(See : How To Make Compost Tea) Fertilizing Tomatoes When Planting But giving them a weak dose when you bring them home is a great idea. It can also be provided when watering seedlings with a weak mixture of compost, worm casting, or liquid fertilizer tea weekly when seeds mature beyond the 4 week mark.įor those that purchase transplants from their local greenhouse or garden center, chances are the plants have been fertilized a bit already. This can be done easily by adding worm castings or slow-release organic fertilizer to the seed-starting mix when planting. A bit of added nutrients placed in the seed starting soil can help tomato plants get off to a great start.