Is the term "fertile ash" a misnomer? I understand that volcanic ash contains minerals and can make soil more fertile.

But I've also heard the term "fertile ash" in some documentary (I have forgotten in what context). Is that a scientific term, or a  misnomer?  

The Oxford online dictionary contains the following definition of "fertile":

http://www.oxforddictionaries.com/definition/english/fertile


  1 (Of soil or land) producing or capable of producing abundant
  vegetation or crops: the fertile coastal plain
  
  1.1 Producing many new and inventive ideas: her fertile imagination
  
  1.2 (Of a situation) encouraging a particular activity or feeling: conditions at the time provided fertile ground for revolutionary
  movements
  
  2 (Of a person, animal, or plant) able to conceive young or produce
  seed.
  
  2.1 (Of a seed or egg) capable of becoming a new individual.
  
  2.2 Physics (Of nuclear material) able to become fissile by the capture of neutrons.


By that definition, ash alone cannot be described as fertile.

Perhaps a philosophical point, but if it's not valid according to this dictionary, and it's not a scientific term either, then I suppose it's more a question of correct English, so maybe I should reword my title to ask if it's a scientific term.
'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.