Who can initiate an optional referendum at the national level?

Zambia
Referendum Act (Chapter 14 of the Laws ofZambia)
(Enacted in 1969, last amended in 1994)
http://aceproject.org/ero-en/regions/africa/ZM/Referendum%20Act%20Cap14.pdf/view
Part 1, Article 2(1).
The President may, if in his opinion it is necessary or desirable so todo, by *statutory order, direct that a referendum be held on any question orquestions specified in the order.