The phrase "also the main source" is correct and usable in written English.
It can be used when referring to an additional primary origin of information or material in a discussion or analysis.
Example: "In addition to the previous studies, this report cites also the main source of the data used in the research."
Alternatives: "additionally the primary source" or "also the primary origin".
Among minerals, sand is extremely valuable and also the main source for building industry all over the world.
It is also the main source for answers to questions asked verbally to the new Google Home, a voice-activated smart speaker that is due to be launched in Britain in June.
The United States was not only the largest weapons supplier last year, but also the main source of weapons to the developing world, accounting for about $14.9 billion of these deals — or 48.6 percent.
Groundwater (springs) is also the main source of irrigation water supply in these important agro-industrial zones.
He was also the main source for Nisman's allegations.
The Energy Department is also the main source of funding for radiation health research -- much like having the tobacco industry determine the safety of smoking.
Moreover, the oceans are a bountiful source of the nation's food.Yet the seas, and what Japan does with them, are also the main sources of friction with the outside world.
Thus, Cairo's LBMs are linked to Al Qalyubiyah and Ash Sharqiyah farms that are also the main sources of fertile eggs and day old birds for southern governorates.
University teaching hospitals were also the main sources of teaching and research output by providing medical education, and conducting clinical research.
National policy documents and all related documents from public and private stakeholders including non-governmental organizations were also the main sources of information.
It also would strengthen bonds with an important ally in a volatile corner of South America — that also is the main source of cocaine shipped into this country and where the United States has very few friends these days.
