The body was discovered in Glindow in the state of Brandenburg, south-west of Berlin, police confirmed.
The Bild newspaper reported the baby was found shortly after midday when the owner of the land called the police.
State prosecutor Christoph Lange said the body would now be investigated. It is not known how the baby died or how long it had been in the compost or what its gender is.
Bild reported that police had arrested the baby's mother on Tuesday afternoon outside a supermarket.
A baby was also found dead in Brandenburg last February in a plastic bag on a plot of land.
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