What is pH?

The unit of measure of nutrient solution acidity or alkalinity is termed "pH". It is a description of the concentration of the H+ ions in the feed solution, substrate or drain water.

The mathematical description is:

pH = -log10[H+] = log10 1/[H+]

This equation tells us that as the concentration of H+ ions What is in a solution increases, the more acid is the solution. Conversely, as the H+ concentration decreases, the more basic the solution becomes.

To control the pH and still retain a certain degree of buffering, the bicarbonate (HCO3) content of the raw water (bore-hole, well or mains) must be determined and adjusted to contain 40 to 60 ppm using one or a combination of acids. The naturally occurring bicarbonate ions in the water act as insurance against wide fluctuations in the pH of the solution. However some sources of water, particularly rainwater contain very little, if any bicarbonate (<6.1 ppm). Feed solutions composed of mainly rainwater will cause the slab solution to have an unstable pH. The addition of potassium bicarbonate to the stock tank will provide some buffering capacity under these situations.

The bicarbonate content of the water and the pH of the water are closely correlated. However, the pH is not a measure of the bicarbonate content in the water and the bicarbonate content is not a measure of the pH of the water.

The pH of the water will rise through the addition of bicarbonate ions due to its reaction with free hydrogen (H+). This will result in the creation of water (H2O) and carbon dioxide (CO2). More importantly however, hydrogen ions, which are what is measured by the pH meter, are removed from the solution. As a result, the pH meter detects fewer hydrogen ions and produces a reading indicating a higher pH.

Why is it important to monitor the pH of the nutrient solution?

The pH of the nutrient solution will determine the availability of certain elements. Generally Iron (Fe) Manganese (Mn) and Zinc (Zn) become less available as the pH is raised from 6.5 to 7.5. At high pH phosphorus will form insoluble phosphate salts which will precipitate out into the solution. Thus P will become unavailable to the plant until the pH is reduced to within normal limits. A balanced uptake of each of the nutrients by the plant is only possible if the root zone pH is within the range 5.5 to 6.0. The primary control of pH in the root zone is through the input (drip) solution. It should be noted that the crop itself has the ability to change the solution pH around its roots quite considerably, especially during early season vegetative growth.

Monitoring the pH in the slab

Monitoring pH in the slab is an essential part of weekly nutritional monitoring and should be undertaken on a daily basis. Sampling should take place in a number of representative slabs in order for a meaningful and accurate assessment to be made. Sampling is easy, simply push a syringe into the slab and extract a small quantity of nutrient solution (100 ml). Record the value and plot this on a graph this will make it easier to identify trends.

Tips for accurate pH measuring:

Calibrate the pH meter regularly using standard solutions.
Check the battery status, low batteries in portable pH meters are often a cause of error.
Always ensure the pH probe is immersed in de-ionised water when not in use.
Store the meter in a cool dry place.

Other learning topics

Irrigation and nutrients

Root zone management

An introduction to root zone management and the role of Grodan’s stone wool growing media in the context of Controlled Environment Agriculture (CEA) and ‘Precision Growing’.