Whether it is soil, water or hydroponic nutrient solution, measuring pH levels is an important aspect of successful gardening and crop production.

Healthy plant growth depends on the proper synthesis of the ideal environment for your fruits, vegetables and ornamental plants. While adjusting the pH in your soil or water can help plants thrive, conversely, the wrong pH can lead to disease or even death.

What is pH?

PH is an acronym for 'potential of hydrogen', which is quantified by the ratio of H+ and OH- ions in water and reflects the degree of acidity or alkalinity of the environment. The standard pH scale ranges from 0 to 14, although these levels can be exceeded. The higher the pH, the more alkaline the substance. Lower the pH, the more acidic the substance is. A pH level of 7.0 is a neutral level.

How can the pH be measured?

Although it is impossible to visually determine the pH level of a liquid, the pH of soil often affects its color. Green soil tends to be more alkaline, while yellow or orange soil tends to be more acidic. Soil pH can be measured with a pH meter that is specifically designed for soil testing or with a pH meter for liquids using a special technique.

Liquid pH can be measured with reagents placed on paper test strips or liquid drops or with a digital pH meter. Testing with reagent strips or drop tests involves using a color-matching method. Although they are initially inexpensive, they end up costing more than a pH meter. More importantly, strips and drops have shelf life, do not provide high accuracy and color matching is an inaccurate and controversial method in practice. For example, most strips show a pH increase in 0.5 increments. Therefore, when using a pH strip, the difference between 7.0 pH and pH 8.0 will only produce two different shades of pink. What about the approximately 7-10% of people who are color blind? A digital pH meter, on the other hand, provides a pH reading on the instrument's display, so there is no need for interpretation: the user simply dips the pH meter into the solution and sees the result.

It is important to note that pH meters for soil and liquid have very different sensors and should always be used accordingly.

How does the pH meter work?

Despite the different types of pH meter electrodes, ranging from inexpensive pocket models to lab models that cost tens of thousands of dollars, the most common is a glass flask with a built-in sensor. The pH meter electrode measures the activity of hydrogen ions by creating a small amount of sensor voltage in the flask. The pH meter then converts the voltage into a pH value and displays it on the instrument's display.

In addition, many digital pH meters have a built-in thermometer that automatically adjusts for any differences from a base 77ºF (25°C). This function is called Automatic Temperature Compensation (ATC).

What is calibration and why is it needed?

Calibration is akin to tuning, and just like a musical instrument that must be tuned from time to time, a scientific instrument must be calibrated to achieve accurate test results.

Although some people may have absolute hearing and can tune a musical instrument without using a tuning fork, the pH meter must be calibrated properly by comparing it to a laboratory-certified standard reference point, more commonly known as a buffer calibration solution. Buffer solutions are a liquid, but can also be purchased in powder form for mixing with distilled water to create a fresh solution at the right time.

Any scientific instrument should be calibrated as close as possible to the level to be tested (measured). If a pH range is to be tested, the instrument should be calibrated in the middle of that range. For example, if testing will be done in acidic solutions, the pH meter should be calibrated to a pH value of 4.0 to achieve more accurate results. Most waters fall within the range of pH 6.0 to pH 8.0. Therefore, to check the pH level of the water, calibrating the instrument to pH 7.0 is sufficient. The three most common pH levels for calibration are 4.0, 7.0, and 10.0. These points cover the pH range of 0 to 14, although other pH calibration point values also exist.

The pH meters are available in one-, two-, or three-point calibrations for accurate results. Some of the pH meters can be calibrated to one point, but manufacturers most often recommend at least two calibration points for optimal testing. The differences depend on the technology used to manufacture the instrument and the type of sensor used.