According to "Vampires, Burial and Death: Folklore and Reality" by Paul Barber, intact human bodies take a surprisingly long time to decompose, even if not subjected to modern embalming techniques.

Since this culture would essentially be using their deceased as an important component in compost (presumably not having large numbers of domesticated animals to provide manure as fertilizer), it would be much more efficient to grind the bodies up and mix them thoroughly with larger amounts of vegetable matter. All the composting advice I've been reading lately says that component particles should be no larger than a quarter-inch in their largest dimension, in order to best encourage breakdown.