Cations of calcium, magnesium, potassium and sodium are referred to as alkaline cations. Their percentage of the exchange capacity is called base saturation. If the proportion of alkaline cations is higher than 70 or lower than 30 percent this is referred to as soils with high or low base saturation. If the soil is supplied with ions from outside (e.g. with acids or fertilizer salts), a part of the adsorbed ions are exchanged.