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Base Saturation

Base Saturation is an Indication of Soil Fertility

Base Saturation is closely linked to soil fertility. The ease with which adsorbed minerals (cations) are released to plants depends on base saturation.

Percent base saturation (BS) is the percentage of the Cation-Exchange Capacity occupied by the basic cations Calcium, Magnesium and Potassium. The three basic cations are essential minerals that play a key role in plant growth. Basic cations are distinguished from the acid cations like Hydrogen and Aluminium.

At an approximate soil pH 5.4 or less, Aluminium is present in a significantly high concentration that hinders growth of most plant species, and the lower the soil pH, the greater the amount of toxic Aluminium. Most plants prefer soil pH 5.5 and higher.

Soils with a high percent base saturation are generally more fertile because:

1. They have little or no acid cation Aluminium that is toxic to plant growth.

2. Soils with high percent base saturation have a higher pH; therefore, they are more buffered against acid cations from plant roots and soil processes that acidify the soil (nitrification, acid rain, etc.).

3. They contain greater amounts of the essential plant nutrient cations Potassium, Calcium and Magnesium for use by plants.

Ideal Base Saturation Ratio

William Albrecht (1888–1974) developed the  formula for ideal ratios of cations in the soil – the Base Cation Saturation Ratio. His breakthrough came after he studied the connection in between poor forage quality and ill health in livestock. 

When Albrecht researched cattle nutrition and observed that certain pastures seemed conducive to good health, he came to the conclusion that the ideal balance of cations in the soil was Calcium 60 to 75%; Magnesium 10 to 20%; Potassium 2 to 5%; Sodium 0.5 to 5.0%; and other cations 5%.

Base Saturation Ratio and Soil pH

Studies show that there is a correlation in between the soil pH and base saturation. As base saturation (BS) represents the percentage of CEC occupied by bases (Calcium, Magnesium, Potassium, and Sodium). The base saturation ratio increases with increasing soil pH (see the figure below) and the availability of Ca2+, Mg2+, and K+ increases with increasing base saturation ratio.

 

Soil pH increases with base saturation ratio (J.L. Havlin, in Encyclopedia of Soils in the Environment, 2005.)