Since your plants aren’t getting nutrients from the soil, you have to add them manually.
Dry nutrients are notably cheaper than their liquid equivalents. There is a great deal of debate about which is more effective. Some hydroponic gardeners swear they see a huge improvement when they switch to liquid while others say you’re only paying for the privilege of having someone else mix the chemicals for you. Everyone agrees liquid nutrients are easier to use. You just pour in the right quantity and walk away. Dry nutrients require carefully mixing, and sometimes it can be difficult to get the solution into a proper suspension in the water instead of having it thin on top and gritty on bottom.
This is another case where price usually makes a difference. The less you spend on dry nutrients, the more time you’re going to need to properly mix them. The higher quality, more expensive dry nutrients are still cheaper than the liquid nutrients and will blend easily. If stockpiling for an emergency, high quality dry is probably a better choice because you can store a larger quantity in a smaller space for a reasonable cost.
That leads you to the major issue of whether you want to purchase organic or synthetic nutrients. Organic nutrients are made from guano, potash, kelp, worm casings, and other byproducts of living things.
Synthetic nutrients are soluble chemical salts created to provide plants with all their nutrient needs.
The only real difference between the two is that organic nutrients are more likely to create subtle changes in the pH of your water. Either one will dissolve into nitrogen, magnesium, and the other chemicals your plants need to be healthy and happy.
If you have the time to set up an experimental hydroponic garden for tomatoes, herbs, or your favorite veggies, experiment with different brands and formulations of nutrients until you find what works best for you and your plants. A lot of hydroponic gardeners swear it’s as much an art as it is a science.
Keep in mind, their goal is to exceed the natural bounty expected by traditional farming, so in most cases, doing badly means only having a yield as high as that which a farmer achieves growing a crop in the soil. There is no question that you’re best off using a commercially purchased nutrient solution. However, in a true emergency situation, you can make your own from 9 oz saltpeter (potassium nitrate), 7 oz Plaster of Paris (calcium sulfate), 6 oz Epsom salts (magnesium sulfate), 4 oz monocalcium sulfate, 1.5 oz ammonium sulfate, and ½ tsp iron sulfate. Wear a mask and gloves when mixing this and keep it sealed in an airtight plastic container. When feeding it to your plants, use 1 tsp per gallon of water.