The acidity and nature of the ash (and leachates derived from the ash) varies between volcanoes and eruptions. Ashfalls can lead to elevated soil sulphur levels and lowered soil pH. These changes in soil composition can reduce the availability of phosphate and other essential minerals and alter the soil's characteristics to such an extent that arable crops and pasture plants will not survive. Where there is acid rain following an eruption, pastures will be scorched and will die.
Ash interaction with soil will have variable effects on pH, soil nutrients, capacity for cation exchange and micro-organism activity dependant upon the ash composition and leachate composition. To date little research has been published discussing these dynamics in detail.