Dry And Liquid Fertilizers
When to use which
A good rule of thumb for most plants, is to mix a balanced dry fertilizer (e.g., 10-10-10) into the soil under new plants when you put them into the container, and use liquid fertilizer for any subsequent fertilizing.
Most liquid or water-soluble fertilizers are from chemical sources, but some organic types are made from fish by-products and seaweed. Liquid fertilizers dissolve quickly in warm water and are used for foliar feeding and drenching the soil around the plants.
A good strategy for fertilizing is to buy a 20-20-20 combination (20% nitrogen, 20% phosphorus, 20% potassium) and use it at half strength (10-10-10) during the early part of the season when the plant is growing roots, leaves, and stems. At mid-season, you can switch to a 15-30-15 or even to a 5-10-10, with more phosphorous and less nitrogen to encourage flower and fruit production.
Much depends on the type of plants being grown, and the needs of each particular plant. A good full-service nursery or your nearby botanical garden employees can advise you on what's best for your particular needs.