The soil’s ability to hold and release plant nutrients is key. One big factor in this is the Cation Exchange Capacity, or CEC, a measure of how many cations or positively charged ions, or cations, can be retained on soil particle surfaces. These cations can be swapped out, or exchanged, freely. The best hosts for cations are clay particles and Soil Organic Matter (SOM). Clay particles typically have a negative charge, so they attract and hold positively charged nutrients. SOM has patches of both positive and negative charges so it can hold both cations and anions (negatively charged particles).
Calcium, magnesium, potassium and sodium are all key nutrients that form cations when they ionize in water. The calcium-to-magnesium ratio, in particular, has a big impact on soil structure. According to Dr. William Albrecht, the ideal soil structure was composed of 65% calcium, 15% magnesium, 4% potassium and 1-5% sodium.
So the CEC is a measure the number of nutrients that soil can hold. It influences soil structure stability, nutrient availability, soil pH and the soil’s reaction to fertilizers and other ameliorants.
A low CEC can accelerate the deterioration of your soil’s pH, as it becomes more vulnerable to things like nitrogen fertilizer and nitrate leaching. Soils with low CEC also suffer from a deficiency in important nutrients like potassium and magnesium, which quickly pass through the soil rather than attaching to negatively charged sites.
Having greater levels of SOM allows nutrients to harbor in the soil, as the higher CEC from the SOM makes the soil is more resistant to leaching and able to retain essential nutrients. Clearly, inputs can be reduced with greater SOM levels because the nutrients aren’t “just passing through.” And as a bonus, SOM also gives the soil higher water-holding capacity.
On the other side of the ion coin, soil doesn’t retain anions like nitrate or sulfate at significant levels, which leaves phosphate as the primary anion of importance within your soil. Anion exchange is important in the process of releasing fixed phosphate within the soil to make it available to plants. What holds phosphate in the soil is the SOM!
So, having greater SOM helps get your CEC where it needs to be to hold and make available cation nutrients, plus it aids in the anion exchange vital to making phosphate available. This gives growers a great buffer or insurance policy when it comes to crops having access to the right nutrients when needed.
