Of Calcium in the Soil – Part 6


O, that egg again!

We’ve arrived at Part 6 if this extraordinary saga of how calcium arrives and behaves in the soil (if I’ve occasionally typed “soul” instead of “soil”,  is it really a typo?). Click to catch up on part 1, part 2, part 3 and part 4 and part 5.


6. Soil base saturation and soil pH

The term “soil acidity” expresses the quantity (expressed in meq/100g) of the acidic cations (cf. part 3) that the soil can hold on to. The percent base saturation – another important term on your soil test results – is the percentage of the soil’s cation exchange capacity (CEC) occupied by the basic cations.

This is from our soil test:


This means that calcium occupies 50.6% of the total exchange sites. In other words, in 100g of my soil, 15.6 meq can hold on to cations, both basic and acidic. Of that, 7.9 meq is occupied, or saturated, by calcium, 1.65 meq by magnesium, 0.64 meq by potassium. So, as far as I can learn from the test results (*), 10.19 meq/100g of soil, or 65.3% of the CEC, is saturated by bases. That leaves 35.3% of the CEC (*) for the acidic cations (hydrogen and aluminum).

(*) Sodium (also a base cation) is not listed on my test results, which means its levels are low, so I don’t have a sodic soil (cf. part 5).

Not surprisingly, the greater the percent base saturation, the higher the soil pH. Because calcium is normally the major cation, by virtue of its abundance taking up about half the CEC (as in our soil), we can say that there is less calcium in acid soils and more in alkaline soils.

But if the soil is very alkaline (pH > 7.0), the high levels of calcium may have negative effects. For one, more calcium taking up the CEC very simply means that there is less room on the colloid for everything else. Secondly, an excess of calcium can no longer be adsorbed onto the colloid. This “free” or unadsorbed calcium begins to accumulate in the soil water and goes on to react with what other nutrients are present.

For instance, the free calcium will readily attract soluble boron (B-), which is an an-ion (a negatively charged ion), and form a nearly insoluble compound with it, thus making the boron less available to plants.

Excess calcium will also tie up, or immobilize into insoluble compounds, cations like iron (Fe++), phosphorus (P+++) aluminum (Al+++), zinc (Zn2+), copper (Cu2+), cobalt (Co2+), and manganese (Mn2+), as well as magnesium (Mg ++) and potassium (K+).

Lastly, calcium also increases the pore space in the soil by flocculation, which, as we saw in part 5, is desirable. But when pore space exceeds 50% of the total soil volume, the soil can dry out much easier, like sand.

In short, too much calcium in your soil and many nutrients become insoluble and thus unavailable to plant roots, and the soil structure is damaged to boot.

But, on the other hand, if the soil is very acidic, and thus if there is not enough calcium, many of the other cations can become excessive and thus toxic. Then calcium applications with limestone are called for. The aim when attempting to adjust soil acidity is never so much to neutralize the pH as to replace lost cation nutrients, particularly calcium.


Next time, in Part 7, I promise, we’ll finally meet the plants, and discover by what magical means they get the calcium out of the soul soil. 

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1 Comment

  1. The CEC on my soil test was something else that was hard to understand. But your explanation makes sense. It also helps explain why my soil is so poor.

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