What effect does vitamin C have on plant growth?

Well yes, a vitamin can help a plant grow. For an explanation go to: please look at the related link below.

AnswerThe experiment referred to above uses a mere four potted plants - two having been given no vitamins, the other two each given a separate mix of vitamins. Neither the vitamin blends nor the amounts given were stated. I realize that this experiment was carried out by children (ages unknown, but definitely children), but I must be critical and simply conclude that the two variables - plants given vitamins versus plants not given vitamins - were not properly controlled.

Plants naturally produce the vitamins necessary for their survival; they do not, however, do the same in regards to minerals. This is the reason that in areas where the soil has become depleted of it's mineral content (which is, unfortunately, the case when land is over-farmed, e.g. "factory farms") synthetic, petroleum-based mineral fertilizer is used.