Soil cation exchange

The protocol involving NaOAc-HOAc at pH 5 was first proposed and used by Jackson (1958) to remove carbonates from calcareous soils to analyze soil cation exchange characteristics (Grossman and Millet, 1961). Other researchers used HOAc for the extraction of metals from sediments and soils (Nissenbaum, 1972 Mclaren and Crawford, 1973). Tessier et al. (1979) first used the NaOAc-HOAc solution at pH 5 to dissolve the carbonate fraction from sediments. Since then, the NaOAc-HOAc buffer has been widely used as a specific extractant for the carbonate phase in various media (Tessier et al., 1979 Hickey and Kittrick, 1984 Rapin et al., 1986 Mahan et al., 1987 Han et al., 1992 Clevenger, 1990 Banin et al., 1990). Despite its widespread use, this step is not free from difficulties, and further optimization is required in its application. Questions arise with regard to this step in the elemental extraction from noncalcareous soils, the dissolution capacity and dissolution rates imposed by the buffer at various pHs, and the possibility that different carbonate minerals may require different extraction protocols (Grossman and Millet, 1961 Tessier et al., 1979). 

AGRICULTURAL CROPS Sewage sludge (kg/ha) Missouri Maximum addition when soil cation exchange capacity (in meq/100 g) is ... 

Soil cation exchange capacity ranges from less than 5 to more than 15 meq/100 g. 

Soil cation exchangeable capacity is an index used both to evaluate the nutrient and water retention ability of the soil and as an important basis for the amelioration of soil and to apply, rationally, fertiliser. Exchangeable cations absorbed by soil colloid include K+, Na+, Ca2+, Mg2+, Al3+ and H+. K+, Na+, Ca2+ and Mg2+ are exchangeable bases. Al3+ and H+ are exchangeable acids and the sum of these ions is known as the cation exchangeable capacity. Exchangeable Cu2+, Zn2+ and Mn2+ are present at negligible concentrations. 

Helling, S. C., Chesters, G., and Corey, R. B. (1964). Contribution of organic matter and clay to soil cation-exchange capacity as affected by the pH of the saturating solution. Soil Sci. Soc. Am. Proc. 28, 517-520. 

FIGURE 4.25 (a) Schematic of soil cation exchange showing the displacement of and NHj" by Fe + and... 

Ammonia/ammonium N (NH/NH ) Ammonia is colorless and readily solnble in water. The Ionic form is present under acidic soil conditions, whereas nnionized form is present under alkaline conditions. Ammonia is also absorbed on soil cation exchange complex or fixed in the crystal lattice of clay minerals. Indnstrially, ammonia is prodnced throngh the Haber process at 400°C and 250 atm (N2 + 3H2 —> NHj). Ammonia is a common source of fertilizers as (1) anhydrous ammonia directly injected into the soil and (2) ammonium salts such as ammonium nitrate or ammoninm snlfate. Ammoninm fertilizer applied to upland or aerobic soils is rapidly oxidized to nitrate. Intensive nse of these fertilizers and excessive rainfall or irrigation have resulted in elevated levels of nitrate in gronndwaters. 

Inorganic forms of nitrogen stored in wetlands include ammonium, nitrate, and nitrite. They can be found in soil pore water. Ammonium may be present on the soil cation exchange complex or permanently trapped within the clay layer. Inorganic N forms are not typically very stable, comprising <1% of the total N within wetlands. 

FIGURE 8.27 Schematic showing partitioning of ammonium between soil cation exchange complex and pore water under aerobic conditions. 

Figure 1. Illustration of soil cation exchange. Equilibrium between cations adsorbed on soil colloid surfaces and those in solution.

FIGURE 10.2 Soil cation exchange and uptake by plants in the example shown, nutrient K is desorbed from soil into the soil solution, in which it is absorbed by a plant root and transported upward into the plant tissue by the osmotic flow of water. The water eventually enters the atmosphere as water vapor through the process of transpiration, leaving the IC in the plant, where it participates in essential metabolic processes. 

CURTIN D. and SMILLIE G.W. 1978. Estimation of components of soil cation exchange capacity from measurements of specific surface and organic matter. Soil Science Society of America Proceedings, 40, 461-462. 

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