Soil solutions elemental forms

Generally, the most common cations in the soil solution are potassium, sodium, magnesium and calcium. Alkali soils are high in sodium and potassium, while calcareous soils contain predominantly magnesium and calcium. Salts of all four of these elements tend to accelerate metallic corrosion by the mechanisms mentioned. The alkaline earth elements, calcium and magnesium, however, tend to form insoluble oxides and carbonates in nonacid conditions. These insoluble precipitates may result in a protective layer on the metal surface and reduced corrosive activity. 

Soil pH is the most important factor controlling solution speciation of trace elements in soil solution. The hydrolysis process of trace elements is an essential reaction in aqueous solution (Table 3.6). As a function of pH, trace metals undergo a series of protonation reactions to form metal hydroxide complexes. For a divalent metal cation, Me(OH)+, Me(OH)2° and Me(OH)3 are the most common species in arid soil solution with high pH. Increasing pH increases the proportion of metal hydroxide ions. Table 3.6 lists the first hydrolysis reaction constant (Kl). Metals with lower pKl may form the metal hydroxide species (Me(OH)+) at lower pH. pK serves as an indicator for examining the tendency to form metal hydroxide ions. 

Organic matter added to arid soils in the forms of sewage sludge and other solid waste is decomposed following the model C = C0 (l-e kt) + Ci (Pascual et al., 1998). The decomposition is initially a rapid process of mineralization, followed by a second slower phase. With decomposition, trace elements originally bound in organic materials are released into the soils and soil solution, and they become available to plants. 

Mercury and the noble metals are found in nature in their elemental forms however, they are generally unreactive and so their occurrence in the soil solution is limited. Some elements, such as sulfur, can be reduced to their elemental state (see Figure 4.8) by soil microorganisms however, they can also easily be both oxidized and the oxidized forms reduced and so are rarely found in their elemental form in soil. 

Potassium is the eighth most abundant element in the Earths crust, which contains about 2.6% potassium, but not in natural elemental form. Potassium is slightly less abundant than sodium. It is found in almost all solids on Earth, in soil, and in seawater, which contains 380 ppm of potassium in solution. Some of the potassium ores are sylvite, carnallite, and polyha-lite. Ore deposits are found in New Mexico, California, Salt Lake in Utah, Germany, Russia, and Israel. Potassium metal is produced commercially by two processes. One is thermochemical distillation, which uses hot vapors of gaseous NaCl (sodium chloride) and KCl (potassium chloride) the potassium is cooled and drained off as molten potassium, and the sodium chloride is discharged as a slag. The other procedure is an electrolytic process similar to that used to produce hthium and sodium, with the exception that molten potassium chloride (which melts at about 770°C) is used to produce potassium metal at the cathode (see figure 4.1). 

In this chapter some of the theoretical concepts used in these models will be outlined. In particular, emphasis will be given to the chemical thermodynamic principles that can be used to predict the stable forms of a given element. Such chemical principles provide the theoretical foundation of the commonly used chemical models. These models can be used to predict the final extent of reaction but not the rate. It is probably fair to say that these laws as basic principles are indisputable scientific fact however, problems arise when we try to apply them to ill-defined complex natural media such as soils and soil solutions where some reactions are kinetically slow and practically irreversible. However inadequate our chemical models are in relation to real-world situations they are the best we have and can be used to give valuable insight and meaning into the processes we observe. 

Essential means that 1) without this element the plant cannot complete its life cycle (i.e., produce a viable seed), and 2) no other element may substitute for the element in question. b Most readily absorbed form (i.e., most soluble in soil solution at suitable soil pH) in parentheses. c Used in relatively large amounts (>0.1% of dry plant tissue). d Used in relatively small amounts (<0.10% of dry plant tissue). 

Ba must get recovered from specific resources (food, soil solution or ambient waters) also, which is not trivial given that bond stability (Ba complexes are very weak as a rule) and ligand selectivity differ considerably (both c and X to the negative) from those elements forming the window of essentiality . 

For many of the more abundant elements, such as Al, Fe, and Mn, precipitation of mineral forms is common and may greatly influence or even control their solubility. For most trace elements, direct precipitation from solution through homogeneous nucleation appears to be less likely than adsorption-desorption, by virtue of the low concentration of these metals and metalloids in soil solutions in well-aerated dryland soils. When soils become heavily polluted, metal solubility may reach a level to satisfy the solubility product to cause precipitation. Precipitation may also occur in the immediate vicinity of the phosphate fertilizer zone, where the concentration of heavy metals and metalloids present as impurities may be sufficiently high. Precipitation of trace metals as sulfides may have a significant role in metal transformation in reduced environments where the solution sulfide concentration is sufficiently high to satisfy the solubility product constants of metal sulfides (Robert and Berthelin, 1986). 

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