Ion in the soil solution

Soil-pH may influence both the concentrations of ions in the soil solution and the charge characteristics of the clay. Dispersion of clays is thus, to some extent, a pH-dependent process. At soil-pH(H2o,i i) values below 5, the aluminum concentration of the soil solution is normally sufficiently high to keep clay flocculated (Al3+ is preferentially adsorbed over divalent and monovalent ions in the soil solution). Between pH 5.5 and 7.0, the content of exchangeable aluminum is low . If concentrations of divalent ions are low, clay can disperse. At still higher pH values, divalent bases will normally keep the clay flocculated unless there is a strong dominance ofNa+-ions in the soil solution. 

Plant roots and root-induced chemical changes in the rhizosphere strongly affect the bioavailability of trace elements (Hinsinger, 1999). First, root-induced changes in the ionic equilibria influence the bioavailability of trace elements. The differential rates of plant uptake of water and ions in the soil solution result in a depletion or an accumulation of the ions in the... 

Soil pH is perhaps the most critical and common soil measurement where a definite amount of water is added before a measurement is made. Soil pH is a particularly complicated measurement because the proton can and does exist as a hydronium ion in the soil solution, as an exchangeable ion on the cation exchange sites, and bonded to various soil constituents. Because of these complexities, a soil sample is usually brought to a standard moisture content before a pH measurement is made. By bringing different soils to a common moisture content, they can be compared and analytical results from different laboratories will be comparable. Although there is a number of ways to measure soil pH, typically it is carried out using a pH meter and a pH electrode. 

Excess Na ions in the soil solution can be inhibitory to certain plant processes. Plant sensitivity to various Na levels in soil is dependent on plant species and stage of plant development. Sodium toxicity to higher plants is characterized by leaf-tip burn, necrotic spots, and limited leaf expansion, thus reducing yield. 

Soil scientists are very interested in the relation between the concentration of an ion in the soil solution and the amount retained by the soil. It is this relation which determines the rate of diffusion of a nutrient to a plant root and which also determines the possibilities of leaching losses from the soil. 

The ions in the soil solution constitutes the active acidity and are measured directly as soil pH values. On the other hand the adsorbed H+ ions held on exchange sites are not subject to pH measurements are termed as reserve acidity , both the forms contribute to soil acidity. Thus soil pH does not reflect the total acidity. However a suitable index which takes into account the reserve acidity of soil is the lime potential, which is calculated as follows ... 

Solutes, electrolytes, and nonelectrolytes in the soil solution are the immediate sources of the elements required by plants for growth. This supply can be continuously renewed by the many mechanisms of ion-soil interaction that remove and add ions in the soil solution (1) mineral weathering, (2) organic matter decay, (3) rain, (4) irrigation waters containing salts, (5) fertilization, and (6) release of ions retained by the colloid or clay fraction of soils. 

The soil is involved with all the essential elements. Most are present as ions in the soil solution and flow into the plant as it absorbs water. Plants obtain hydrogen, carbon, and oxygen from water and air, but soils provide water-holding capacity, provide pore space for O2 and CO2 movement between plant roots and the atmosphere, and supply CO2 to the atmosphere through the decay of organic matter by soil microorganisms. 

Animals derive their essential elements from plants. The ability of plants to supply these elements to plants and animals depends on a combination of factors availability of the ions in the soil solution, plant selectivity at the soil solution/root interface, and ion translocation from root to plant top. The system is good but not perfect. The concentrations of essential elements are occasionally too high or too low for animals, because plants can tolerate a much wider range of elemental concentrations than can animals. Plant contents of iron, for example, tend to be lower than ideal for the human diet. Grazing animals in a few semiarid parts of North America may suffer from high selenium, in Australia from low cobalt, and formerly in Norway from low phosphorus availability because of unsatisfactory amounts in soils. The supply of essential elements by plants to animals is generally adequate cases of too little or too much are noteworthy because of their rarity. 

Figure 3.6 shows the effect of this solid solution mixing on the aqueous solubility of a substance assuming ideal mixing, g = 1. Mixing has little effect on the IAP/ ratio of a substance until its mole fraction is <0.5. The reduced aqueous solubility due to this mixing should be pronounced for trace ions in the soil solution such as phosphate and the trace metals, but insignificant for Si, Al, and Fe. 

Probably the most important and distinctive property of soils is that they can retain ions and release them slowly to the soil solution and to plants. The retention prevents concentrations that are too high and too low. The evolution of plants has taken advantage of this buffered range of ion concentrations that soils make available in the soil solution. Over most of the earth s surface, the availability of these ions in the soil solution is adequate, but not necessarily ideal, for plants. Crop and horticultural plants and a desire for maximum yield place greater demands on the soil and may require adjusting the native soil solution. Adjustments by fertilization, liming, and salt removal are usually temporary. The soil and climate tend to return the soil to its native state. 

The strongly retained cations in soils include many of the essential microelements and also the toxic cations. The concentrations of these ions in the soil solution are low and they are apparently retained by two means. One group is the cations that in aqueous solutions precipitate as insoluble oxides and hydroxyoxides. The root zone of a typical agricultural soil might contain as much as 300 000 kg ha-1 of Fe and Al, but their plant availability is only a few kg ha-1. 

The effects of low pH on plant growth are generally caused by increases of toxic ions, or decreases of essential ions, in the soil solution. Such effects can also arise from nutritional imbalances because the concentrations can increase or decrease as soil acidity changes. 

The active acidity is the instantaneous condition of free hydrogen ions in the soil solution. It results most frequently from dissociated free acids or dissociated acid salts occurring in the soil. It is expressed by the pH value, and determined by colorimetric or potentiometric methods. It should be determined in fresh samples as far as possible, preferably in the field or as soon as possible after transporting the sample into the laboratory, to obtain the actual acidity under natural conditions. 

The influences of competing ions in the soil solution on the relationship between sorption of Mo by soils and Mo uptake by grass were investigated by Smith, Brown, and Devel (1987). Plants did not show any toxicity symptoms with addition of high amounts of Mo (up to 100 mg L ). Sorption data fit the Freundlich isotherm (Pasricha and Randhawa, 1977). Soils with the highest amounts of CaCO, sorbed the least amount of Mo CL in the soil increased Mo sorption, whereas SO/" decreased it. 

Several investigators have reported that considerable dinoseb is volatilized from soil surfaces 168, 169, 170, 186, 187, 188), The vapor pressure of dinoseb is slightly higher than 1.0 X 10" mm Hg at room temperature, and thus dinoseb would be somewhat volatile (Table IV). Vaporization depended upon the soil moisture content, soil temperature, and whether or not the soil was limed. Dinoseb was more readily volatilized from wet soils than dry soils and the amount lost increased with an increase in soil temperature. Liming the soil makes the soil solution less acid and drives the reaction shown in Equation 11 to the right, and the resulting predominant phenate ions in the soil solution are not as volatile as the molecular phenol species. 

The pH of natural waters is determined by the presence of inorganic acids and bases, such as HCO3 and COs, which would be present in higher concentrations than any organic contaminant. A similar situation prevails in soils where the pH results from the interaction of soil minerals with ions in the soil solution. Under these conditions an organic acid or base would not have any significant effect on the pH of its environment, however, conversely the environmental pH will determine the ratio of the neutral and charged species... 

An important feature of the biogeochemistry of trace elements in the rhizosphere is the interaction between plant root surfaces and the ions in the soil solution. These ions may accumulate in the aqueous phases of cell surfaces external to the plasma membranes (PMs). In addition, ions may bind to cell wall (CW) components or to the PM surface with variable strength. In this chapter, we shall describe the distribution of ions among the extracellular phases using electrostatic models (i.e. Gouy-Chapman-Stem and Donnan-plus-binding models) for which parameters are now available. Many plant responses to ions correlate well with computed PM-surface activities, but only poorly with activities in the soil solution. These responses include ion uptake, ion-induced intoxication, and the alleviation of intoxication by other ions. We illustrate our technique for the quantitative resolution of multiple ion effects by inserting cell-surface activities into nonlinear equations. 

Hie parameters ois, oos and at, reflect the disposition of ions in the soil solution after they have become incorporated into the interfacial region. Therefore, these surface charge densities represent the net charging effects of the surface speciation of the ions. By analogy with the use of speciation models (ion-association models) to estimate the distribution of ionic charge in aqueous phases like soil. solutions, surface speciation models (surface... 

Inhibition of K uptake by high concentrations of Ca ions in the soil solution. 

---