Ion Exchange
Ion exchange is when soil grabs and swaps tiny charged bits that plants need to grow.
Think of it this way
A spice rack holds jars in fixed spots. Each jar can swap one spice for another, but only one at a time. Soil works the same way. It holds charged bits in fixed spots and trades them for bits that plant roots offer up.
Explaining ion exchange by grade level
Dirt holds on to bits of plant food, like a hand holds a ball. When a plant root comes close, the dirt trades one bit for a new one. This swap feeds the plant. Clay dirt is best at this, since it can hold more bits.