Conventionally-grown vegetables do tend to have higher nitrate content than their organically-grown equivalents. However, there are numerous other factors that play a role as well. I would therefore have said "partially" instead of "largely".

ETA: We did in fact analyze our data in light of this issue, but the results were skewed by the uneven distribution of conventional/organic products across the different categories (i.e., powders, mixed drinks, concentrates, bulk juices).