Weathering and Soil Development

The particle surfaces and soil solutions created by weathering tend to be more simi lar chemically than the composition of the parent minerals. This relative uniformity is of great benefit to the development and maintenance of life. The- small variation among different soils and soil solutions, however, affects plant growth and is often emphasized because of humanity s concern for maximum or optimum growth of agricultural plants. Compared to the vagaries of weather, pests, prices, and other production factors, soil chemical variability is relatively small and within a farmer s control. 

Compound Granodiorite0 (Granitic) (%) Basalt0 (%) Shale6 %) Sandstone6 (%) Limestone6 (%) 

6From F. J. Pettijolin. 1957. Sedimentary Rocks, 2d ed. Harper Row, New York. 

Some minerals remain virtually unweathered despite their inherent instability, because them dissolution rate in water is exceedingly slow. Quartz particles larger than several micrometers in size (fine silt) remain in soils for so long that quartz appears to be the most stable state for soil silicon, When finely divided into clay-sized particles, however, quartz persists only slightly longer in soils than does clay-sized feldspar. Feldspar disappears from the sand and silt fractions relatively rapidly. 

To illustrate, imagine a drop of pure water falling on the surface of albite (NaAlSi30g) feldspar. This igneous mineral is unstable in water at room temperature and pressure. The first ions or molecules to dissolve achieve a high degree of randomness because they can roam in the droplet unhindered by other ions  

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Based on the mineralogy controlling weathering and soil development, sensitivity of ecosystem to acid deposition is assessed with a comprehensive consideration on the effect of temperature, soil texture, land use and precipitation. The results show that the most sensitive area to acid deposition in China is Podzolic soil zone in the Northeast, then followed by Latosol, Dark Brown Forest soil and Black soil zones. The less sensitive area is Ferralsol and Yellow-Brown Earth zone in the Southeast, and the least sensitive areas are mainly referred to as Xerosol zone in the Northwest,... 

Silicon reactions are central to rock weathering and soil development. Silicon is the soil component lost in greatest amount from rock minerals during weathering, and the transformations of silica into secondary minerals are the major reactions of soil development. The sand fraction of soils is usually >90% quartz (SiC>2), the most prevalent form of Si in soils. Highly weathered soils may contain as little as 20% Si (Table 2.1a). A1 and Fe ore deposits are essentially highly weathered soils from which most of the Si has been lost. 

Despite such reservations, the thermodynamic data now available provide a reasonably satisfactory picture of solution conditions under which well-defined minerals form and decompose during weathering and soil development. The equilibrium between kaolinite and gibbsite, for example, can be written... 

Based on the mineralogy controlling weathering and soil development, sensitivity of ecosystems to acid deposition is assessed with the comprehensive consideration on the effect of temperature, soil texture, land use and precipitation. The results show... 

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