Making Fertilizer Decisions

According to Wikpedia, the definition of Cation Exchange Capacity or CEC is the maximum quantity of total cations, of any class, that a soil is capable of holding, at a given pH value, for exchanging with the soil solution.

I once read that CEC of your soil is like a bucket. Some soils are large buckets and some are small buckets. The higher the CEC the larger the bucket and the greater the nutrient holding capacity. Sandy soils with little organic matter are small buckets and clay soils with a lot of organic matter are large buckets. Soil organic matter can retain cationc nutrients by cation exchange and chelation, and can be a source of nutrients when decomposed.

The CEC of your soil directly affects the amount of fertilizer you should use and the frequency with which you should apply. Based on your soil type and the percent of organic matter in the soil and the relative strength of positively and negatively charged nutrients in your soil, you can devise a sound soil treatment plan that accomplishes the most with the least.

Cations have a positive charge and Anions have a negative charge. Here are some soil nutrients in the group they belong:

                        Cation                                     Anion

                        Hydrogen (H +)                      Phosphate (H2PO4 -)

                        Ammonium (NH4 +)              Nitrate (NO3 -)  

                        Sodium (Na +)                        Chloride (Cl -)

                        Potassium (K +)                      Sulfate (SO4 - -)

                        Calcium (Ca ++)                     Boron (H3BO3 -)

                        Magnesium (Mg ++)               Bicarbonate (HCO3 -)

                        Copper (Cu ++)

                        Iron (Fe +++)

                        Aluminum (Al +++)

Along with soil texture and organic matter, pH also has an effect on CEC. And of three factors, usually only pH can be changed. Soil pH changes the CEC because the soil has exchange sites that become active as the pH increases. Soil CEC could be expected to increase up to 50% if the pH was changed from 4.0 to 6.5 and nearly double if the pH increased from 4.0 to 8.0.

Think of it like this, your soil works like a giant magnet, attracting cations, and repelling anions- positively and negatively charged nutrients. Knowledge of this process can be used to develop appropriate soil treatment and fertilization plans for your specific type of soil.

You can estimate your soil CEC using a basic formula based on your soil type and percentage of organic matter or you can have a qualified representative from SFG help you by pulling grid sampling and getting a more accurate representation of your soil.

Soils with low CEC (small buckets) need more frequent but smaller applications of fertilizer. Soils with high CEC (large buckets) can go for longer periods of time before needing fertilizer but need larger quantities to refill their exchange sites.

Contact one of your qualified SFG agronomists and have them help you develop a lime and fertilizer plan for your specific soil type.