Analysis Of The Seasonal Changes In Soil Ph Variations

Seasonal variability of soil pH is driven by fertilizer rate, timing of fertilizer application, root and bacterial activity, and soil organic matter (Murdock and Call, 2006). Soil pH is observed to be highest at or near mid-winter and lowest at late summer (Slattery and Ronnfeldt, 1992). Variation of soil pH can be more than one unit from spring to fall (Angima, 2010).

Because soil pH is by its nature operationally defined, any subsequent calculations are also operationally defined. pH is is a logarithmic relationship to concentration, defined as pH = -log [H+] (negative log of hydrogen ion concentration). pH is converted to theoretical hydrogen ion concentration in the solution using [H+] = 10−pH. A small error in the determination of soil pH could potentially lead to much larger errors in calculated NH3 compensation points due to the logarithmic relationship between pH and H+ concentration. Unlike the theoretical definition of pH, there is a more practical definition that is linked to the analytical method used to measure the pH of a solution.

The basic principle of electrometric pH measurement is determination of the activity of the hydrogen ions by potentiometric (voltage) measurement using a glass electrode and a reference electrode. The glass electrode, actually a membrane electrode, is comprised of a thin- walled bulb of special pH-responsive glass sealed to a stem of high resistance glass. The internal reference electrode is permanently immersed in a reference buffer solution of known pH. The voltage generated between the two electrodes is a direct indicator of the pH difference between the reference solution and the solution in which the glass electrode is immersed. The electrometric pH reading is a product of complex interactions between the electrode and the soil suspension; differences in the soil:water extraction ratio, the electrolyte concentration of the soil suspension, and the spatial placement of the electrode can all affect measured pH (Thomas, 1996).

The first source of error comes from the methods for measuring soil pH, or more specifically, from not knowing what method of measurement was applied to soil samples. These methods differ according to the extracting agent (H2O, CaCl2, KCl) and the ratio of soil to solution (1:1, 1:2, and 1:5) (Soil Survey Staff, 2009; Libohova et al.,2013; GlobalSoilMap, 2015). Selection of the method of measurement is related mainly to the soil conditions, especially the seasonal variation of soil moisture that affects the loss, formation or accretion of salts (Thomas, 1996; Kisselet al., 2009).

The pH determined in water is usually higher by 0.2 to 0.4 units compared with the pH determined in solutions containing salts, such as CaCl2, KCl (Brady & Weil, 1999). Soil pH measured in a suspension of salt solution tends to reduce the seasonal effect of soluble salt concentration by providing Ca2+ions, which displace the hydronium and aluminum ions from the colloid surfaces (Brady & Weil, 1999; Kisselet al., 2009).

The resulting pH measurement is relatively constant throughout the season and analytically more stable and accurate (Kissel et al., 2009).