Nitrogen Leaching

• All organic materials have carbon and nitrogen. However, sawdust has a carbon to nitrogen ration of 500 to 1. This is one of the reasons it is so effective as a weed block: it strips the soil of nitrogen and leaves none for plants. Unfortunately, this nitrogen robbing extends to nearby plants along the border of the garden. Unseen, underneath the garden path, roots are reaching out in search of food, and for all the energy it takes to branch out close to or under the path, they are being rewarded with nitrogen-bankrupt soil.
  
  

Crusting

• An advantage of an organic material for garden paths is that water will evenly filter down through the material and won't pool in odd places or create runoff problems on either side of the path. This isn't the case with sawdust, since it has a habit of forming a crust at the top that is impermeable. This means excess storm water doesn't filter evenly, but runs across the surface creating gouges in the path or the garden beds on either side, moving sawdust, mulch and soil away from where it belongs.
  
  

Rapid Decomposition

• Unless a constant source of sawdust is available, it is not a good long-term option for garden paths. It breaks down rapidly, demanding the labor and time of continuously bringing in new sawdust and spreading it on the path.
  
  

Wind Erosion

• Sawdust is light and blows away easily. This means it can get into nearby beds, robbing the plants there of nitrogen, and requiring more work to keep up the path. In dry, windy areas, sawdust is more of a liability than a feasible garden path material.