The phrase "also the primary source" is correct and usable in written English.
It can be used when referring to an additional source of information that is considered the main or most important one in a given context.
Example: "In addition to the articles, the interview with the author is also the primary source for understanding the book's themes."
Alternatives: "additionally the main source" or "furthermore the key source".
Also, the primary source of immune-modulating melatonin has yet to be determined in birds.
Cole was also the primary source for Stephen Davis's 1985 biography "Hammer of the Gods," a book that solidified the posthumous image of Led Zeppelin as heathen reprobates.
One of the most popular apps is Google Search, which is also the primary source of Google's advertising revenue and therefore is vital to Google's business model.
Family remittances have become the largest income source for thousands of households, and also the primary source of foreign exchange (remittances are equal to about 80% of exports of goods and services).
The chief flaw in the gold standard is also the primary source of its support among gold bugs: It dramatically curtails the government's ability to print money to alleviate economic trouble.
Plant cell walls are also the primary source of cellulose, the most abundant and useful biopolymer on the planet.
With a small but highly competent professional staff, the IEA has also become the primary source for the world's energy statistics and is playing a key role in the climate debate.
Recent study is exposing the soil microbial community as not only decomposing SOM, but also providing the primary source of chemicals for its formation.
Models by Julio Ángel Fernández suggest that the scattered disc, which is the main source for periodic comets in the Solar System, might also be the primary source for Oort cloud objects.
Thus, the results indicate that shallow groundwater discharge also was the primary source of FMeHg within the reach under these study conditions, accounting for 80% or more of the observed increase in FMeHg flux.
They provide a reflection of cellular redox activity which is principally a product of cellular energy metabolism, which also provides the primary source of reactive oxygen species which are thought to be important in the pathogenesis of diabetic complications [ 22].
