Update March 26th
Weekly Shonen Sunday has announced the details of Arakawa's new manga.

Scan of WSS
Scan2

The title is "Gin no Saji ~Silver Spoon~" and it's about a school life at an agricultural high school in Hokkaido.

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According to Weekly Shonen Sunday magazine, Arakawa Hiromu will start a new manga series from the volume 19 to be released on April 6th. Details of the manga will be announced later.

Sunday's other new line-ups
Mitsuda Takuya's new boxing manga was also announced to start on March 16th.

New pokemon manga "Pocket Monster RéBURST" will start on March 9th.

ref. Comparison of the top 3 Weekly Shonen Manga Magazines: The number of copies printed (2008 - 2010)

Sources: Scan of WSS, JMPA