"Plant food" or "Plant feeder" are one of a multitude of marketing gimmicks under which psychoactive research chemicals are frequently sold. Despite the way such products are labelled, these chemicals have absolutely no legitimate value for the proposed purpose.
Psychoactive research chemicals occupy a legal grey area in most jurisdictions. Although often not explicitly controlled substances, distribution with intent for human consumption may violate various laws and statutes. In the United States for example, distribution of many research chemicals would contravene the Federal Analog Act if there is demonstrable intent for human consumption. In other nations laws regarding public safety and pharmaceutical licensing may be breached if unregulated and unapproved chemicals are sold for consumption.
In an attempt to minimize the risk of legal action research chemicals are therefore typically labelled as 'not for human consumption'. Some distributors take this a step further by proposing spurious applications for which these chemicals might be used. This extends the facade that a distributor is unaware that a product may be intended for human consumption.
The most common examples of such gimmicks are "plant food" and "bath salts", both of which have been typically used to market beta-ketone stimulant research chemicals such as mephedrone and MDPV (though there is no reason that other chemicals with a range of effects could not be sold under a similar gimmick). Despite these marketing claims there is no supportive evidence that the products are fit for the proposed purpose. Beta-ketones are not a nutritionally balanced supplement to support plant growth.