Do volcanos really create fertile soil?
As always: It Depends.

Assuming enough water and sunshine, crop growth rate boils down to the concept of limiting nutrients. These may be: nitrogen (via ammonia or nitrates), phosphorous (via phosphates), potassium, and sometimes others.  In typical continental settings (i.e. subduction related vulcanism), lavas may be enriched in potassium and phosphorous but not nitrogen. So there will be some effect from rock composition.

Perhaps more important, there will be a refresh effect. If we have regular ash falls, this will 'top up' the soil as compared to normal soils, replacing nutrients. This is a similar effect to flood plains. 

So, to answer, it tends to be true that volcanic eruptions increase fertility, but the most effective kind are from explosive ash-fall vucanism you get in regions like Italy or the US Cascades. The kind of flood-plain basalts you get in Iceland probably don't help.