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Soil pH - What is it and how do we measure it?

26 May 2020
Type Media Article

Soil pH is a measure of hydrogen ion (H+) activity in the soil solution. Maintaining the soil pH at the optimum level increases the microbiological activity of the soil and results in better soil nutrient recycling/availability. Brendan Healy & Karen Daly have more information

Soil pH plays a key role in soil fertility and is important because it influences several soil factors affecting plant growth, such as nutrient availability, nutrient leaching, toxic elements, soil bacteria and soil structure. Maintaining the soil pH at the optimum level increases the microbiological activity of the soil and results in better soil nutrient recycling/availability. Put simply, soil pH is essentially a measure of hydrogen ion (H+) activity in the soil solution. A pH value is defined as a value between 0 and 14 expressing the acidity or alkalinity of a solution on a logarithmic scale, i.e., the pH value is equal to −log10 [H+], where [H+] is the hydrogen ion concentration in moles per litre (mol/L).

As the activity of H+ in the soil solution increases, the soil pH lowers in value. Soils with pH values below pH 7 are acidic in nature. Products such as orange juice, coca-cola and cheese are acidic. Those with pH values above pH 7 are termed as alkaline. Products such as toothpaste, bleach and baking soda are alkaline. Soils at pH 7 are said to be neutral. Water is an example of an item that can be pH neutral. The majority of soils in Ireland are acidic in nature ranging in pH value from 5.0 to 7.0 and a list of crops with their optimum pH is shown in Table 1. Nutrients are mostly available to plants between pH values of 5.5 to 7.0.

Crop/PlantOptimum pH
Beet, Beans, Peas and Oilseeds 7.0
Cereals and Maize 6.5
Grassland 6.3
Grassland (High Molybdenum) <6.2
Potatoes 6.0

Table 1: Optimum soil pH for a range of crops

pH measurements are carried out on soil sample solutions in Soil Prep. Lab. 1 in Johnstown Castle to determine how acidic or alkaline a soil sample is. Two workstations are available in Johnstown Castle for the pH analysis of soil solutions. The workstation (see Figure 1) has an autoanalyser that is pre-programmed through the accompanying software and can measure the pH of a maximum of 128 samples in a single run.

Figure 1: pH autoanalyser workstation

Soil samples are dried and sieved to 0.2 mm in Johnstown Castle for the purpose of pH and other analyses. A 10ml soil scoop is used to take a representative 10ml aliquot from the soil sample and this is carefully placed in a plastic tube.  A dispenser is used to add 20ml of deionised (pure) water to the 10ml of soil in the tube. The soil solution is manually stirred with a rod and allowed to sit for 5 minutes. The soil solution in each tube is then placed on the autosampler rack in the order they were pre-programmed.

The pH autoanalyser system requires calibrating before it can determine the pH of a soil solution. Stable solutions known as buffers with pH values of 4.0, 7.0 and 10.0 are used to calibrate the pH system. Once calibrated, the pH of soil solutions can be determined from the calibration data.

The probe holding a pH electrode, temperature probe and a stirring rod is programmed to dunk into each sample (see Figure 2). The pH electrode then takes a stable voltage reading from the sample. This data along with the temperature reading recorded by the temperature probe is sent back to a software program that displays the pH value proportional to the voltage produced using the saved calibration data.

The pH value of the control (known) soil sample is checked throughout the sample run to ensure the analysis is running smoothly. A check on the calibration data is also performed using a buffer solution with a pH value of 6.0. Soil pH is a useful parameter for management of soils and it is often accompanied by lime requirement which we also describe in this series.

 Figure 2: pH analysis of soil solution