Caliche soil

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. 

The details of the soil profile studied by Vanden Heuvel (1966) show how the magnesio-silicates are associated with the caliche layer, that occurs in the soil profile itself (in the instance studied it covers the upper horizons). Table 3 shows the clay mineral assemblages identified. 

Potassium nitrate, essential in the manufacture of black gun powder, was produced by the Chinese, who had developed gun powder by the tenth century AD. The process involved the leaching of soil in which nitrogen from urine had combined with mineral potassium. By the early 1800s, potassium nitrate had become a strategic military chemical and was still produced, primarily in India, by using the ancient Chinese method. The caliche deposits in Chile are the only natural source of potassium nitrate (2). These deposits are not a rich source of potassium nitrate, purifying only to about 14% as K O. 

CALICHE (Nitrate). The gravel, rock. soil, or alluvium cemented with soluble salts of sodium in the nitrate deposits of the Atacama Desert of northern Chile and Peru. The material contains from 14 to 25% sodium nitrate, 2 to 3% potassium nitrate, and up to I i sodium iodaic. plus some sodium chloride, sulfate, and borate. At one time, this was an important natural fertilizer. 

Figure 7.42. Comparison between (A) an idealized plot of variation in 8180 and 813C for carbonates subjected to vadose and phreatic meteoric diagenesis (after Lohmann, 1988) with (B) the meteoric alteration trend observed for the Key Largo Limestone, Florida, U.S.A. (after Martin et al., 1986). The critical trend in isotopic composition is termed the meteoric calcite line. This trend may be modified at the water recharge surface where evaporation is an important process, caliche is formed and the diagenetic phases are depleted in 13C derived from soil-gas CO2. Another modification can occur distally to the recharge area where precipitating carbonate cements may have isotopic ratios nearly equivalent to dissolving phases.

Numerous theories exist as to how the Chilean deposits formed and survived. It has been postulated that the unique nitrate-rich caliche deposits of northern Chile owe diein existence to an environment favorable to accumulation and preservation of the deposits, rather than to any unusual source of the saline materials (2). The essential conditions are an extremely arid climate similar to that of the Atacama desert in the 1990s, slow accumulation during the late Tertiary and Quaternary periods, and a paucity of nitrate-utilizing plants and soil microorganisms. 

Rocks and soils enriched in bioavailable calcium carbonate (limestone, caliche, etc.). Hard waters, waters affected by acid-rock drainage. Cement, concrete, fly ash, many other industrial/ commercial materials or by-products. 

One type of probe is a hand-held stainless-steel hollow auger, which has soil-air vent holes drilled into the shaft near the bottom and is fitted with a gas sample port near the top (Lovell, 1979). Another type of hollow probe has a straight shaft with a gas sample port at the top and soil-air vent holes at the bottom this probe is pounded into the soil by means of a captive hammer that slides up and down the shaft to either drive the probe into or remove it from the soil (Dyck, 1972 Chemical Projects Limited, Toronto, Ontario, Canada, written common., 1972 Lovell, 1979). Both the hollow auger and hollow hammer-probe are efficient dynamic soil-gas samplers in light soils. Neither sampler works well in stony or hard, compacted soils, or in soils containing layers of caliche attempts to use probes in these soils may either bend the probe or disturb the soil to the extent that the soil-gas sample is diluted by atmospheric air. Soil gas can not be sampled in wet soils with these probes. 

In the southwestern United States and elsewhere in arid or semiarid climates, rainfall may largely evaporate at the land surface, or it may infiltrate into shallow soil horizons, evaporating there or as it returns toward the surface because of capillary forces. Resultant carbonate mineral layers are called caliche and may occur in and on soils, along with layers of chert (impure, precipitated SiOj) and iron(III) oxides, similarly formed, which are called silcrete and ferricrete. 

Figure 7.3 A generalized profile of a forested soil developed in a humid climate with a moderate temperature, showing a caliche (CaCOj) layer between the B and C horizons. From Fundamentals of Soil Science, 3d ed.

Figure 7.4 Plot of the relation between rainfall and depth to the top of carbonate mineral (caliche) accumulation in soils derived from loess. From H. Jenny and C. D. Leonard, Soil Sci. 38 363-81. 1934 by Soil Science Society of America. Used by permission.

Aridisols Soils of dry climates and deserts. Often contain wind-blown dust. A and B horizons thin with little organic material. Calcium carbonate (caliche) accumulations generally present, sometimes with gypsiferous or saline horizons. 

ScHLESiNGER, W.H. (1985) The formation of caliche in soils of the Mojave Desert, California. Geochim. Cosmochim. Acta, 49, 57-66. 

In the subsoils of arid and semiarid soils, Ca commonly precipitates as cakite (CaCC>3) rather than being leached away. It is found as indurated layers (caliche and other local names) in many arid soils and as more diffuse CaC03 in Aridisols and Mollisols. Precipitation of CaCCTj in soils is affected by the rates of soil water movement, CO2 production by roots and microbes, CO2 diffusion to the atmosphere, and water loss by soil evaporation and plant transpiration. CaCC>3 layers are also derived from upward movement and evaporation of Ca-rich waters. Calcium carbonate accumulations can amount to as much as 90% of the mass of affected soil horizons. Gypsum precipitates in some arid soils, despite being about 10 x as water soluble as Ca carbonate. 

Small amounts of weathered solutes reprecipitate in lower soil horizons. Examples include clay accumulation in the B horizon, silica pans (impermeable layers of soil particles indurated with silica), and the wide-spread caliche horizons of CaCC>3 accumulation in arid regions. Most solutes, however, reach the sea, where precipitation of other secondary minerals removes most of the weathered solutes except Na+, Cl-, and Mg2+. Marine sediments, in turn, are slowly convened into igneous, metamorphic, or sedimentary rocks, which form new soil parent material. Such element recycling has circulated ions many times from land to sea during the earth s histoiy. 

Type A means cohesive soils with an unconfined, compressive strength of 1.5 ton per square foot (tsf) (144 kPa) or greater. Examples of cohesive soils are day, silty day, sandy clay, clay loam and, in some cases, silty clay loam and sandy clay loam. Cemented soils such as caliche and hardpan are also considered Type A. However, no soil is Type A if ... 

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