Positively charged ions in plant nutrition are called cations: Calcium: Ca+2 Magnesium: Mg+2 Potassium: K+ Sodium: Na+ Hydrogen: H+ Aluminum: Al3+ Iron: Fe2+ Manganese: Mn2+ Zinc: Zn2+ Copper: Cu2+ Cations are attracted to negatively charged soil particles, called the clay-humus-complex, because opposite charges attract. The soil's ability to hold onto cations is called its cation exchange capacity (CEC). Soils with a higher CEC can hold onto more nutrients. The CEC of a soil depends on its clay and organic matter content. Sandy soils have a low CEC because they have larger particles and pore spaces, and a lower capacity to retain nutrients. Clay soils have a higher CEC because they have smaller particles and pore spaces, and clay particles have a higher surface area Negatively charged nutrients in plant nutrition are called anions. Some common anions include: Phosphate (PO43-), Sulfate (SO42-), Nitrate (NO3-), and Chlorine (Cl-). Anions are the opposite of cations, which are positively charged nutrients. The relationship between anions and cations in plants can cause significant changes in pH levels in the growing system. When plants absorb more anions than cations, the pH increases, and when they absorb more cations than anions, the pH decreases. Here are some other things to know about anions: Anions are held and retained by soil particles, but to a lesser extent than cations. Anion adsorption is dependent on pH and increases as soil pH decreases. Phosphates and sulfates are adsorbed more strongly than nitrates and chlorides. Anions are more likely to be lost from the soil through leaching, with the exception of phosphate and sulfate to a lesser degree. Over-application of a negatively charged element followed by excessive water will quickly move that element through the system Among the micronutrients, Fe, Mn, Cu, Zn, and Ni are taken up by plants in their cationic forms, and B, Mo, and Cl are taken up by plants in their anionic forms.