamm和Mehra-Jackson提取物的可加性原理，我们建议对Schwertmann准则K_（Sch）=（Fe_（ox）：Fe_（dit（paral）））进行修改，该准则有时会超过1，到K_（ Sch）〜m = Fe_（ox）：（Fe_（ox）+ Fe_（dit（seq））），始终小于1。修改后的Schwertmann准则的值与Cis-Ural森林的颜色更好地吻合与初始Schwertmann准则相比，CIE-L * a * b *光学系统表示的土壤。在草原土壤中，使用修改后的标准可以用相当可接受的K_（Sch）〜m = 0.68-0.83代替K_（Sch）= 1.11-1.37的非逻辑值。
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