This plant was discovered on the property of my workplace. Is this plant poisonous? The soil is more clay like and the area in question gets a lot of sunlight. These berries are still green but one was found and looked more blueish-purple.
— and thank you for sending such a clear, defined photo! Always much appreciated by our ASK team at Toronto Master Gardeners, thank you.
This is a lovely discovery: a wild grape vine. Your instinct to question whether or not the berries are poisonous is very good.
Please read a prior TMG descriptive overview of WILD GRAPES including identification, wild grape varieties vs Vitis vinefera (planted by growers in the wine-making industry), and differentiation from other grape family members, specifically Virginia Creeper, Parthenocissus quinquefolia. There’s even a bonus mention about the possibilities of jelly-making, so, to answer your question: the fruit is not poisonous. As you mention, the immature green fruit will ripen throughout the summer months, and will soon ripen to darker purple berries. Likewise, the tender grape leaves are edible — the most common use, a la Meditereenee, would be steamed grape leaves, stuffed and rolled with rice, meat and spices!