Is the term "fertile ash" a misnomer?
'Fertile ash' is not a scientific term, but neither is it a misnomer. Unless the ash is deposited really hot - hot enough for the particles to weld together as 'ignimbrite', then the ash deposit has high porosity and permeability. As soon as water infiltrates, the anhydrous minerals start weathering to clay - generally smectite, or mixed layer clays. This released aqueous magnesium, silica, and low concentrations of other elements crucuial to plants. Thus, both the texture and chemistry of volcanic ash is conducive to plant growth.

When the volcanic island of Surtsey emerged from the Atlantic, just south of Iceland, in the 1970s, plants started to establish themselves in the volcanic ash within a matter of months.