pHiguring Out pH

Not something to be ignored if you’re a hydroponic gardener, pH is a measure of the acidity of an aqueous (water) solution. By understanding what it is, it is easier to understand how to properly use pH measurements to prepare the right nutrient solutions for your plants.

First off, any discussion of pH begins with its scale. The pH scale goes from 1-14. It is logarithmic, not linear, which means the difference between the numbers that are far apart is exponentially greater than the numbers that are close together.

For example, a difference of 1 may have a value of 10 units, but a difference of 2 indicates a value of 100 units. Therefore, the difference between a pH of 6.2 and 6.4 is close to negligible, but the difference between 6.2 and 4.2 can mean the difference between life and death for your plants.

The Chemistry of pH

Water (H₂O) molecules have two hydrogen (H) atoms and one oxygen (O) atom, giving it a neutral state with a pH of 7. Water molecules will sometimes donate a hydrogen atom to another atom.

For example, if a water molecule gives a hydrogen atom to another water molecule, then instead of two water molecules (H₂O and H₂O) they become a hydroxide (HO) molecule and hydronium (H₃O) molecule.

This is a normal property of water, and the fact that the molecules tend to cancel each other out by more swapping is why it is considered to have a neutral pH. In other words, water is considered to be both a weak acid and a weak base.

If more of the water molecules are converted to hydroxide, the solution is basic, and has a pH value greater than 7. Mild bases tend to taste bitter, and are commonly used in cleaning products.

If more of the water molecules are converted into hydronium, the solution is acidic, and has a pH value less than 7. Mild acids are commonly used in daily life. For example, citric acid gives orange juice more flavor, and phosphoric acid adds a bit of a sour snap to the flavor of some soft drinks.

As a word of caution, strong acids (with a pH closer to 0) and strong bases (with a pH closer to 14) should be used with care. Human skin contains a lot of water molecules that are strong candidates for conversion to hydronium (acid) or hydroxide (base), which is generally an uncomfortable experience for anyone who experiences this.

Testing the pH of a Hydroponic Solution

There are a few ways to go about testing your garden’s pH values. Common ways to test the pH of a nutrient solution include strips and drops, or a hand-held pH meter. Strips of litmus paper react by turning different colors when exposed to either an acid or a base. There are litmus drops that work similarly by changing the color of a small sample of the solution being tested. In both cases, the color displayed is matched to a chart and the pH value is estimated visually.

Testing the pH can also be done with a battery-operated pH meter. A pH meter has a higher up-front cost and requires some maintenance, but lasts a lot longer than consumable items like litmus paper and provides a more precise readout. The pen type of pH meter is popular with gardeners, giving them a fast, easy-to-read digital result.To ensure the accuracy of your pH meter, it should be calibrated occasionally against solutions of known pH to ensure correct readings.

Plant pH Requirements

Different plants have different pH requirements and preferences, but in general, a pH value of around 6.2 is suitable for soil-based plants, while 5.6 or so is better for hydroponic systems. Micro-organisms in an organic garden will make some pH adjustments on their own, but with hydroponics and other systems with low-to-no organic micro-organism reliance, maintaining the correct pH levels becomes more important.

The lower end of the acceptable pH range for plants tends to favor micronutrient availability, but plants in mineral soils with a pH of less than 5.5, or in hydroponic media with a pH of less than 5, may experience problems with calcium and magnesium washing away, or with the aluminum and manganese becoming too soluble and overwhelming the plant.

Adjusting pH Levels

To adjust the pH of a nutrient solution, first test the solution’s pH to see how far out of whack things are. If the pH is too high, the solution is too basic, so simply add an acid to lower the pH. If the pH is too low, which is a less common occurrence, add a base to raise the pH.

There are products out there designed especially for adjusting your pH levels, often referred to as “pH up” and “pH down.” Follow the manufacturer’s suggestions (or your personal experience) on the amount to add per gallon to make an adjustment.

It is better to add too little pH adjuster and have to repeat the process than to add too much and overshoot your target pH. Remember, since the pH scale is logarithmic, the closer the pH gets to the target value, the smaller the steps needed to make adjustments.

So what exactly are in those products used to adjust pH values? To make a nutrient solution more acidic (lower the pH), substances are added that will increase the amount of hydronium relative to the amount of hydroxide.

For example, phosphoric acid (H₃PO₄) is sometimes used to lower the pH of a nutrient solution. Since it is an acid, it increases hydronium when mixed with water. Phosphoric acid, when mixed with water, forms hydronium and dihydrogen phosphate (H₂PO₄), which further converts to more hydronium and hydrogen phosphate (HPO₄), and finally becomes even more hydronium and phosphate (PO₄).

This ability to release so much hydronium, leaving only phosphate, is why phosphoric acid is primarily used to adjust pH, but there are other chemicals that can also generate hydronium when mixed with water that also work.

To bring pH levels back up, products containing potassium-hydroxide are often used. Other options include adding lime to raise pH levels, or sulfur to lower them.

Understanding the basic principles of pH removes a lot of its mystique and helps growers know what to expect from it. By keeping your garden’s pH level within the desired range for what you’re growing, you will increase your success, as keeping the pH levels closer to the ideal levels for the right plants optimizes nutrient uptake, leading to better, healthier yields.