How to measure the pH of your substrate?

Regardless of the substrate in which you grow, measuring pH is essential to know if your plants are getting all the nutrients they need.

 

What is pH?

The pH is a coefficient that indicates whether a solution is acidic or alkaline. The acronym “pH” means Hydrogen potential and represents the concentration of hydrogen ions (H+) present in certain solutions. The values of the pH can be between 0 and 14. Solutions with a pH less than 7 are acidic and solutions with a pH greater than 7 are alkaline or basic. Pure water is neutral (pH 7), being neither an acid nor a base.

Some examples of acids are vinegar or lemon juice, while bleaches or baking soda are alkaline.

Why is important to know the pH?

pH of the soil is a key value used in agronomy and gardening and it’s just as important as knowing the composition of the soil (content in clay, silt, sand…). Plants get the feed they need from the soil, and nutritional elements availability within soil varies with pH. It means that if the soil is too acidic or too alkaline the plant wouldn’t be able to absorb all the nutrients. The ideal range of pH for most of the plants is 6-6.5. In this slightly acidic range almost all elements (nitrogen, phosphorus, calcium…) are available to be absorbed by the roots.

 

Light blue color represents the ideal range for most of the plants.

 

How can I measure the pH?

The pH of aqueous solutions can be measured with a pH meter or an indicator. Both tools can be acquired at specialized stores or on the Internet.

  • pH meter. Instrument that measures the potential difference between two electrodes: a reference electrode and a glass electrode that is sensitive to hydrogen ions. They are instruments that have a metal rod that when introduced into the solution measures the pH and indicates its value on a digital screen. It’s a very precise tool suitable for professional growers. It must be calibrated frequently.
  • pH test paper. It’s a strip of coloured paper impregnated with a mixture of qualitative indicators for pH determination. Test paper changes its color depending on the pH. It’s an economical but less accurate method.

Procedure

  1. Take substrate samples. Samples should be taken from the soil around the roots, so they will be from lesser or greater depth depending on the size of the roots of your plant. Take samples of soil from different parts of your garden or pot and mix them all evenly.
  2. Add distilled water. Introduce the mixture in a container and add distilled water. Dilution ratio varies according to the country, but from Atami we recommend to mix 1 part of substrate and 1.5 parts of distilled water (1:1.5). So, if you have 100 ml of sample, you should mix it with 150 ml of distilled water.
  3. Stir well and let stand for 1-2 hours.
  4. In case you’re using a pH meter, just introduce the rod in the test solution and hold until readings stabilize, usually 10 to 20 seconds. pH meter has to be previously calibrated following the manufacturer's instructions.

If you’re using pH test paper simply immerse a strip in the solution until color development is complete. Then compare the developed color to the sequence chart on the package, and read the pH.

What to do if the pH is too high or too low?

First of all, repeat the procedure to make sure that the measurement is correct. Don’t take samples from the same places as previously. Once you’re sure the pH you’re reading is the same as the actual pH of the substrate, you can take measures to change the pH of the soil according to the requirements of your crop.

If the pH is too high (alkaline soil)

When you want to change the pH of the alkaline soil to make it a little more acidic, you can use sulfur powder or iron sulfate mixed with the substrate. Another very reliable option is to add blond peat, which has a very acid pH (3,5)

If you are growing pot plants, the best option to acidify the soil is by adding product prepared with phosphoric acid or nitric acid to the nutrient solution, with very low doses of these products being sufficient. In case you prefer more environmentally friendly fertilizers, citric acid is ideal for this purpose.

If the pH is too low (acidic soil)

When, on the other hand, you want to raise the pH of an acid soil, you can add some ground limestone or quicklime to the substrate, mixing everything well. Calcareous water is highly recommended to raise the pH of small spots. If the tap water in your region is hard, which means it is rich in calcium and magnesium, you can use it to water your plants. On the contrary, if the tap water of your region is soft, you can add Calmag to the irrigation water.

In case you want to basify the substrate of pot plants, it would be sufficient to use caustic potash preparations in low concentrations together with the nutrient solution.

It is very important to bear in mind that different types of rain, as well as the time of year in which they occur, and the level of contamination of the environment, influence the pH level of the soil. For this reason, it is very important to be able to make periodic pH measurements of the substrate with which you are working, so that the elements you plant and grow can grow without any problem.

Curious fact

The hortensia (Hydrangea) flowers can be pink, white, or blue, depending on the pH of the soil. In quite acid soils, with pH between 4.5 and 5, the flowers become blue; in less acid soils, with a pH between 6 and 6.5, the flowers acquire a pink color; and in alkaline soils with pH around 8, the flowers grow white.