Rates of Weathering and Soil Development

Temperature and moisture are the major environmental variables affecting weathering rates. Assuming similar chronological ages and parent materials, the difference in composition of the South Dakota, North Carolina, and Costa Rican soils of Table 7.2 illustrates the effects primarily of temperature. Weathering is much fester in the warm climate of Costa Rica than in the cold winters and short summers of South Dakota. North Carolina s climate is intermediate between the two. 

The rate of water movement through soils determines the rate at which weathered solutes are removed from the vicinity of soil particles. Weathering can continue even when the rate of movement is slow, such as in poorly drained soils. Lack of water, however, almost totally arrests soil development. Desert soils can be old chronologically, yet young in the sense of soil development. 

Jenny (1941) proposed that soil development be regarded as the result of five soil-forming factors climate, topography, biosphere, parent material, and time. In a qualitative sense, the weathering rate is related to these factors by 

Converting this expression into a quantitative equation, however, is beyond our present capabilities. None of the four factors has been adequately described numerically. Climate, for example, is an ill-defined integration of the intensity, duration, and seasonal distribution of temperature, moisture, and evaporation. Deposition of airborne salts and dust and parts of erosion should also be included as subtle parts of climate. The parent material factor includes chemical composition, mineral composition, crystal size, and rock fabric (stincture). 

The relative importance of each factor in Eq. 7.6 also varies with local and regional conditions, In a peat bog or on a steep mountain slope topography clearly has a prominent role. For a young alluvial soil, on the other hand, parent material is usually the dominant factor. In desert and polar soils, the biosphere s role is comparatively small. 

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Climate. The rate of weathering partly depends on the climate. For example, the wide variations of temperature and the high rainfall of the tropics make for much faster soil development than would be possible in the colder and drier climatic regions. 

Hale, 1961). If it is held that lichens initiate plant succession and soil development in certain situations, such as plane rock surfaces, the slow rate of growth of lichens does not preclude their ability to function as biogeo-physical and biogeochemical weathering agents, to accumulate nutrients, and to be responsible for the accumulation of organomineral material. In such situations soil formation is itself a rather slow process. 

The high concentration of dissolved metals in the Rio Solimoes is contrasted by the solute-deficient waters of the Rio Negro which drains the highly weathered lateritic and podsolitic terrains of the Central Amazon. Due to a lack of exposed rock, the intense chemical weathering of the humid tropics over millions of years, and the low rates of weathering in conjunction with the development of thick, siliceous and aluminous soils, the suspended sediment are typically cation-depleted, consisting almost entirely of quartz and kaolinite (27), while the dissolved load is dominated by silicon, with extremely low levels of major cations and trace metals. 

Based on predicted weathering and erosion rates of the region, we estimate the profile to be several million years old. Because the soil has developed in situ, the topmost grains have reacted with water for the greatest extent of time. With depth, the total "lifetime" of the particles as soil decreases. This implies that the topmost quartz surfaces should be "reactively mature" (all fines removed, deep grown-together etch pits) and the bottom-most quartz surfaces should be "reactively young" (plentiful fines, fresh surfaces). ... 

The primary objectives of mass-balance studies are (i) quantify the mass fluxes into and out of watershed systems (ii) interpret the reactions and processes occurring in the watershed that cause the observed changes in composition and flux (iii) determine weathering rates of the various minerals constituting the bedrock, regolith, and soils of the watershed and (iv) evaluate which mineral phases are critically involved in controlling water chemistry to help develop models of more general applicability (i.e., transfer value). 

The rate of phosphate loss from soils by weathering is about the same as the overall weathering rate, so the total amount of phosphate in soils tends to remain constant throughout soil development. The availability of phosphate to plants, however, decreases as soils become more acid and the proportion of phosphorus as aluminium and iron phosphate increases. 

The phosphate content of soils tends to remain roughly constant during soil development. Phosphate is only slowly leached from soils, at about the same rate as silica loss, so the total phosphorus content of soils varies little with soil maturity. The form of phosphate, however, changes from predominantly apatite (Cas(OH,F)(P04)3) in igneous rocks to AI(III) and Fe(III) phosphates in moderately to strongly weathered soils. 

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