The Soil Solution
The Soil Solution
The soil solution was introduced in Section 1.2 as a liquid water repository for dissolved solutes. Speaking more precisely, one can define the soil solution as the aqueous liquid phase in soil having a composition influenced by exchanges of matter and energy with soil air, soil minerals, and the soil biota. This more precise concept identifies the soil solution as an open system, and its designation as a phase means two things: (1) that it has uniform macroscopic properties (for example, temperature and composition) and (2) that it can be isolated from the soil profile and investigated experimentally in the laboratory. Uniformity of macroscopic properties obviously cannot be attributed to the entire aqueous phase in a soil profile, but instead is associated with a sufficiently small element of volume in the profile still large enough to include many pores. As is the case for soil humus (Section 3.2), the problem of isolating a sample of the soil solution without artifacts is an ongoing challenge to soil chemistry, but several techniques for removing the aqueous phase from soil into the laboratory have been established as operational compromises between chemical accuracy and analytical convenience. Among these techniques, the most widely applied in situ methods are drainage water collection and vacuum extraction, whereas the common ex situ methods include displacement by another fluid and extraction by vacuum, applied pressure, or centrifugation. The in situ techniques are influenced by whatever disturbance to a soil profile and, therefore, natural aggregate structure and water flow patterns, has occurred because of apparatus installation. They yield a sample of the soil solution from a largely undefined profile volume and they differ in whether they provide the flux compositionor the resident compositionof a soil solution. A flux composition, which is relevant to chemical weathering and, more broadly, to solute transport in soils, is measured in a soil solution sample obtained by natural flow into a collector, as occurs in a pan lysimeter (Fig. 4.1).
Keywords: Arrhenius equation, Davies equation, Marion–Babcock equation, chemical species, electrical conductivity, hydration complex, molar gas constant, phenolic hydroxyl, rate coefficient, soil solution
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