The following six methods were chosen to compare their effectiveness, rapidity and simplicity for iron-oxide removal from soils:( a) Tamm's ammonium acid oxalate method, 16)23) Jeffries' magnesium ribbon method, 12) 13) (d) Karim's 14) and Haldane's" zinc powder method, and (e) Mehra and Jackson's dithionite-citrate-bicarbonate method. 11) Soil samples used are some red soils and podzols in Japan (Table 1 ). The air-dry soil (<2mm) is carefully ground and passed through a 1-mm sieve (round

more »

... ). A 2-gm sample (<1mm) is placed in a 300-m1 Erlenmeyer flask, a 300-m1 tall beaker or a 100-m1 centrifuge tube for iron-oxide removal. Each original procedure has been followed with only the slightest and unimportant modifications, except that the amounts of reagents for extraction were reduced to onefifth of the original directed ones in the method of Truog et al. (130ml of water, 1ml of sodium sulfide solution, 2gm of ammonium chloride, etc. were used for a 2-gm sample). Organic matters in an extracted solution are decomposed by the sulfuric and nitric acids treatment and the dehydrated silica is separated by filtration. Iron in the hydrochloric acid solution is determined volumetrically by the stannous chloride reduction-potassium dichromate oxidation method, 19) using sodium diphenylamine sulfonate as indicator (as in Karim's or Haldane's method). The results obtained are shown in Table 2 . Tamm's method generally gives much lower values than others for most samples. Roughly speaking, the other five methods