Effects of Electrolytes and Soil-to-suspension Ratios on pH in Acidic-coarse Textured Soil

Soil pH measurement is the standard and routine chemical laboratory for soil fertility evaluation, due to its ease of measurement and influence on other properties of agricultural soil (Minasny et al., 2011; Miller and Kissel, 2010). Soil pH affects plant nutrient availability, organic material decomposition, microbial activity, element toxicity, pollution mobility and soil weathering (Kome et al., 2018; Miller and Kissel, 2010; Kissel et al., 2009). These soil fertility properties play key roles in sustainable agriculture (Power and Prasad, 1997). Because soil pH influences most of the other soil fertility parameters, it has been named the master variable of soil science (Kome et al., 2018).
       
Electrolytes and soil-to-suspension ratios are the measurement factors that dominantly influence the variability of soil pH (Minasny et al., 2011). Electrolytes employed in soil pH determination include water, 0.01 M CaCl₂ and 1 M KCl; with soil-to-suspension ratios of 1:1, 1:2.5 and 1:5 w/v (Thomas, 1996). However, recommendations for measuring soil pH vary from country to country or even between laboratories. In the United States, water is used as an electrolyte (Soil Science Division Staff, 2017); however, certain laboratories recommend 0.01 M CaCl₂ (Rayment and Higginson, 1992), with a soil-to-suspension ratio of 1:1. In New Zealand, water (Metson, 1956) and 0.001 M CaCl₂ (Edmeades and Wheeler, 1990) with a soil-to-suspension ratio of 1:2.5 were recommended. Laboratories in Australia recommend water or 0.01 M CaCl₂ using a 1:5 soil-to-suspension ratio (Slattery et al., 1999; Minasny et al., 2011). In Thailand, varied soil pH measurement methods are employed among different laboratories. Water, 0.01 M CaCl₂ and 1 M KCl are used as electrolytes (Therajindakajorn, 2011; Land Development Department, 2004); with 1:1 (Land Development Department, 2004) or 1:2.5 soil-to-suspension ratios (Therajindakajorn, 2011).
       
Soil characteristics; e.g., texture, cation exchange capacity, organic matter content and other physicochemical properties; as well as soil management, such as the application of mineral fertilisers, manure and other soil amendments, can influence pH (Weil and Brady, 2017; McLean, 1982). Approximately 30-40% of the world’s agricultural soil is identified as acidic (Samac and Tesfaye, 2003; von Uexküll and Mutert, 1995). Agricultural soils in Thailand and more specifically Northeastern, are generally coarsely textured, indigenous acidic and low fertile due to their highly weathering and intensive uses in agricultural approaches (Vityakon, 2007).
               
Given the diverse nature of soils, each soil necessitates a tailored approach for pH measurement. This principle holds true for acidic-coarsely textured soil, an important soil type found in tropical regions, which required a particular pH measurement procedure. The objective of this study was therefore to evaluate the effects of electrolytes and soil-to-suspension ratios on pH of typically acidic-coarsely textured soil.

Soil
 
In order to achieve an inherently acidic-coarsely textured soil with a diverse pH range, this study focused on isohyperthermic Aeric Kandiaquults, which are naturally acidic soil commonly found in Northeast Thailand. These soils were subjected to varying amounts of cricket feces-derived liming material. Thirty soil samples were examined, then air-dried and subsequently sieved through a 2-mm sieve before laboratory analyses. The physicochemical properties of the soil are presented in Table 1.
 

 
pH measurement methods
 
Two factors of soil pH measurement methods were investigated; electrolytes (water, 0.01 M CaCl₂ and 1 M KCl) and soil-to-suspension ratios (1:1, 1:2.5 and 1:5 in w/v). The ionic strengths (I) of 0.01 M CaCl₂ and 1 M KCl were equivalent to 0.03 M and 0.5 M, respectively, regarding the calculation procedure demonstrated by Lehmann et al., (1996) and Sposito (2008):
   
Where,
z_i and c_i = valency and molar concentration of an ion i.
       
The soil-to-suspension ratios of 1:1, 1:2.5 and 1:5 were achieved by adding 5, 12.5 and 25 ml of each electrolyte to 5 g of soil, respectively. Soil pH was subsequently determined following the standard method described by McLean (1982). Briefly, a soil suspension was agitated for 30 min, then allowed to settle for 30 min before the pH of the suspension was measured on a pH meter (Eutech PC 700, Thermo Fisher Scientific, Massachusetts, USA).
 
Laboratory measurements for initial soil properties
 
Electrical conductivity (EC) was measured in a soil-to-water ratio of 1:5 w/v. Soil organic carbon determination was performed using the Walkley and Black method (Nelson and Sommers, 1982). Total nitrogen was determined by the Kjeldahl method (Bremner and Mulvaney, 1982) on a micro-Kjeldahl distillation apparatus (Pro-Nitro S 4002851, JP Selecta, Barcelona, Spain). Phosphorus was extracted using a Bray-2 solution and determined via a UV-Vis spectrophotometer (Hitachi U-5100, Hitachi High-Tech Corporation, Tokyo, Japan) using a wavelength of 820 nm (Jones, 2001). Cations of K, Ca, Mg and Na, were extracted in 1 N NH₄OAc at pH 7 (Pansu and Gautheyrou, 2006) and measured on a flame atomic absorption spectrometer (Agilent 240FS AA, Agilent Technologies, California, USA). The effective cation exchange capacity (ECEC) was achieved by the sum of K, Ca, Mg and Na, according to Ketterings et al., (2014). Soil texture was assessed using the pipette method (Kroetsch and Wang, 2008). Hydrogen ion concentrations in the soil were achieved using the titrimetry method following Pansu and Gautheyrou (2006).
 
Statistical analysis
 
Delta pH (∆pH) was obtained from the difference between pH measurements under differing soil-to-suspension ratios within the same electrolyte. Regression analysis, through the PROC GLM procedure (SAS Institute Inc., 2004), evaluated the relationships of EC with ∆pH and pH measured under a soil-to-suspension ratio with H⁺ concentration. The error of each method was estimated by the root mean square error (RMSE), which is defined as:
   
Where,
Ŷ_i and Y_i  are the soil pH measurements obtained by each method and H⁺ concentration, respectively. Statistical significance was at p≤0.05.

Effects of electrolytes and soil-to-suspension ratios on soil pH
 
Soil pH varied depending upon the electrolytes and soil-to-suspension ratios (Fig 1). Under the same soil-to-suspension ratio, soil pHs were observed in descending order: water > 0.01 M CaCl₂ > 1 M KCl. Decreased pH, measured in CaCl₂ and KCl solution relative to those in water, resulted from a greater dissociation of H+ from exchangeable sites replaced by CaCl₂-derived Ca²⁺ and KCl-derived K⁺, bringing about increased aqueous H⁺ (Kome et al., 2018; Minasny et al., 2011). Meanwhile, water suspension determined mainly existing H⁺ in soil solution and readily dissociated H⁺ (Wilke, 2005). The pH in KCl, which was lower than that in CaCl₂, resulted from the higher ionic strength of 1 M KCl (I = 0.5 M) than that of 0.01 M CaCl₂ (I = 0.03 M), rendering more substantial H⁺ displacement of the electrolytes (Edmeades and Wheeler, 1990). Ionic strength consists of the combining functions of the concentration of an ion and its valency (Lehmann et al., 1996; Sposito, 2008). The higher ionic strength of an electrolyte produced the higher replacing power of an ion derived from that electrolyte in replacing H⁺ on the exchangeable sites of soil colloid surfaces (Sposito, 2008; Hao et al., 2019).
 

 
Increasing suspension volumes brought about increased soil pH in all electrolytes (Fig 1). Kome et al., (2018) and Merl et al., (2022) determined that raised suspension volumes decreased H⁺ concentrations, termed the dilution effect.
       
Increased EC significantly decreased ∆pH regardless of the type of electrolyte (Fig 2), indicating that the differences in soil pH, resulting from the different soil-to-suspension ratios, lowered with rising ionic strength. Notably, the ionic strength increased as EC increased (Dolling and Ritchie, 1985). Miller and Kissel (2010) and Kome et al., (2018) stated that declined ΔpH under increased EC was attributed to the salt effect. The higher EC was attributed to higher concentrations of cations and anions in soil (Miller and Curtin, 2008). The excess cations might displace the high amount of H⁺ and Al³⁺ on the exchangeable sites of soil colloidal surfaces, either low or high suspension volume (Sumner, 1994). The results implied that the soil-to-suspension ratios significantly affect soil pH measurement, particularly in low fertile soils which exhibit low EC (Sumner, 1994; Dolling and Ritchie, 1985), similar to the tropical coarsely textured soil studied herein. However, this result was not the case for ΔpH_1:5-1:2.5 in 1 M KCl, which has yet to be explained.
 

 
Optimal methods in soil pH measurement
 
Regression analysis between pH and H⁺ concentrations, as well as their R² and RMSE were employed to evaluate the optimal pH measurement method for acidic-coarse textured soil (Fig 3A-I). Increasing the suspension volumes decreased R² and increased RMSE of the pH measured in water (Fig 3A, B, C) and 0.01 M CaCl₂ (Fig 3D, E, F). The results indicated that higher suspension volumes lowered precisions of the pH measured in these two electrolytes. Our findings were in line with those of Kome et al., (2018), who demonstrated that increasing the suspension volumes from the soil-to-suspension ratios of 1:2.5 to 1:5 resulted in lower prediction precision, as assessed by R², which decreased from 0.957 to 0.921 in water and from 0.855 to 0.844 in 1 M KCl. A more constant equilibrium of H⁺ and Al species in lower soil suspension volumes might bring about a higher precision measurement method than that of the higher volume counterparts. The instability of H⁺ and Al in soil has been reported to affect the equilibrium of soil suspension (Dahlgren and Walker, 1994), which may influence the precision of the pH measurement within the current study.
 

 
Ionic strength increased as the suspension volume increased (Sonmez et al., 2008). Increases in the ionic strength of particularly variably charged soil brought about increased negative and positive charges (Black and Campbell, 1982). At a higher ionic strength, due to higher electrolyte concentration, Dahlgren and Walker (1994) demonstrated that Al and H⁺ on the exchangeable sites were displaced and then released into the aqueous phase of the soil suspension by cations. The displaced Al in the aqueous phase consequently hydrolysed, producing additional H⁺ as Al³⁺ + H₂O = AlOH²⁺ + H⁺ (Adams, 1971). Dahlgren and Walker (1994) claimed that the H⁺ product rapidly adsorbed onto a negative charge which increased due to the increasing ionic strength. Withdrawal of H⁺ rendered lower H⁺ activity in the aqueous phase, causing further hydrolysis of aqueous Al and, as a result, manifesting supersaturation and precipitation of Al species. Positively charged Al species were then adsorbed on the negative charges. This process reasoned that the equilibrium of suspension with a higher ionic strength was less constant, resulting in less precision of the higher suspension volumes in the pH measurements.
               
The highest R² (0.850) and lowest RMSE (0.010) were observed in pH_{KCl (1:2.5)} (Fig 3H). As soil pH is the determination of H⁺ concentration in soil (McLean, 1982), the optimal method for soil pH measurement in an acidic-coarse textured soil used in the current study was pH_{KCl (1:2.5)}. However, due to the high amount of the chemical KCl used to prepare 1 M KCl (74.55 g KCl /L), the soil pH measurement of pH_{KCl (1:2.5)} would be costly. An alternative method, suitable for a cost-saving laboratory, was the soil pH measurement in 0.01 M CaCl₂ with the soil-to-suspension ratio of 1:1 (pH_{CaCl2 (1:1)}), as their pH values were similar to those measured in pHKCl (1:2.5) (Fig 1), concomitant with a low amount of CaCl₂ (1.47 g CaCl₂ /L). The soil pH measurement method of pH_{CaCl2 (1:1)} owned R² of 0.766 and RMSE of 0.013 (Fig 3D).
 

The results demonstrate that the descending order of soil pH, measured in water > 0.01 M CaCl₂ > 1 M KCl. The pH increased with increasing suspension volumes in all three electrolytes.
 
Decreasing the suspension volumes increased the precision of pH measurement, where water and 0.01 M CaCl₂ were utilised as the electrolytes. The most optimal pH method recommended for soil fertility evaluation of an acidic-coarsely textured soil was the soil-to-suspension of 1:2.5 in 1 M KCl. The alternative method suggested for a cost-saving laboratory was the pH measurement in 0.01 M CaCl₂ at a ratio of 1:1.