Fertilizer, soil pH and Cation Exchange Capacity

By Dave Nanda, 
Director of Genetics and Technology 
for Seed Consultants, Inc.

Soil pH, and Cation Exchange Capacity (CEC) — how are they related and do they affect fertilizer inputs? Some of the facts below should clarify their relationship.

• Soils are made up of sand, silt, clay and organic matter. The CEC of a soil tells us about the texture of the soil. Soils with higher clay and organic matter content have higher CEC values. The CEC value of the soil in a field is fairly constant but can be changed over time with the addition of organic matter, through the use of cover crops and manure, for example.

• Positively charged particles are called cations and negatively charged particles are called anions. The CEC is the measure of how many negatively charged sites are available in the soil.

• According to David Mengel of Purdue University, most common soil cations are calcium, magnesium, potassium, ammonium, hydrogen and sodium. Common anions are chlorine, nitrate, sulfur and phosphate.

• The cations that are held on the clay or organic particles can be replaced by other cations and therefore they are called exchangeable. For example, potassium can be replaced by cations such as calcium or hydrogen.

• The total number of cations a soil can hold is the soil’s CEC. Higher CEC value of a soil indicates higher negative charge and the greater capacity of that soil to hold more cations.

• The relative proportion of acidic and alkaline or basic ions on the exchange sites determines a soil’s pH value.

• High CEC soils generally do not need to be limed as frequently as low CEC soils. CEC can also affect the frequency of nitrogen and potassium fertilizer applications.

• Sandy soils with lower pH are more subject to leaching of nutrients.