Yes that is correct - best advice always from EvElmand
May give one more advice: If the ground is hard (as it is now when it is more or less dryed out) you may use a "pælebor" = drill into the soil.
http://www.xl-byg.dk/produkter/haven/haveredskaber/paelebor/
These are cheap and you may get in in a size just a little bigger than your posts.
Then fill in some sand afterwards, and wash it down with the waterhose. It will then fill the hole completely and strong.
Another easy solution is to buy "tentorstål" - Lengths so, that you may hammer them 60 cm into the ground.
http://www.bygmax.dk/8-mm-kamstal-laengde-6-m.html
Cut with angle-grinder.
In corners (Where you need diagonals to stiffen the corner) you may connect with "rørbøjler" http://lehaka.dk/u-boejler-varmgalvaniserede/
Tentorstål is not an beauty-solution, but it may be fast and you get a strong and flexible fence.
Greetings A.