Soils support plant growth through ion exchange

Cation exchange = process that allows plants to gain nutrients
Negatively charged soils hold cations of calcium, magnesium, and potassium
Cation exchange capacity = a soil’s ability to hold cations, preventing them from leaching, thereby increasing their availability to plants
A useful measure of soil fertility
Greatest in fine soils

Many pollutants are positively charged, notably heavy metals such as cadmium, lead, and mercury
These cations are attracted to negatively charged clay and humus particles in soil
Make it difficult to remediate soil
Also means they are held in soil instead of being released into aquatic ecosystems