CHEMICAL STUDIES OF PODZOLIC ILLUVIAL HORIZONS

Summary Resin‐treated humus extracts, adjusted to predetermined pH values, were mixed with solutions of FeCl 3 , FeCl 2 , or NiCl 2 , at the same pH. At C/Fe(III) ratios of 3 and 6, maximum precipitation of humus and Fe occurred at pH 2‐3, i.e. the approximate precipitation pH of hydrous ferric oxide. Above pH 3, humus was almost completely dispersed; Fe was partially precipitated at pH 5 without extensive co‐precipitation of humus. At ratios of C to Fe(III) found in the B horizon of a New Zealand podzol (C/Fe(III) = 22), no flocculation of humus occurred at any pH value from 1 to 5. Using FeCl 3 and AlCl 3 as a mixed flocculant (C/Al =12, C/Fe(III) = 5·8) complete flocculation of humus occurred from pH 1·8 to pH 4. Comparison of the results with data from pure Al and Fe systems suggested that Fe was the active precipitant at pH3 while Al was active from pH 3 to pH 4. Flocculation of humus by Fe(II) was complete at pH 6‐7, but only at C/Fe(II) = I. At C/Fe(II) = 2 flocculation was partial, the maximum occurring at pH 5. As with the ferric systems, maximal flocculation appears to occur at about the precipitation pH of the metal ‘hydroxide’. Nickel (C/Ni = 3) did not flocculate humus at any pH from 1 to 8. Thus greater amounts of divalent ions are needed to flocculate humus, in agreement with the Hardy‐Schulze effect. Experiments testing the effect of atmospheric oxidation of ferrous‐humus systems on flocculation were, in general, inconclusive, although slight precipitation of humus was observed following aeration of a ferrous‐humus system at pH3. Changes in pH occurred on mixing humus with all metal solutions studied, whether flocculation occurred or not. These changes were negative below a critical mixing pH which depended on the metal species used, and positive above this value. The critical pH values for Fe(III), Fe(II), and Ni were 2·8,6·8‐7·8 (depending on reaction time), and 8·0 respectively; for a mixed Fe(III)‐Al system the critical pH was 4·2. An interpretation of these changes is suggested in the case of ferric iron, that is based on that previously postulated for Al. Semi‐quantitative experiments showed that Ni, Fe(II), and Al ions possess additive properties in flocculation of podzol humus at pH 2·96. Organic matter which contained slightly less than the critical amount of Al necessary for flocculation was still dispersed but highly sensitive towards the addition of small amounts of Fe(II) or Ni which by themselves were without effect. Freshly precipitated hydrous ferric oxide did not sensitize humus towards flocculation by Al at pH 3·2, although considerable amounts of Fe(III) could be co‐precipitated by adding Al in excess of the critical concentration.