Nutrients are made from mineral salts. Salts always contain at least 2 elements. For example kitchen salt is made from Sodium (Na) and Chloride (Cl) in the form of NaCl.
When NaCl dissolves in water the Na and Cl will split.
Na has a positive charge (Na+). Positively charged ions are called cations.
Cl has a negative charge (Cl–). Negatively charged ions are called anions.
The cations and anions in your feed tank can be measured by a EC/ppm meter because of their electric load. EC stands for electronic conductivity. It’s a way of measuring how conductive the water in your feed tank is (how quickly it allows electricity to pass through the water in your feed tank). A high EC means there is a high amount of cations and anions in your feed tank.
If water with Na+ and Cl– in it evaporated, the cation Na+ and anion Cl– would combine again to form NaCl. The Na+ acts like the north pole of a magnet and the Cl– like the south pole of a magnet, they stick together and become a solid salt.
A plant up takes nutrients in preferential order which is called a redox. This means plants like certain elements more than others (luxury consumption).
An overview of a redox. The order is left to right.