Why does dirt kill saw blades?
"Dirt", really soil, is made up of components that varies by locale, but almost always contains a significant amount of silica and other silicates, in the form of sand or finely ground quartz. It also contains other hard minerals in addition to softer organic materials.

Silica and silicates are the same materials that are used in abrasives, such as sandpaper and grinding compounds. 

Running a sharp metal blade through abrasives dulls them.