Home » Understanding Basic Soil Chemistry: Why We Care so Much About Cation Exchange Capacity

Understanding Basic Soil Chemistry: Why We Care so Much About Cation Exchange Capacity

Editor’s note: This is an excerpt from Don Schriefer’s Agriculture in Transition, which is just one of many classic texts on soil health in the Acres U.S.A. library. Schriefer was an educator consultant from DeMotte, Indiana, who developed a “systems” approach for farm management that related tillage to the management of soil aeration, soil water and residue decay.

A FARMER SHOULD KNOW SEVERAL THINGS about soil chemistry to help him understand his soil laboratory data. We will start with a study of the different types of soil nutrients and how they react within the soil. These nutrients fall under two categories: positively charged cations and negatively charged anions, and are listed below.


  • Ca++ (Calcium)
  • Mg++ (Magnesium)
  • K+ (Potassium)
  • H+ (Hydrogen)
  • NH4 (Ammonium)
  • Trace Elements


  • PO4= (Orthophosphate)
  • SO4= (Sulfate)
  • NO3− (Nitrate)

We also need to understand the term cation exchange capacity. Cation indicates that we are referring to positively charged elements.


Soil is composed of sand, silt, clay, humus or any combination of these. These all carry various degrees of negative sites, and the number of negative sites determines how many positively charged nutrients (cations) each can hold.

The measurement of the negative sites is given as its CEC (cation exchange capacity). This is often said to be the soil’s “pantry size.”

The soil test lab assigns a CEC number to each soil sample. In effect, this figure is the measurement of how many negative sites are available in 1 acre of soil at a depth of 7-1/2 inches. One acre, 7-1/2 inches in depth, will weigh approximately 2 million pounds

The CEC number on your soil tests represents a combination of one or more soil types found in a particular soil area. Sand has a very small “pantry” to hold cations, while the humus pantry is as big as a gymnasium.

Figure 1: Table of Cation Exchange Capacities

A complete soil analysis will also provide a percent base saturation figure for the cations calcium, magnesium, potassium and hydrogen. Trace elements are too small to assign a percent base saturation number.

The percent base saturation figure reports the percent of the negative sites that are filled with each cation. The negative sites need to be filled with a balance of cations. For example, a soil would produce nothing if its pantry were filled to 100% with any one of the cations.

Figure 2 represents what most of us consider a reasonable soil balance. Notice that of the major elements, calcium occupies the vast majority of the pantry, potassium the least. In natural conditions, calcium and magnesium are very important elements and occupy most of the pantry.

Figure 2: Suggested C.E.C. Saturation

Hydrogen is the acid element, and the pH number on your soil test is a measurement of hydrogen activity. When a pantry is filled 10 to 12% with hydrogen, the base soil pH will be approximately 6.3 to 6.5. (Base pH is my way of describing the “off season” pH when there are no acids being produced by roots or activities of soil life.) If your soil tests show a pH of 7.0 or higher, you will notice the hydrogen percent base saturation is zero. If the percent base saturation of hydrogen is higher, the soil pH will necessarily be lower.

Let us now look at pH. This is necessary because many farmers are still applying limestone based on the pH numbers alone. We must first ask the question, why do we lime? Some would say that its purpose is to adjust the soil pH.

Looking at the chemistry of limestone, we notice it either contains calcium alone (calcitic limestone) or a combination of calcium and magnesium, which is called dolomitic limestone. The calcium and magnesium are coupled to a carbonate (CO3) as CaCO3 and MgCO3.

The carbonates are responsible for raising the soil’s pH and the calcium and magnesium enter the pantry and add to the percent base saturation of these two elements. The carbonates raise the soil pH by combining with and removing hydrogen acid elements.

Figure 3 shows just how deceiving a soil’s pH can be. Notice that each soil sample has the same pH. As a result, they all have the same percent base saturation of hydrogen. This is expected because hydrogen, after all, is the acid element.

Figure 3: Soil pH and % Base Saturation

Look at the differences in pantry-fill of calcium and magnesium. The bottom sample is very low in calcium and extremely high in magnesium. Someone who believes applications of limestone should be based on pH alone might recommend both sample areas be limed to raise the soil pH to 6.5 and 7.0. They would also recommend the highest neutralizing limestone in the area. This would be dolomite limestone that contains both calcium and magnesium. A magnesium limestone raises the soil pH higher than a calcium limestone because magnesium displaces more hydrogen.

Imagine someone recommending a magnesium (dolomitic) limestone for the second sample area, which is already carrying twice the amount of magnesium that it needs and is also very low in calcium. If they do this, they simply must not have a good grasp of nutrient balance. Another reason we should not apply limestone based on pH alone is that a soil changes from day to day, particularly during the growing season.

Figure 4 show how much a soil pH can change during the season.

Figure 4: how soil pH changes in a season

Notice how low the pH can go during the peak of the growing season. This is because the root hairs are giving off lots of hydrogen, which lowers the soil’s pH. The soil microorganisms also release several organic acids through their normal activities. These activities result in a temporary lowering of the pH. As the seasonal root and soil life activity declines, the pH returns to its base level. The question is, how much limestone would be recommended if the soil tests were done in late October, as compared to early July?

We recommend limestone for the purpose of balancing calcium and magnesium in the soil. If a client’s soils need magnesium, a dolomite limestone is used. Regardless of the soil pH number, we have often suggested applications of a calcium limestone because of excessive magnesium in a soil.

It is a grievous error to offer a farmer a soil test that only provides the levels of phosphorous (P), potassium (K) and soil pH. A good thing to remember is that a locally available limestone often becomes the wrong one to use on your soils.

I have not attempted to make specific fertility recommendations in this unit. It is too complex without knowing the soil chemistry of a specific farm along with the crop history and exact knowledge of the nitrogen and fertilizer delivery systems used.

Learn More

Learn more about cations and anions in soil with Don Schriefer’s book Agriculture in Transition, available at the Acres U.S.A. Bookstore online.