It is fairly fertile in that the decay process adds nutrients to the soil that plants love to soak up. Gravel is made of, well, gravel. Particle size is anything larger than 2.0mm. Sand is the stuff you see at most beaches. In terms of texture, it is made of large particles, 0.02-2.0mm in size.
Water drains through sand very quickly.
Silt has particles that measure from 0.004-0.006mm. The grains in silt look like tiny pieces of rock. Silt will generally float on the top of a layer of water and will take time to settle out of the mixture. Clay is the stuff that pottery is made of. In terms of texture, it is made of particles smaller than sand, less than 0.002mm in size.