Podzol spreads with air/solid block in between

The Bug

Podzol spreads on the grass around the tree even though there is a block of air in between. The same happens when there are blocks placed under the 2×2 dirt.

Steps to Reproduce

• Build the setup as shown in the attachment below. setup.png
• Apply some bone meal to the spruce saplings so that they grow into a tree.
• Take note as to whether or not the podzol generated from the tree, spread over air blocks.

Observed Behavior

Podzol is able to spread over air blocks.

Expected Behavior

Podzol would not be able to spread over air blocks.