06/15/2026
Weekly Garden Tip By Isaac Walker
Fertilizers - Part 2: pH and CEC
Last week, we discussed the role of fertilizers in the garden. Now, letโs take a more scientific approach to understanding why we sometimes need fertilizers and how plants can obtain those nutrients.
Start by imagining a particle of soil. This particle carries a charge, and most likely, it will be negative, as are most North American soils. Conveniently, most molecules that are nutritious to plants have a positive charge, and since opposite charges attract, these molecules can form weak bonds with the soil particles. Plants can access those nutrients by swapping positively charged hydrogen atoms for the nutrients bonded to the soil. This is called cation exchange (the term cation refers to any positively charged molecule). The cation exchange capacity (CEC) of a soil is how much potential it has to bond with nutrients and then trade those for H+.
CEC is mainly influenced by a few factors. Firstly, a soil has to contain nutrients in order for cation exchange to take place. A depleted soil will not do well simply due to a lack of nutrients. Additionally, soil density, texture, and makeup (sand vs silt vs clay vs organic matter) can change how many nutrients it can hold.
Lastly, soil pH helps determine how easily nutrients can be taken up by plants. The pH of a soil is its concentration of hydrogen. The higher the concentration, the lower the pH. For most plants, a pH between 6.0 and 7.0 is optimal for cation exchange. With too much hydrogen in the soil, nutrients will not be able to bond, and a higher pH can reduce the availability of micronutrients such as iron (Fe) and manganese (Mn). Maintaining the proper pH can be done through testing your soil and adding necessary amendments.
To sum things up, just like choosing the right fertilizer is important, it is also important to be aware of your soilโs CEC and how you can improve it, if necessary. Some soils will need fertilization to make up for a low CEC, and understanding your soilโs CEC can help you use fertilizer efficiently and effectively.
