Fertilizers, whether they are composed of materials that are natural or synthetic, are materials that when applied to soil or used in hydroponics, supply one or more nutrients essential to the growth of plants. What you buy is contingent on what you are growing. For example, if you are growing leafy-type varieties where your intent is to eat the leaves, versus the root or fruit, you may want to consider a fertilizer that is higher in Nitrogen (N) compared to that of Phosphorous (P) or Potassium (K).
Have you compared the NPK labels of granular versus liquid fertilizers?
Generally, you will find that the N number is higher in granular products than their liquid counterparts. The numbers on the granular label indicate the percentage or weight per 100 pounds of material. So, a 10-30-10 NPK means there are ten pounds each of Nitrogen and Potassium and thirty pounds of Phosphorous.
Compared with liquid fertilizers you could see a set of NPK numbers of 7-21-7. Or, perhaps a more common example would be the so-called balanced granular fertilizers, such as 10-10-10 or 20-20-20. So, which should you buy?
In short, it doesn’t matter. When deciding on whether to buy a granular or liquid product, you are not comparing weights or percentages per 100 pounds, but the ratio of nutrients. In my examples above, 10-30-10 and 7-21-7 have the same ratio: 1-3-1. Likewise, 10-10-10 has the same ratio as 20-20-20, that is 1-1-1. Look for the ratios to make your decision. And as long as the relative amount of nutrients are the same you should do fine.
Just remember it might be useful to read and follow the directions before you add the product to your hydroponic system.
Rather than compare granular to liquid, compare granular to granular and liquid to liquid. If one liquid fertilizer has an N of 5 and other 7, select the product with the higher N if your goal is to grow and eat the leaves-the vegetative portion of the plant. For example, you may want a 7-4-10 product for vegetative use and a 4-8-7 for fruit or flower production. And remember you can use hydroponic fertilizers, granular or liquid, with hydroponics and with your soil-based plants.