Methods of pH determination in calcareous soils: use of electrolytes and suspension effect.
Soil pH is fundamental to the understanding of soil systems, because it is an indicator of many reactions in the soil (Moore and Loeppert 1987). Measurement of soil pH is highly technique dependent, in that many soil parameters and experimental factors affect the values obtained. A considerable amount is known about pH values in non-calcareous and non-saline soils; however, it is essential that reliable and accurate methods for soil pH determination are also developed for calcareous saline soils, especially from arid and semi-arid regions.
The difference in pH between the sediment and supernatant produced during soil pH determination is termed the 'suspension effect' (McLean 1982). If the soil suspension is allowed to settle, the pH as measured in the suspension liquid is often higher than in the sediment layer. The suspension effect is influenced by the extent to which electrodes encounter clay and humus particles and the soil C[O.sub.2] is in equilibrium with atmospheric concentrations, and the magnitude of liquid junction effects (Foth and Ellis 1988). The suspension effect is minimised by measuring soil pH using 0.01 M CA[Cl.sub.2] or 1 M KCl solutions instead of water (Sumner 1994). In addition, the use of Ca[Cl.sub.2] or KCl solutions has the advantages of(i) decreasing the effect of the junction potential of the calomel reference electrode, (ii) equalising the salt content of soils, and (iii) preventing dispersion of the soil (Tan 1995).
Many researchers have studied the effect of different electrolytes and suspension on soil pH in acidic soils (McLean 1982; Conyers and Davey 1988; Aitken et al. 1990). In most arid and semi-arid countries, soils are mostly alkaline and calcareous. There are few studies describing the effect of electrolyte addition on pH determination in calcareous soils. Therefore, the objective of this study was to assess the effects of different added electrolytes, with and without stirring of the soil suspension, on the pH of calcareous soils.
Materials and methods
Thirty surface (0-0.10 m) and subsurface (0.10-0.20 m) samples were collected from cultivated soil. Soils were selected from agricultural areas to have a wide range of pH and salinity. All samples were air-dried at 25[degrees]C and passed through a 2-mm sieve.
Soil pH was determined in distilled water (p[H.sub.w]), 0.01 M Ca[Cl.sub.2] (p[H.sub.ca]), 1 M KCl (p[H.sub.k]), and 0.01 M Ba[Cl.sub.2] (p[H.sub.ba]) for all samples. Soil: solution ratios of 1 : 1, 1 : 2.5, and 1 : 5 (w/v) were used for each electrolyte. The suspension was shaken manually every 10 min for 30 rain. Values of pH were recorded: (i) after 1 min of stirring and (ii) after 1 min of stabilisation (i.e. allowed to stand for 1 min to obtain a clearer suspension in these sandy soils). All pH determinations were performed at room temperature (20 [+ or -] 2[degrees]C) using a Jenway 3020 pH meter and Philips combine glass/calomel electrode type CE1, which had previously been calibrated at pH 7.0 and 9.0. Electrical conductivity (EC), sodium adsorption ratio (SAR), cation exchange capacity (CEC), CaC[O.sub.3], CaS[O.sub.4], and texture were also determined for the bulk sample.
Analysis of variance (ANOVA) was performed using SAS (Statistical Analysis System 1985) statistical packages to assess the significance of electrolyte and stirring treatment effects on pH as measured in saline (EC >4 dS/m) and non-saline (EC <4 dS/m) soils. The analysis design was completely randomised with 2 replicates and 2 readings (stirred and non-stirred) for each combination of electrolyte and dilution ratio.
Results and discussion
ANOVA of pH data
There were significant differences in pH when soils were suspended in different electrolytes (Table 1). This suggests that soils react differently when soil suspensions are modified by electrolyte addition compared with water alone.
The highest pH (8.56) was p[H.sub.w] and the lowest p[H.sub.ca] (8.12) as average values of all dilution ratios. No significant difference was found in soil pH between electrolytes of different composition. In a study of 576 acid soils Gascho et al. (1996) found mean p[H.sub.w] values of 6.34, p[H.sub.ca] 5.62, and p[H.sub.k] 5.21. In this study, mean p[H.sub.w] values in calcareous soils were also higher than p[H.sub.ca] and p[H.sub.k].
Soil pH values were significantly affected by dilution ratio and soil salinity (Table 1), whereas there was no significant effect due to stirring at the time of measurement. These results agree with the findings of Yu and Ji (1993), who reported that increasing the soil to water ratio affected the dissociation of adsorbed ions and, thus, increased pH of the suspension.
Effect of electrolytes on pH
Average p[H.sub.w] was significantly higher than the pH values in electrolyte solutions (Table 2). Electrolytes appeared to release more [H.sup.+] ions from soils than distilled water. This release is probably due to ion exchange between cations held in solution and on soil exchange sites. In particular, soils contain varying amounts of exchangeable [H.sup.+] and [Ca.sup.2+] ions, which were exchanged more readily by electrolyte solutions than distilled water. Conyers and Davey (1988) suggested that in neutral soils, at least, direct desorption of [H.sup.+] by [Ca.sup.2+] was the dominant mechanism of pH decrease.
One of the effects of electrolyte addition on pH compared with water is probably the reduction, at least initially, of the double layer potential surrounding the negatively charged soil surfaces. The result would be an increase in the number of [H.sup.+] ions released into the suspension and hence a decrease in the measured pH values.
A liquid junction effect is another probable reason for the lower pH values obtained in electrolyte solutions than distilled water. The liquid junction effect arises from the unequal diffusion of electrical charges across the junction between electrolyte and internal electrode solution. The effect is usually greater when measuring pH in water than when electrolyte solutions are used (Moore and Loeppert 1987). Hence, when water is used soil pH values tend to be higher than when measured in electrolyte solutions, since charges are carried more equally across the junction.
Schofield and Taylor (1955) pointed out that addition of 0.01 M Ca[Cl.sub.2] to the soil solution has less effect on solution ionic strength than 1 M KCl, and ionic strength has an inverse relationship with soil pH. In contrast, additions of KCl and Ba[Cl.sub.2] tend to concentrate dissolving solutions whereas water addition simply dilutes the suspension. The use of electrolyte solutions has the advantage of decreasing the variability in soluble salts content between soils, and so the pH values obtained are less dependent on the soil solution ratio. Further, the addition of electrolytes probably maintained flocculated conditions during measurement, compared with addition of water alone, especially in sodium rich soils.
Addition of CA[Cl.sub.2] electrolyte probably stabilised the dissolution of soil minerals containing CaC[O.sub.3], through the common ion effect, more than when KCl and Ba[Cl.sub.2] were added. Thus, variations in Ca2+ ion concentration in soil suspensions following KCl and Ba[Cl.sub.2] additions may have influenced the pH values in these electrolyte solutions. However, the position of the equilibrium between soil CaC[O.sub.3] and CA[Cl.sub.2], KCl, and Ba[Cl.sub.2] may also have been influenced by the time that elapses between electrolyte addition and pH measurement.
Soil pH values measured in Ca[Cl.sub.2] solutions were probably less dependent on surface exchange properties than those obtained in KCl, since p[H.sub.k] values are negatively related to the CEC values of soils (p[H.sub.k] = 8.68 - 0.055logCEC, [R.sup.2] = 0.68). The extent of exchange of K for Ca is an additional factor leading to variable Ca concentration in soil suspensions and, hence, reduces the predictability of the common ion effect on the solubility of CaC[O.sub.3].
There were no significant main order differences in pH values when measured in either stirred (s) or non-stirred (q) suspensions. The main order effect of suspension state, however, did not distinguish between the separate effects of added electrolytes or soil salinity (Table 3).
Stirring suspensions slightly reduced pH values in nonsaline soils (Fig. 1). Only in water was the difference between stirring and non-stirring significant (P< 0.05). Foth and Ellis (1988) also found that stirring soil suspensions in water reduced pH values relative to non-stirring.
[FIGURE 1 OMITTED]
There were no significant differences between pH of each suspension state of saline soils (Fig. 2). This means that salinity appears to act in a manner similar to added electrolytes, in that pH values were stabilised in both stirred and non-stirred conditions. This stability probably arises from the control of the degree of soil flocculation produced by either soil salinity or added electrolyte. In the case of non-saline soils, added electrolytes appeared to stabilise fluctuation in pH readings and reduce the suspension effect. In addition, the suspension effect could be related to the CEC of the soil. As McLean (1982) reported, the difference in pH between stirred and non-stirred conditions is usually greatest when CEC is high, due to the presence of weakly bound exchangeable ions and low electrolyte concentrations.
[FIGURE 2 OMITTED]
The pH of the soil suspension was usually lower than that of the supernatant produced when water was separated from the soil by centrifugation or gravitational forces (Tan 1995). The [H.sup.+] concentration was presumably higher at soil surfaces than in the bulk solution. In a soil suspension, the electrode registers the concentration of [H.sup.+] ions both in the solution and in the extension of the double layer. When pH measurements are performed in the supernatant solution, the electrode measures only the [H.sup.+] ion concentration of the bulk solution (Tan 1995). In addition, Conyers and Davey (1988) found that suspended solids sometimes reduced the apparent pH, particularly in water although not usually in salt solutions. The measured pH values in their study were not stable until the suspension had settled. The pH of a soil suspension includes a liquid junction potential of up to 0.2 pH units in the water extract of the soils (Conyers and Davey 1988). The results from the present study tend to agree with Sumner (1994), who found that the common practice of stirring soil water suspensions while measuring pH is likely to result in the largest possible junction potentials.
The pH determinations of calcareous soils have been observed to be frequently affected by stirring (Clark 1964). These effects were generally more pronounced with calcareous non-saline soils. The pH values of soils were found to decrease with stirring. With increasing electrolyte concentration, the differences between the stirred and non-stirred values decreased. The suspension effect can be minimised by the 0.01 M Ca[Cl.sub.2] solution instead of water. The results presented in Table 3 confirm these earlier findings and suggest that Ba[Cl.sub.2] and KCl behave similarly to Ca[Cl.sub.2] in suppressing the suspension effect by both, probably by reducing the junction potential and stabilising the degree of flocculation in the soil solution.
Addition of electrolytes (i.e. Ca[Cl.sub.2], KCl, and Ba[Cl.sub.2]) to soil water suspensions decreased the soil pH significantly compared with distilled water alone. The variation in pH was probably due to several factors including a release of H+ by [Ca.sup.2+], [K.sup.+], and [Ba.sup.2+] ions to the soil suspension, liquid junction potential effect, and salinity effect. Consequently, in the presence of electrolytes, pH values were less dependent on the soil solution ratio and stirring than in water. Further, the addition of electrolytes probably maintained flocculated conditions during measurement.
The suspension effect (i.e. pH values obtained in stirred or non-stirred suspensions) was not significant in electrolyte solutions at all dilution ratios. There was, however, a small suspension (P <0.05) effect when measured in water.
Use of electrolytes gave less variable values of suspension pH than water. Added electrolytes minimised the liquid junction potential, reduced fluctuations in pH measurements with time, and decreased the salt effect (i.e. pH values were less affected by dilution ratios).
In view of the almost universal use of water to measure soil pH it remains unlikely that soil laboratories will readily accept using added electrolytes. However, the pH value for a soil remains highly dependent on the analytical technique and procedure used. Some of the variability encountered in measuring soil pH could be reduced if standard procedures were to include the use of added electrolytes. The effects of the many factors that affect measured soil pH values need to be investigated before an official procedure can be recommended for any user.
Aitken RL, Moody PW, Mckinley PG (1990) Lime requirement of acidic Queenland. 1. Relationship between soil properties and pH buffer capacity. Australian Journal of Soil Research 28, 695-701. doi:10.1071/SR9900695
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Manuscript received 29 June 2004, accepted 1 March 2005
A. Al-Busaidi (A,B), P. Cookson (B), and T. Yamamoto (A)
(A) Arid Land Research Center, Tottori University, 1390 Hamasaka, Tottori 680-0001, Japan. (B) Department of Soil and Water Sciences, College of Agricultural and Marine Science, Sultan Qaboos University (SQU), PO Box 34 Al-Khod 123, Oman.
Table 1. Analysis of variance for salinity, electrolytes, dilution ratios, and state (stirred and non-stirred) of measurements of different soils--dependent variable, pH Source of variation d.f. Sum of squares Mean squares Salinity 1 4.407 4.407 Electrolyte (ELE) 3 21.537 7.180 Dilution ratio 2 13.703 6.851 State 1 0.035 0.035 ELE*Dilution 6 1.856 0.309 Saline*ELE 3 7.345 2.448 Saline*Dilution 2 0.005 0.003 Saline*ELE*Dilution 6 0.144 0.024 Model 24 49.033 2.043 Error 695 22.227 0.032 Total 719 71.260 [R.sup.2] 0.688 CV 2.160 Root MSE 0.179 Source of variation F-values P-value Salinity 137.80 0.0001 Electrolyte (ELE) 224.48 0.0001 Dilution ratio 214.23 0.0001 State 1.11 0.2930 ELE*Dilution 9.67 0.0001 Saline*ELE 76.56 0.0001 Saline*Dilution 0.08 0.9229 Saline*ELE*Dilution 0.75 0.6111 Model 63.88 0.0001 Error Total pH mean 8.278 Table 2. Descriptive statistics for the whole dataset Soil Non-saline (EC <4 dS/m) property Min. Max. Mean s.d. E[C.sub.1:1] (dS/m) 0.29 2.15 0.93 0.52 E[C.sub.sp] (dS/m) 0.67 3.70 1.72 0.88 SAR 2.66 9.53 6.04 2.37 [pH.sub.wa] (1:1) 8.39 8.93 8.61 0.16 [pH.sub.wa] (1:2.5) 8.67 9.25 8.89 0.18 [pH.sub.ca] (1:1) 7.75 8.13 8.02 0.11 [pH.sub.ca] (1:2.5) 7.96 8.29 8.12 0.10 [pH.sub.k] (1:1) 7.72 8.40 7.93 0.21 [pH.sub.k] (1:2.5) 7.94 8.73 8.20 0.25 [pH.sub.ba] (1:1) 7.95 8.33 8.12 0.11 [pH.sub.ba] (1:2.5) 8.05 8.56 8.21 0.15 Na (mmol/L) 5.44 21.74 12.85 6.09 Ca (mmol/L) 1.25 6.25 5.80 1.51 Mg (mmol/L) 2.08 8.33 3.03 1.95 CaC[O.sub.3] (g/g) 0.33 0.60 0.43 0.08 CaS[O.sub.4] (cmol/kg) 1.00 1.60 1.35 0.21 CEC (cmol/kg) 1.96 16.30 9.37 5.55 % Clay 6.00 35.00 16.09 8.91 Soil Saline (EC >4 dS/m) property Min. Mean Max. s.d. E[C.sub.1:1] (dS/m) 4.50 34.75 10.98 9.64 E[C.sub.sp] (dS/m) 4.01 86.80 26.57 24.57 SAR 7.31 52.10 20.47 11.67 [pH.sub.wa] (1:1) 7.88 8.64 8.18 0.23 [pH.sub.wa] (1:2.5) 8.03 8.96 8.40 0.27 [pH.sub.ca] (1:1) 7.78 8.25 8.01 0.16 [pH.sub.ca] (1:2.5) 7.95 8.36 8.14 0.14 [pH.sub.k] (1:1) 7.68 8.37 7.98 0.18 [pH.sub.k] (1:2.5) 7.88 8.63 8.20 0.19 [pH.sub.ba] (1:1) 7.82 8.39 8.06 0.17 [pH.sub.ba] (1:2.5) 8.00 8.44 8.19 0.14 Na (mmol/L) 27.17 565.22 156.18 146.71 Ca (mmol/L) 6.25 100.00 37.83 28.07 Mg (mmol/L) 4.17 166.67 70.29 63.68 CaC[O.sub.3] (g/g) 0.28 0.58 0.41 0.07 CaS[O.sub.4] (cmol/kg) 1.00 10.00 2.01 2.46 CEC (cmol/kg) 2.17 21.74 9.29 5.97 % Clay 4.00 30.00 11.58 7.16 Table 3. Mean pH values of non-saline (EC <4 dS/m) and saline (EC >4 dS/m) suspension in stirred and non-stirred states Electrolyte Non-saline soils Saline soils Stirred Non-stirred Stirred Non-stirred Ca[Cl.sub.2] 8.13 8.14 8.11 8.12 Ba[Cl.sub.2] 8.23 8.24 8.17 8.18 KCl 8.22 8.23 8.20 8.21 Water 8.80 8.85 8.31 8.32
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|Title Annotation:||Short Communication|
|Author:||Busaidi, A. Al-; Cookson, P.; Yamamoto, T.|
|Publication:||Australian Journal of Soil Research|
|Date:||Jul 1, 2005|
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