Strange material/marks on garden shed shelf

These odd looking fragments are on the middle shelf of the shed and the cardboard box in front of it. (Sorry, the photo appear sideways for some reason).  I put the cardboard box of glass jars there in January so it's appeared since then.

On the shelf above it  - close to the roof - was an unopened box of grass fertilizer. The box looks damp and has crusted over with white speckles. All the other cardboard boxes are fine. There's no stain where the box was resting though.

Any ideas what I'm looking at - lawn feed art?