Since I use Coco Coir made from coconut husk for my seed starting medium, I need to use some plant food once I transplant the seedling to a larger pot as this medium doesn’t have the necessary nutrients. I usually use a balanced and organic plant food. For balanced plant food, I look for plant food that has the same number for Nitrogen (N), Phosphorus (P) and Potassium (K) such as 2-2-2.
Nitrogen is great for the green color of the leaves.
Phosphorus is wonderful for the roots and for flower and fruit development.
Potassium is a must for overall plant growth.
For the plant food, you can use a liquid, granular or slow release option. I like to use the liquid or granular option in the grow room because of faster results.
Instead of the balanced formula, some folks pick a plant food that has more phosphorus to focus on the roots development for seedlings. I don’t think you can go wrong with either formula.