Saprolite soils

The preceding description of soil formation is extremely generalized and does not include many soil types. For example, peat soils are predominantly composed of partially decomposed plant remains. In arid areas, upward water flow and evaporation at the soil surface can produce a hard caliche soil. In very warm and humid areas, leaching and organic matter decomposition occur quite rapidly and soil minerals can become highly weathered, resulting in saprolite soils. In ecosystems in these areas, biomass, instead of soil particles, may become the major repository of plant nutrients. 

At the Endeavor (formerly Elura) Mine (43 km NNW of Cobar), the main lens of the Elura Zn-Pb-Ag orebodies is subcropping with a small patch of gossanous float present in an essentially flat landscape. There Pb in saprolite (>50 ppm in an area 1.4 x 1.0 km) and lag (>50 ppm in an area 2.5 x 1.6 km) define the underlying mineralisation. However Zn in soil is anomalous for >1 km to the southwest of the main lens (Lorrigan... 

Owing to their extremely low solubilities in an aerobic environment, goethite and hematite remain unchanged over geological time spans. They may, therefore, store information about the environment in which they formed. Al substitution may be one such piece of information. Thus, medium to high Al substitution has been observed in goethites from tropical and subtropical soils, bauxites and saprolites (Fitzpatrick Schwertmann, 1982 Schwertmann Kampf, 1983 Curi Franzmeier,... 

April et al., 1986 Kirkwood and Nesbitt, 1991 Land et al., 1999). Likewise, Cleaves (1993) found watershed solute discharge in the Piedmont of the eastern USA, to be 2-5 times faster than for past periglacial periods represented by long-term saprolite weathering. He attributed these weathering rate differences to past periods of lower precipitation, colder temperatures and lower soil gas CO2. 

Weathering in older soils produces decreasing permeabilities due to in situ secondary mineral formation and the development of hard pans and argillic horizons. Such zones of secondary clay and iron-oxides are clearly evident in the increased aluminum and iron at a depth of a meter in the Panola regolith (Figure 3). Low permeabilities in such features are commonly related to the absence of continuous pores due to the formation of thick cutans of clay (O Brien and Buol, 1984). This process is enhanced by physical translocation and collapse of saprolite structures (Torrent and Nettleton, 1978). 

Weathering of primary minerals as a source of nutrients for trees results eventually in almost complete depletion of these minerals near the soil surface. In soils of the Appalachian area in the southeastern USA, layers of saprolite (undisturbed weathered rock), up to tens of meters in thickness, develop below most of the biologically active soil and thus are relatively protected from disturbance by bioturbation. However, even in saprolites weathering is not complete, and tree roots are still able to get enough nutrients from the saprolite for growth (Velbel, 1985). 

In a humid subtropical area such as the Parana Basin in Brazil, the weathering profile is 30 m thick and has a lower yellow part which is a saprolite. Above this the upper red soil called Terra Rossa consists of kaolinite and iron oxides (Benedetti et al., 1994). The rock being weathered in this area is a 140-million-year-old basalt. Benedetti et al. (1994) found that the calculated weathering rate is increased by a factor of 1.3-5 by including biomass uptake and release of calcium, magnesium, and potassium in their weathering model. 

Milnes, A.R., Wright, M.J. Thiry, M. (1991) Silica accumulations in saprolites and soils in South Australia. In Nettleton, W.D. (Ed.) Occurrence, Characteristics and Genesis of Carbonate, Gypsum, and Silica Accumulations in Soils. Special Publication 26. Madison, WI Soil Science Society of America, pp. 121-149. 

DBCP Sorption on Soil and Saprolite. For the Kunia site, sorption values were measured on samples from several depths in Boreholes 2 and 3 as well as from three shallow depths near the well. The borehole sorption data, obtained by flow equilibration, are given in Table I. Batch equilibration gave results which were about 20% higher for samples with the highest sorption, but the batch method had inadequate precision when sorption was low, as for samples 2-1, 2-3 and 3-2. The precision of the flow-equilibration method for DBCP is limited only by the GC analysis. Zero values in Table I indicate sorption Kd < 0.01 ml/gm. Of the several borehole samples, only sample 3-1 showed substantial sorption of DBCP. In both boreholes there was little sorption below 1 meter. The diminished sorption of DBCP with increasing depth appears to be related principally to a decrease in organic carbon with depth. 

Halloysite Phyllosilicates Volcanic ash granitic (feldspathic) saprolite R Ephemeral in intensely weathered soils... 

Kretzchmar, R., W.P. Robarge, and A. Amoozegar. 1995. Influence of natural organic matter on transport of soil colloids through saprolite. Wat. Resour. Res. 31 435-445. 

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