Choosing the "food" that your plants get and don't get comes down to the hydroponic nutrients you decide to use.
Hydroponic nutrients come in two main categories, base nutrients, and additives. While the base nutrients provide the main source of vital elements like minerals and vitamins, additives bring in the extras that should be provided in small amounts only as needed.
Most base nutrients can have multiple parts which are often denoted for either the Vegetative (growth) stage or Blooming (flower) stage. The additives typically come in smaller amounts and can be added when your plant is either showing deficiencies in a certain element or if you're trying to arouse certain characteristics out of your plants, such as taste or flower size, or even root growth.
Be sure to match up your growing medium with the appropriate form of hydroponic nutrients, whether it be soil, coco, or growing hydroponically in inert materials like Rockwool or Hydroton clay pellets. Most of the larger brands come in one or all three different flavors to match with your chosen medium. While others are meant to be used universally no matter which method you chose.
Nutrients for use in the soil are typically made to supplement and eventually replace those nutrients that were originally available in the soil mix itself. While those made for coco and hydroponics are all the nutrients your plants will ever receive and should be used all throughout the life of the plants. Many brands come with two parts meant for each phase of growth from vegetative growth to flowering.
Hydroponic nutrients also come in two options, Organic and synthetic. Organic nutrients may contain larger particulate matter that can clog up pumps, tubing, sprayers, and misters, and are therefore more appropriate for use in soil, soilless media, and coco coir, where the nutrient solution may not necessarily be being reused or recirculated. Synthetic nutrients make up the majority of nutrients available and are typically formulated to be used in hydroponics systems but can often also be used in coco coir and soil as well, depending on the brand and formula being used.
All hydroponic nutrients come labeled with the ratio of nitrogen, phosphorous, and potassium (N-P-K), they respectively correspond to the percentage of these materials found in the fertilizer that is readily available to the plant immediately. So synthetic nutrients are typically labeled with higher numbers than organics due to those elements being in a more readily available form versus organics which can take longer to break down.
Additives come in many flavors, covering the entire plant's life from root growth to flower sweetening, each with its own very specific purpose and usage including:
As with any hydroponic nutrients, it becomes very important what your pH is at all times due to plants being able to take up specific nutrients in specific pH ranges only. Which is why for hydroponics you should keep your pH at 5.5 to 6.5 and 6.2 to 7.2 for soil. pH Up and pH down are also available to bring your water & nutrient solutions to the proper pH.
Check out our Nutrient Buyers Guide to help you find the best nutrients for soil, soilless, and hydroponic gardening.