Cation Exchange Capacity - CEC

Cation Exchange Capacity, you may have seen this phrase on your soil test, or even during your lawn care research. Cation Exchange Capacity was first discovered by two English chemists, Thomas and Way in 1850 when they noted that certain soils had a greater ability to absorb ammonia fertilizers than others.

The definition of cation exchange capacity in the simplest terms is the soil’s ability to exchange cations. In this post, we will explore the basics of Cation Exchange Capacity, also known as C.E.C., and why it matters when it comes to your soil and your fertilizer program.

Cations are positively (+) charged elements that have more protons than electrons. Some positively charged elements include Calcium, Copper, Hydrogen, Potassium, Magnesium, Iron, Zinc, Sodium, and more. There are also negatively(-) charged ions that contain more electrons than protons and these are referred to as anions.

Many soil particles will have a negative charge, and the amount of negative charge depends on the soil type. These types of soils include sand, silt, clay, and more. A soil with a low C.E.C. will have less negatively charged particles than a soil with a higher C.E.C. Sandy soils will typically have the lowest number of anions (negative charges), while certain types of clay soils will have a higher number. The higher the number of anions, the greater the ability of the soil to hold nutrients for plant absorption. Two similar charges (+ and +) will repel each other, while two opposite charges (+ and -) will attract each other. This type of attraction between cations and anions allows for organic matter or soil to hold nutrients until they can be used by the plant.

Since soils are negatively charged and plant nutrients can be both positively and negatively charged, cations will be more attracted to the soil, or more available since they are more bound to the soil by way of opposite charges. Anions are more easily moved through the soil and can easily be drained out by water from excess rains or irrigation.

In the chart below, we review what nutrients are considered cations or anions and their mobility in the soil.

Most mobile elements are anions, and the somewhat mobile and immobile elements mostly hold a positive charge. Application of negatively charged elements followed by excessive watering will allow for those elements to quickly move through the soil.

Even though Phosphorus is an anion, this element is not water-soluble, and therefore this element can be easily leached from soils.

Even though nutrients can have positive or negative charges, the CEC of a soil has a major effect on fertilization and nutrient absorption. Cation exchange capacity and nutrient availability are directly related. A soil that has a lower CEC will not be able to hold as many plant nutrients. This can result in the loss or leaching of nutrients. A low CEC soil will also be more susceptible to nutrient deficiencies.

Soil pH is important for CEC because as pH increases (becomes less acidic), the number of negative charges increases, thereby increasing CEC. To figure out your cation exchange capacity, a lab conducted soil test is needed. The cation exchange capacity of soil is difficult to modify but can be improved over time by adding organic matter. Organic matter can include peat moss, topsoil (purchased from a garden center), compost, and manure. But, because the cation exchange capacity is inherently dependent on the soil’s texture/structure, it is difficult to alter significantly without adding other materials to the soil.

Certain types of soils have higher or lower cation exchange capacity. Generally, sands have the lowest cation exchange capacity, and organic soils will contain the highest number of cations. The color of soil can indicate fertility potential, with darker soils generally having a higher CEC, but this is not always the case and also depends on the soil texture.

There are several disadvantages to having a soil with a low cation exchange capacity, which are:

• Increased nutrient deficiencies
• Susceptible to pH extremes
• Decreased crop yields
• Harder to establish turfgrass/plants
• Requires more frequent fertilizer applications

There are several advantages to having a high cation exchange capacity soil:

• Increased nutrient availability
• Greater retention of nutrients
• Reduced nutrient loss by leaching
• Soil pH less likely to change
• Requires less frequent fertilizer applications

A high cation exchange capacity is desired in soil because the soil will be able to hold cations from fertilizers for some time before being absorbed by the plant. Having a high CEC also aids in the buffering of pH, which helps keep the pH from changing. Having a high CEC soil aids in nutrient leaching into groundwaters, therefore helping to protect the water quality. This nutrient-holding capacity will also help growers avoid a severe nutrient deficiency. 

Cation exchange capacity is important for gardeners and agriculturalists to understand to effectively formulate a nutrient program for turfgrass or crops while also maintaining responsible application of nutrients to minimize environmental impact and maximize crop success. Without an effective nutrient program for plants, yields will diminish and nutrient deficiency will prevail in soils and show in crops. Overapplying nutrients can happen and it can negatively affect plants and soil. If your lawn or garden is struggling, look to get a soil test to determine what is going on before taking action. Take note of your soil type, and add organic matter regularly.