Electrometric Measurement of Soil pH

Soil acidity is known to exert adverse effect on crop growth by its effect on nutrient availability and microbial activity. Measurement of soil pH only is not a true representation of soil acidity. This is due to the fact that soil pH which is a measure of active acidity is subject to change due to a number of factors. Substantial amoimts of soil acidity reside in the soil solid phase, in the interlayer spaces, as solid phase minerals and in the fimctional group of soil-organic fraction. All these contribute to the pool of active acidity in response to any shift in the thermodynamic equilibrium and thus helps in maintaining the buffering capacity of soils. 

In the pH measurement the reference and indicator electrodes are immersed in a heterogeneous soil suspension comprising of dispersed solid particles in an aqueous solution. If the solid particles are allowed to settle down, the pH can be measured in the supernatant liquid or in the sediment. Placement of electrode pair in the supernatant normally gives a higher pH reading than placement of the same in the sediment. The difference in soil pH reading is called the Suspension effect . 

In practice a measured quantity of soil is shaken with a convenient volume of water or salt solution imder consistent conditions and the pH of the suspension is determined electronically 

Standard certified buffer tablets are available for a whole range of pH-values and these are most convenient. One tablet dissolved in a specified amount of water, usually 100 cc provides a solution of known pH to within about 0.02 of a unit. Buffer tablets of pH 4.0, 7.0 and 9.2 are mostly used now a days for instrument calibration. For 1(N) KCl Analytical Grade salt is used. 

ApH 0 implies negatively charged clay surface ApH 0 implies positively charged clay surface 

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