The following is from Wikipedia; you might find it interesting (might not!). Logs are take-up-lots-of-room bulky and intact masses of wood react much differently in the dirt than do smaller chunks like sawdust.

Hügelkultur

The practice of making raised garden beds filled with rotting wood It is in effect creating a Nurse log, however, covered with dirt. Benefits of hügelkultur garden beds include water retention and warming of soil. Buried wood becomes like a sponge as it decomposes, able to capture water and store it for later use by crops planted on top of the hügelkultur bed.

The buried decomposing wood will also give off heat, as all compost does, for several years. These effects have been used by Sepp Holzer for one to allow fruit trees to survive at otherwise inhospitable temperatures and altitudes