When you research aquaponics and hydroponics, you will probably encounter terms that are unfamiliar to you. In particular, around the pH. We know that this is important and we know that it affects the way our plants grow in the system. But what does that mean exactly? Why does this have such a big impact on the success of our operations? Let’s dig a little deeper.
pH is a measure of acidity, which is important because nutrients are more or less available at different levels of acidity. This means that you will need to find the ideal point on the pH scale where all nutrients are available and plants get what they need.
The pH is represented by a scale from 0 to 14 (0 being the most acidic and 14 the most basic). The term « pH » means « potential hydrogen » because hydrogen (H+) and hydroxide (OH-) ions are at the origin of a basic or acidic solution. pH tests measure the concentration of these two substances in a solution. The pH is measured on a logarithmic scale (we will come back to this later).
That’s when things get complicated. As you know, water is composed of hydrogen and oxygen. Pure water has a pH of 7, which means that it contains an equal number of hydrogen and hydroxide ions because there is nothing else in the water to break the atoms.
When water molecules decompose (which occurs when a nutrient mixture reacts with water), water becomes acidic or basic. We measure the concentration of hydrogen in relation to hydroxide; the critical measure is the concentration of hydrogen ions.
Looking at the figures
Some solutions contain more hydrogen ions than hydroxide ions, which makes them acidic. These solutions would have a pH of 0 to 6.9. Others contain more hydroxide ions, which makes them basic, so they would have a pH of 7.1 to 14. If a solution has an equal concentration of hydronium and hydroxide ions, it is neutral (pH 7). Let us look at these figures more closely.
The pH numbers represent a logarithmic scale. So if your pH meter reads 6.5, it does not mean that there are 6.5 hydrogen ions in your system. This means that the hydrogen ion concentration is five times higher than a neutral solution (pH 7). An acid solution can contain a hundred trillion times more hydrogen ions than a basic solution, which can be quite heavy to represent numerically. That’s why scientists use the pH scale. Each complete change of a unit in a pH number represents a difference of ten times the actual concentration. Thus, a pH of 0 has a hydrogen ion concentration ten times higher than a pH of 1, which has a concentration ten times higher than a pH of 2, and so on.
What does this mean for your system?
It is important to understand how acidic or basic your solution is because it says a lot about nutrient availability and can help you identify deficiencies. pH is also one of the reasons why many people choose to use a reverse osmosis filter, which can remove minerals and therefore the buffer capacity (we will get there) from water.
In most cases, nutrients are mainly available in a pH range of about 5.8 to 6.5. You can ensure that the pH of your system is in the right range. Let’s take iron as an example:
Iron comes in two forms: ferric and chelated
Ferrique is not available for plants. Chelated iron, on the other hand, is available for plants. Iron fluctuates between these two forms in your system. Chelated iron can quickly become ferric iron in a high pH system due to the way it reacts with hydrogen ions. Therefore, in higher pH ranges, plants can quickly develop iron deficiencies.
Conversely, adding nutrients to your system can affect pH. In most cases, aquaponic and hydroponic nutrients lower the pH (or make the solution more acidic). Some nutrients react with hydrogen and hydroxide ion concentrations, and others bring hydrogen or hydroxide to the system.
You can also influence the pH by adding pH down (an acid) or up (a base). The pH down is mainly phosphoric acid, which is the most stable and least harmful to your plants. It is also safe for fish, which means you could use it in an aquaponics system.
Buffer capacity
Buffer capacity is the ability of a solution to resist a change in pH. This occurs in hydroponics or aquaponics, when the water used is hard water, which contains a lot of carbonates. Carbonates are extremely effective in buffering water at high pH. So if you have a consistently high pH in your system, it is probably because the carbonates in your solution increase its buffer capacity. Have your water source analyzed and if you find a high carbonate content, you will probably want to use a reverse osmosis filter to remove these minerals.
Here are some familiar elements and their ph that will help you to orient yourself:
| Associations | Protection contre |
|---|---|
| Tomate et calendula | altenariose sur la tomate |
| Poivron et maïs | virose du poivron |
| Carotte et aneth ou coriandre | mouche de la carotte |
| Chou et laitue ou épinard | altise des crucifères |
| Chou et trèfle | mouche et piéride du chou |
| Chou et tomate ou céleri | piéride du chou et teigne des crucifères |
| Fraise et ail ou poireau | tarsonème |
| Poireau et carotte ou céleri | teigne du poireau |
| Poireau et trèfle | thrips du poireau |
| Concombre et vesce | mildiou et fusariose sur les concombres |
| Concombre et céleri ou épinard | aleurode du concombre |
| Carotte et calendula ou oeillet d'inde | aleurode, nématodes et psylles |
| Carotte et oignons | mouche de la carotte, psylles et thrips |
| Chou et calendula ou oeillet d'inde | piéride du chou |
| Aubergines ou tomates et oeillet d'inde | nématodes et noctuelle |
This does not mean that you must put these elements into your system to change your pH levels. Actually, that would be a bad idea.
Remember that the acidity of your solution must be relatively stable, as it affects the availability of nutrients to plants. Take a look at the pH table to determine the acidity at which the plants in your system can use the nutrients you provide them. Be strategic in the dosage of your system and don’t forget to take into account the buffer capacity.
