Healthy Soil - You Must First Measure It Before You Can Manage It. (Part Three)

Cation Exchange Capacity (CEC):

The Cation Exchange Capacity (CEC) of your soil is essential. If your soil analysis doesn’t calculate the CEC, you cannot properly adjust the cation balance of your soil. In addition, until the cations are in proper proportions, the soil pH will never be corrected. 

In simple terms, the CEC is the texture of your soil. It is composed of different proportions of sand, silt, clay and humus. More than that, it is the calculation of your soil’s ability to hold nutrients for plant use; this is called Cation Capacity. With this information, you can make sound fertilizing and amending decisions for the soil and the crops grown in it. This measurement is a key guideline as to what the proper levels of calcium, magnesium, potassium, sodium, etc. should be for ideal soil cation balance. This is critical, as cation proportions directly affect the soil pH. Using the soil CEC as a guide, will reduce the risk of fertilizer applications that are more than what the soil has capacity to manage.

Opposites attract: Before going further, there is some important background information we need to cover. All the interactions between the soil, plants and nutrients are electrical in nature. The electrical charges are positive and negative, with opposite charges attracting and like charges repelling. Positively charged (+) nutrient ions are called “Cations” and negatively charged (-) ions are “Anions.” Cations and anions can have multiple positive and negative charges respectively. The more charges on each ion, the stronger it’s electrical reactivity to soil particles and other ions. Some cation examples are calcium (Ca2+), magnesium (Mg2+), potassium (K+) and some anion examples include nitrate (NO3-) and sulfate (SO42-). Soil particles have a negative charge, while humus can have negative or positive charges. In the last SoilMatters, we discussed the Soil Base Saturation Cation Percentage. Combining the Base Saturation Percentage information with the calculation of your Soil Cation Exchange Capacity gives you the information needed to adjust your Soil Cation Balance to ideal proportions for optimum performance. Begin this process with a current soil analysis and the guidance of a certified crop advisor who is experienced with Soil Cation balancing. I mention this because not all advisors are familiar with this method.

Think of the electrical charges between soil and cations in terms of a cow magnet (Figure 1). The magnet has negative charge sites just like soil particles. When the magnet is pulled through soil, it attracts positively charged metallic pieces and takes on a “hairy” appearance. This magnetic-attraction continues until all the negatively charged sites are satisfied and equilibrium is achieved. The metallic bonds simulate how cations attach to a soil particle. A soil particle is said to be “saturated” when all the negative sites are connected to cations.

Figure 1

The metallic bonds are like static-electrical charges of varying strengths. The stronger the positive charge of each cation, the stronger the bond to the soil particle. When a cation has two positive charges, it is divalent. A divalent cation has a stronger affinity to soil particles than monovalent cations (mono = single). Calcium and magnesium are divalent cations, whereas potassium is monovalent. One calcium cation can displace two potassium cations and two hydrogen cations can displace one calcium or magnesium cation, because of the positive charges. The other important detail is that the charges always interact until charge equilibrium is reached.

Figure 2

Let’s get Physical: When you consider the physical size of sand, silt and clay particles they are quite different. You can see sand particles without a microscope, but silt and clay you cannot. Think of a golf ball as a soil particle, with each dimple having a negative charge for cation attachment (Figure 2).

If a clay particle is the size of a golf ball (see Figure 3), then a silt particle would range in size between 100 – 100,000 times larger diameter and a sand particle would fall between 100,000 – 10,000,000 times larger than the golf ball (clay particle). The larger the particle size, the fewer particles that will fit in a cubic foot of soil. Fewer particles of soil mean less negative charge sites and less cation holding capacity. The opposite is true with smaller silt and clay particles having much greater surface area and greater number of cation attachment sites. This is why sandy soils need to be fertilized in smaller application rates and more frequently than fine textured soils. They have less nutrient holding capacity. When there is less electrical charge per volume of soil, the soil-cation bonds are weaker and cations are attached more loosely to the soil particles.

In the top 6-7½ inches, on an acre, there are roughly 2,000,000 pounds of soil. This depth of soil is called the aerobic zone. This area is where the majority of hair-root growth and microbial activity happens. Returning to the golf ball example, when you think of how many golf balls will fit into an acre of soil 6 inches deep and how many dimples that represents, in terms of negative attachment sites for cation fixation, it’s significant. Soil particles are much smaller than golf balls, but this represents well  how the soil nutrient inventory or Cation Capacity of the soil works. With all negatively charged sites attached to cations, the challenging part is getting the cations into proper proportions for the soil type and efficient nutrient release and availability for better soil health and plant benefit.

Regular Sampling: By incorporating annual sample-pod soil analysis into your Nutrient Management Plan and applying that information to soil inoculations with beneficial microbiology, you are addressing all three of the major contributing factors for Soil Health; Physics, Chemistry and Microbiology. These three parts are equally important and vital to improving and maintaining soil productivity. 

Figure 3

In my work with growers, I typically see their soil CEC increase over the course of 18-36 months, depending on the conditions. This may sound far-fetched, but by applying organic acids and microbial inoculations, the soil becomes more biologically-friendly which stimulates rapid microbial reproduction. Think about it this way, the soil CEC is a measurement of its nutrient holding ability. The CEC calculation cannot discern between soil fixed nutrients or the nutrients absorbed by beneficial bacteria, fungi or yeast species. The final analysis is a total of all that is measurable in the soil sample. As you increase the microbiology of the soil, you increase the its nutrient holding capacity without changing the composition of the soil. Remember, when a nutrient is absorbed into the cell of a microbe, it is bio-available and can then be easily absorbed into the cell of a plant.

In the final part of this SoilMatters-Healthy Soil series, we will discuss Soil Respiration Rate and its use in accurately measuring microbial population and soil health, in order to better manage its improvement and productivity.

There are many affordable labs that perform these types of analyses. If you are not working with one already, allow me to help you incorporate this technology into your operation and improve your soil health and productivity.

Here’s to your crops’ success!