In growing media, pH changes faster than in soil. Although various growing medias are available with different baseline/starting pH levels, the effect of fertilization and irrigation on their pH levels can be significant.

Soil pH below 5.5 can result in reduced yields and damages to the crop. Under these pH conditions the availability of micronutrients such as manganese, aluminum, and iron increases, and toxicity problems of micronutrients might occur.

On the other hand, at low pH, availability of other essential nutrients, such as potassium (K), calcium (Ca), and magnesium (Mg) is decreased and might result in deficiencies.

Common Causes of Low Soil pH

Before applying products to increase pH, make sure that the low pH is not caused by an inappropriate fertilization regime. By making some minor adjustments, you can solve the acidity problem.

This is especially true for growing media (soilless media). The ammonium/nitrate ratio is a major factor that determines the media pH and can be controlled by proper fertilizer application.

In soils, intensive fertilization with ammonium-based fertilizers or ammonium-forming fertilizers (urea) may lower soil pH. Other factors that can affect soil pH include:

  • Parent material - Type of rocks from which the soil developed.
  • Rainfall - Soils under high rainfall conditions are more acid than soils formed under dry conditions.
  • Soil organic matter - Soil organisms are continuously decomposing organic matter. The net effect of their activity is that hydrogen ions are released and the soil becomes more acidic.
  • Native vegetation - Type of the native vegetation under which the soil was formed affects the pH of the soil. Soils formed under forest vegetation tend to be more acidic.

Read: How to Test the pH Level of Your Soil

Raising Soil pH Using Lime

The most commonly used technique to elevate the soil pH is agricultural lime. The solubility of lime is relatively low, so if it is only applied to the soil surface, it will only affect the top layer of the soil.

In soilless growing, lime needs to be incorporated into the media prior to planting and the process is usually logistically difficult. Waiting until after planting only makes it more complicated because the lime needs to then be individually applied to each grow box or plant. Again, due to its very low solubility, it's impossible to apply it through irrigation.

Raising Soil pH Using Potassium Carbonate

Unlike lime, potassium carbonate is highly soluble and therefore can be applied by drip irrigation. Due to its high solubility, potassium carbonate can be easily distributed throughout the root zone together with irrigation water and reach deeper soil profile. In both soils and growing media, potassium carbonate can rapidly affect chemical reactions in the root zone, thus elevate root zone pH.

Irrigation with water that has a low buffering capacity (low bicarbonate content) might drastically decrease pH levels in growing media. In this case, and especially when using inert grow media, pH drop can present a constant problem.

Applying potassium carbonate periodically, or even regularly as part of the fertilization program, can prevent the pH drop.

Potassium Carbonate as a Fertilizer

Potassium carbonate also contributes potassium to the nutrient content of the irrigation water. When applying potassium carbonate through the irrigation water, it is important to keep the pH below 7.0 to avoid emitter clogging.

In some cases, growers need to increase the buffer capacity of the irrigation water, while keeping pH levels low enough. It is possible to add potassium carbonate to the water, acidifying the water in the process.

The acid neutralizes some of the carbonate ions, while the pH level stay low enough to prevent emitter clogging.

Read next: Balancing Act - Understanding the Ups and Downs of pH