Volcanically derived soils are usually more fertile, but not always.
Parent material is a significant factor when it comes to soil fertility. Soils which are derived from, or are on a basaltic base tend to be more fertile and ideal for certain types of farming, whereas soils which are derived from and/or ontop of a sedimentary base such as sandstone are usually less arable. This has alot to do with drainage and the soils ability to retain moisture, minerals and essential elements such as N and P. Basalt is relatively impermeable in comparison to most sedimentary rocks, so minerals are not leached as easily. However, even the most fertile volcanic soils can be degraded by erosion, leaching, over irrigation, salinity, vegetation removal etc..