Fetching details from the dictionary...
one-tenth hectare
「一段」 has another reading with different meanings.
畑の広さは一段だ。
The field's area is one-tenth hectare.
この土地は一段あります。
This land is one-tenth hectare.
合計で一段の土地が必要だ。
A total of one-tenth hectare of land is needed.