In growing media, pH changes are much more rapid than in soils. Although various growing media are available with different baseline/starting pH levels, the effect of fertilization and irrigation on their pH levels can be enormous.

Soil pH below 5.5 might 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 K, Ca and Mg is decreased and might result in deficiencies.

Common causes of low soil pH

Prior to applying materials that increase pH, make sure that the low pH is not caused by an inappropriate fertilization regime. Often, an adjustment of such a regime may solve the acidity problem.

This is especially true for growing media (soilless media). The ammonium/nitrate ratio is a major factor that can determine the media pH, and it 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 affecting 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 - the 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.

Raising soil pH using lime

The most commonly used technique to elevate the soil pH is applying agricultural lime. Solubility of lime is relatively low, so if it is applied only to the soil surface, it usually affects only the top layer of the soil, not more than a few centimeters deep.

In soilless media, lime should 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 should then be individually applied to each growing container or each 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 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. Therefore, potassium carbonate can be regarded as a fertilizer and its K contribution should be considered. When applying potassium carbonate through the irrigation water, it is important to keep the pH below 7.0 in order to avoid emitter clogging.

Sometimes growers need to increase the buffer capacity of the irrigation water, while keeping pH levels low enough. In this case, it is possible to add potassium carbonate to water, and at the same time to acidify the water. The acid will neutralize some of the carbonate ions, while the pH level will still be low enough to prevent emitter clogging.