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Simple Soil pH Science with the LAQUA pH Pocket Meter

Written by Janice M on . Posted in Agriculture & Soil

Why not experiment with easy pH science techniques to increase plant growth? This article will simply explain what soil pH is; what effect soil pH has on nutrient availability and plant growth; how to measure it and optimise soil conditions.

It is vital to know the pH of soil, because most South African soils are acid and in order for crops to be successful, soil remediation is often vital.

The most advantageous soil pH range for good growth in plants will vary for different crops.

Generally, a soil pH range of 6.0 to 7.5 is acceptable for most plants because most nutrients become available in this pH range.

Soil pH can be determined by mixing a soil sample with water and then measuring the resulting aqueous solution.

What is Soil pH?

A measure of the acidity or alkalinity of the soil. pH 7.0 is neutral on the pH scale. Below 7.0 is acidic and above 7.0 is alkaline.

Table 1: Soil pH and Plant Growth


Source: Colorado State University – Garden Notes

Soil pH affects nutrients available for plant growth:

  • In highly acidic soil, aluminium and manganese can become more available and more toxic to plants, whilst calcium, phosphorus and manganese are less available to the plant
  • In highly alkaline soil, phosphorus and most micro-nutrients become less available

When designing or planting new gardens, it is helpful to check the soil pH because different plants thrive in different soil pH ranges. Measuring the pH can indicate whether the soil is suitable for the plants chosen, or if it needs to be adjusted to produce optimum plant growth.

Table 2: Fruit and Vegetable Preferences for pH


How to measure Soil pH with the LAQUA pH Meter:

Method based on US EPA Method 9045D.

Also applicable for measuring pH of waste samples, which may be solids, sludges or non-aqueous liquids.
If water is present, it must constitute less than 20% of the total volume of the sample.

  1. Calibrate the pH Meter according to the manufacturer’s instructions, using at least two pH buffers that bracket the expected sample pH, eg 4, 7 and / or 11.
  2. Add 20 ml of pure water to 20 g of sample in a beaker or container. Stir for 5 minutes, then cover.
  3. Let the soil suspension stand for about 1 hour. (Alternatively, filter or centrifuge off the water phase.)
  4. Measure the pH of the water phase. Record the pH value and the temperature.

Tips for Accurate Results:

  1. Standard buffer solutions for calibration and samples should be measured at the same temperature.
  2. If the electrode is coated with oily material from a sample, clean it with detergent and warm water.

Effects of pH on Nutrients:

Remember, a soil pH of 6.0 to 7.5 is acceptable for most plants because most nutrients become available in this range.

Table 3: Soil pH and Nutrient Availability


Source: bluedaleplantsonlineblog

About the nutrients:

  • Nitrogen, phosphorus and potassium are the primary nutrients needed in fairly large quantities
  • Calcium, magnesium and sulphur are secondary nutrients required by the plant in lesser quantities
  • Zinc and manganese are micro-nutrients required by the plant in very small amounts
  • Most secondary and micro-nutrient deficiencies are easily corrected by keeping the soil at the optimum pH value
  • Highly acidic soil slows down the activity of important soil micro-organisms and this can result in slow decomposition of organic matter and an accumulation of bound nutrients, especially nitrogen. (Soil microorganisms prefer a pH of 5 to 9.)

How to Increase Soil pH:

  • Apply a material that contains lime (calcium carbonate), such as ground agricultural limestone or wood ashes to increase the soil pH
  • Wood ashes contain high amounts of potassium and calcium, small mounts of phosphate, boron and other nutrients. Although not as effective as limestone, these can hugely increase the soil pH with repeated use
  • The finer the limestone, the quicker it will work
  • Different soils need different amounts of limestone

Table 4: Approximate Amount of Finely Ground Limestone Needed to Raise the pH of a 7-inch (18 cm) Layer of Soil: 


Source: Vossen. P. Changing pH in Soil. 2002. University of California Cooperative Extension

How to Decrease Soil pH:

  • Aluminium sulfate and sulfur are common materials used to make soil more acidic
  • Aluminium sulfate is preferred because it changes the soil pH as soon as it dissolves in the soil. Too much is toxic to plants.
  • Sulfur takes some time to work, as it needs to be converted to sulphuric acid by soil bacteria
  • Ammonia based fertilisers and organic matter can decrease soil pH

Further Reading:

November 2018.

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