An ion or group of ions having a positive charge such as the sodium ion in salt (Na⁺Cl⁻). Cation exchange is the interchange between a positively charged cation in the soil water and a negatively charged ion on a clay surface. The most important ions held in this way are the four important plant nutrients sodium, potassium, calcium, and magnesium. Cation exchange capacity (CEC) is the overall net negative charge of clay minerals per unit mass of soil, usually expressed as milliequivalents (meq) per kg of oven-dried soil.