When looking it up I saw (I don't think it was on this site) a huge thread war over the topic so I thought maybe someone would have given it a tryNot sure why you need a side by side study of lemon juice as a ph down. it doesn't hurt the plant and it works, so why bother?
PS, besides being organic, an added benefit to using citric acid or lemon juice is that it levels out higher than it initially reads. According to Soma, since different nutes have different ph levels for optimum absorption that range from 6.2 to 6.6, using citric acid allows you to drop the ph to 6, then as ph raises over the next 8 hours the plant gets some time with ph at optimum levels for nutrient absorption of various nutrients.If you're growing in soil and want to grow organic, lemon juice or citric acid powder both work well for adjusting water ph. In my case, we're on a well and my tap water is on the soft side (50 ppm) with a ph in the high 6's. Once I add calmag to up the ppm of dissolved solids to an ideal range (around 150-200 ppm) it's up to around 7. 1 teaspoon of lemon juice per quart of water takes it down to around 6 for me, but different waters can behave differently depending on the types of dissolved solids so be sure to use a ph test kit to ensure you've got your water where you want it. Also, be sure to add the lemon juice or citric acid powder last if you're adding liquid nutes like nitrogen or calmag to your water since some nutes can change the ph of the solution. I've never grown hydro so I can't comment on it's effectiveness in that medium.