Why in the world do plants need vitamins ? The truth is that plants don't need vitamins because they are the organisms which produce them. Consuming vitamins is vital to us - humans - because we cannot make them within our bodies. However, the vitamins we eat come naturally from plants or other organisms who have eaten plants as well. So do plants need vitamins ? No, they don't. The question now would be : do plants benefit from the addition of vitamins to their roots and/or leaves ?
The fact is that there is simply absolutely NO scientific evidence published in a peer reviewed journal that points this out to be the case. I carried out an extensive search for any scientific literature that evaluated the effect of vitamins on plants and I came up with nothing. No one has studied this and no one has ever claimed that there is any benefit whatsoever gained from adding vitamins to nutrient solutions. Why do sellers do this then ? The fact seems to be that people buy solutions that have vitamins more, just because we have been educated to believe that vitamins are good. So if they are good for us, then maybe for our plants too.
The truth is that whatever benefit is gained from adding vitamins is not documented or accurately studied by an unbiased third party so up until now there is no scientific evidence to proof that vitamins do anything more than add to the price tag of whatever hydroponic nutrient you are getting. The fact is that as many other substances, vitamins may just be used - either by microorganisms within the solution that cannot produce them - or they may simply decompose as oxygen reacts with them. So next time you are going to buy your nutrients you should ask yourself... Is there really any benefit to this additive ?