Base Saturation

Your soil is filled with positively and negatively charged bits of plant food. The percentage of that food being held by soil particles is called its base saturation.

Crystalized magnesium (CSIRO) CC BY 3.0

Of course, it’s not that simple. The chemical reactions going on in soil are enough to make a chemist’s head spin. But we are here to simplify and understand, so let’s get started!

Electrified plant food

Plants use electrically charged mineral bits, called ions, as food. The negatively charged bits (anions) are usually found floating around in water. The positively charged bits (cations) attach themselves to soil particles, which are negative charged. Those soil particles have a certain number of electrical charges that can attract minerals. That number is referred to as its cation exchange capacity. The number of those attachments being used is its base saturation.

Playing the percentages

There is some crazy math and lab work involved with calculating base saturation, but we can leave that to the experts. Most soil test results will list separate base saturation percentages for calcium, magnesium, and potassium. Don’t be confused by the fact that these numbers do not add up to 100%. Hydrogen and sodium have been omitted. But what do these percentages tell you?

Balancing act

When the charges of soil nutrients are out of balance, plants cannot absorb what they need to thrive. It doesn’t matter if a nutrient is present if the net electrical charges are wrong. If most of the nutrients in your soil are negatively charged, all of the positively charged bits will be able to connect, leaving many negative bits hanging in isolation. Those leftover minerals impact soil pH.

​Base saturation and soil pH

Base saturation measures the number of non-acidic, positively charged bits in a soil sample. That’s why it is called “base” saturation. There are also acidic positively charged bits. Soils with a high base saturation have lots of those acidic, positively charged bits lying around unattached. The more loose acidic bits laying around in the soil, the lower the soil pH.

Negatively charged soil samples with high base saturation (left) and low base saturation (right), due to variations in acid cations (orange) and base cations (blue). Drawing by Kate Russell.

Using base saturation numbers

Soil test results will tell you how much of each plant nutrient is present and base saturation percentages. One thing you might see is an excessive amount of a nutrient but a normal base saturation percentage. How is this possible? Again, it goes back to electrical charges. Say you have a ton of calcium, a positively charged mineral, but the calcium base saturation is normal. This happens because other charged particles are also present. They can block the excess bits from connecting with anything. Or, there may not be enough negatively charged soil particles available. You need to use both the actual mineral levels and the base saturation percentages when deciding on whether or not to add fertilizer.

This post is an oversimplification of an extremely complex topic, but it is accurate enough to help you get the most out of your soil test results. Soil tests cost around $25 and are worth every penny.