Soil pH Measurements

Soil pH measurements can be ambiguous. Two factors that affect soil pH measurements are the soil-solution ratio and the salt concentration. Increasing either factor normally decreases the measured soil pH because H and A1 cations on or near soil colloid surfaces can be displaced by exchange with soluble cations. Once displaced into solution, the A1 ions can hydrolyze (Eq. 10.2) and further lower the pH. Preferential retention of hydroxy aluminium polymers by soil colloids drives the hydrolysis reactions further toward completion and leads to lower pH. Increasing the neutral salt concentration to 0.1 or 1 M can lower the measured soil pH as much as 0.5 to 1.5 units, compared to soil pH measured in distilled water suspensions. 

In soil suspensions at low salt concentrations, extraneous (junction) potentials can affect pH readings (Appendix 10,1). The most plausible explanation for junction potentials is that K ions in the KC1 bridge of the reference electrode diffuse more rapidly, and Cl ions less rapidly, when negatively charged soil particles are near the bridge. One answer is to place the reference electrode in the clear supernatant solution and the glass electrode in the settled clay suspension (where H+ is concentrated) to obtain valid soil pH measurements. Junction potentials are essentially eliminated at salt concentrations greater than 0.01 M. 

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Because standardization is an essential component of soil pH measurement, care must be taken to have good buffers. If the buffers show any indication of contamination, such as material floating in them, soil, or microbial growth, they... 

Fouling of the reference electrode or the reference side of a combination electrode is a common problem in soil pH measurements. Fouling can be caused by salts, organic matter, and clay. Each electrode manufacturer will provide specific cleaning procedures that help to keep electrodes functioning properly however, in many cases, no amount of cleaning is effective and the electrodes need to be replaced. 

Smee, B.W. 2003. Theory behind the use of soil pH measurements as an inexpensive guide to buried mineralization, with examples. Explore, 118, 1-19. 

This procedure gives only a rough approximation of the soil pH. More quantitative do-it-yourself soil pH measuring kits are available at your local gardening store. 

Soil pH measurement is usually affected by a number of factors viz. salt concentration, soil-water ratios, suspension effect etc. The use of O.Ol(M) CaClg solution yields stable readings in pH-measurements. The hydrogen ions in the soil system are distributed between solid and liquid phases as follows. The dynamic equilibrium may be represented as follows. 

A second or reference electrode is necessary to complete the electrical circuit. The reference electrode is sometimes welded to the pH electrode so that the pair look like a single electrode. Reference electrodes are too often taken for granted their spurious potentials are a common source of error in soil pH measurements. A typical reference electrode is also sketched in Fig. 10.5. The wire dipping into the liquid mercury makes electrical contact with the pH meter, and current flows from the electrode to the solution phase through the reversible reaction ( ° = 0.268 V at 25° C) ... 

Another simple way to minimize Ej in soil pH measurements is to allow the tip of the reference electrode to contact only the supernatant solution above the colloidal phase. The rates of K+ and Cl- diffusion are then unaffected by the colloid. The glass electrode, on the other hand, can be placed either in the supernatant solution or in the colloidal suspension. The H+ activity is the same in both phases, and the glass electrode is unaffected by the presence of the colloid. 

You have generated hazardous waste from your soil pH measurements. Dipose of properly in accordance with the Office of Radiation, Chemical and Biological Safety (ORCBS) guidelines. 

Ion selective electrodes have historically been used in soil testing laboratories to conduct standard chemical soil tests, especially soil pH measurement. Many researchers in the 1970s and 1980s concentrated on the suitability of ISEs as an alternative to routine soil nitrate testing. More recently, researchers whose end goal was a mobile macronuttient sensing system have reported on laboratory tests of components of such systems. 

Samples that contain suspended matter are among the most difficult types from which to obtain accurate pH readings because of the so-called suspension effect, ie, the suspended particles produce abnormal Hquid-junction potentials at the reference electrode (16). This effect is especially noticeable with soil slurries, pastes, and other types of colloidal suspensions. In the case of a slurry that separates into two layers, pH differences of several units may result, depending on the placement of the electrodes in the layers. Internal consistency is achieved by pH measurement using carefully prescribed measurement protocols, as has been used in the determination of soil pH (17). 

Measurement of some of these parameters identifies the risk of a particular type of corrosion, for example pH measurements assess the risk of acid attack and redox potential measurements is used to assess the suitability of the soil for microbiological corrosion, a low redox potential indicates that the soil is anaerobic and favourable for the life cycle of anaerobic bacteria such as to sulphate-reducing bacteria. Other measurements are more general, resistivity measurements being the most widely quoted. However, as yet no single parameter has been identified which can confidently be expected to assess the corrosion risk of a given soil. It is therefore common practice to measure several parameters and make an assessment from the results. 

Based on Table 3.6, Cu and Pb may form Cu(OH)+ and Pb(OH)+ at pH 8.0 and 8.4, respectively, while Zn may form Zn(OH)+ at pH 9.0. However, Cd can form Cd(OH)+ at pH 10.1. This indicates that at normal pH ranges of arid and semi-arid soils, Cu and Pb, and to some extent, Zn may be present in their hydroxide complex ions, while Cd is less likely to be in the hydroxide form unless at higher soil pH (Table 3.5). Sauve et al. (1997) reported that free Cu activity in soil solution of Quebec and New York decreased with higher pH. Fotovat et al. (1997) reported that Zn(OH)2° and ZnCC>30 contributed to a considerable proportion of the total solution Zn (about 16-21%) in soils with high pH (7.56-8.99) from South Australia. Ma and Lindsay (1990) reported that the Zn activities measured in 10 Colorado arid soils can be expressed by the relationship with log K° = 5.7 0.38 (Fig. 3.1) ... 

Heavy metals in rice and soils as well as several physicochemical indicators of soils including Organic matter (OM), pH, Cation- Exchange Capacity (CEC) and soil granularity were analyzed. As, Cd, Cr, Cu, Hg, Mn, Ni, Pb, Se, Zn and OM of soils, and As, Cd, Cr, Cu, Hg, Mn, Ni, Pb, Se and Zn of rice were determined at the central chemical lab of the Institute of Geophysical and Geochemical Exploration. The CEC of the soils was measured at the analytical institute of... 

The parameters related to soil-related, data (to define soil solution status such as pHss, DOM or DOC, SPM) influences on HM toxicity for biota. These data should be either measured (in a few plots only) or simulated since this information is mainly not perennially monitored for forested areas of Russia. Soil pH data (water or KC1 extraction) are more available parameters as depending on soil type. The same FAO soil map with added attributive tables containing soil pH values can be used for this purpose. 

It would not be until the development of the pH meter and pH electrode that soil scientists had a good way to measure soil pH. In 1934, A. O. Beckman introduced the first pH meter and started a company to build and sell the meter. This sparked an intense study of soil pH and its relationship to plant nutrient availability [14],... 

For these reasons, a standard method of measuring soil pH is chosen and all phenomena related to pH or involving pH are related to this standardized pH measurement. The most common method is to use a 1 1 ratio of soil to water, typically 10 mL of distilled water and 10 g of soil. In this method, the soil and water are mixed and allowed to stand 10 minutes and the pH determined using a pH meter. 

This method does not measure exchangeable protons attached to cation exchange sites therefore, it is also common to use a salt solution (either KC1 or CaCl2) instead of distilled water in determining soil pH. The K+ or Ca2+ in the solution exchanges with exchangeable hydronium, thus bringing it into... 

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. 

As with pH measurements, a specific amount of water or ionic strength adjusting solution is added to soil, mixed, and allowed to stand. For some analyses, the resulting solution is filtered to separate the soil from the liquid. 

In addition to the simple Eh-pH graph shown in Figure 9.3, three-dimensional Eh-pH graphs can be produced. Known quantities of a pollutant can be added to a number of different soil suspensions and its degradation at different combinations of Eh and pH measured. In this way, the optimum conditions for the decomposition of the pollutant in question can be determined. An excellent example of this is the decomposition of pentachlorophe-nol and hexahydro-l,3,5-trinitrol,3,5-triazene in soil and water under various Eh-pH conditions as illustrated in the papers by Petrie et al. [9] and Sing et al. [10]. 

Figures 6.15 and 6.16 give the profiles of Fe(II) and Fe(III) concentration and pH measured by Begg et al. (1994) and Kirk and Bajita (1995). Blocks of reduced soils were placed in contact with planar layers of rice roots, with the roots separated from the soil by fine nylon mesh which they could not penetrate.

The pH measurements of the mixed contents were made immediately after taking the soil and liquid samples. The presence of a very large amount of suspended sediment affected the accuracy and reproducibility of these pH measurements. The average values for each container are summarized in Table II. The deposited pesticides, the formulating agents and the decomposition products did not change the pH from that observed for ambient water except for... 

Definition. Soil pH is defined as the negative logarithm (to the base of 10) of the H+ activity in the soil solution measured under the stated conditions. 

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