Float trays are made from expanded polystyrene beads under high temperature and pressure.  As trays age 
these beads tend to want to go back to their original shape.  The picture below shows a new tray at the 
bottom left, a one year old tray at the top (showing holes left by root penetration) and a poor quality tray 
at the bottom right.  If roots penetrate the tray and then become diseased by pythium soft rot,  it will be 
difficult to sterilize inside those tiny crevices left by the roots.