How to raise Soil pH

Many growers have a problem of low pH of their soil or growing media. Some soils are acidic by nature and, in other cases, low pH is the result of prolonged and intensive fertilization and irrigation.

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, aluminium and iron increases and toxicity problem 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): ammonium/nitrate ratio is a major factor that can determine the media pH, and it can be controlled by proper a fertilizers 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 continously 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 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.

 

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