If you began your seedlings in a seed starter kit or small starter cells like speedling trays, it is important to recognize that these small soil cells have little or no soil to derive nutrients from. Fertilizing them regularly will keep food available to them. Seed starter book recommendations: seed to seed
Now that your tomatoes & peppers are up they need to be fed. Fertilize them at least twice a week with a kelp-based product, such as Maxi-crop, Algamin Kelp, or Bio Link 3/3/3. Follow the directions on these products for seedling fertilization. It may be important to know that the Algamin Kelp and Omega are having some organic certification problems. If you want to be certified organic you might want to check in to this.
These seed starter kits are designed for getting the seeds up quickly and planting them into the garden or transplanting into bigger pots. Once your plants have formed a few leaves and the roots have filled the cell it is recommended that you transplant your starts into a larger 3 or 4” pot, using a good potting soil mix. A key to healthy plants is to provide an environment that allows continued growth. If plants get root-bound or run out of nutrients and are not replanted soon this can create stress that may continue after being planted in the soil. Symptoms would be slow growth, smaller plants, less vigor, small and/or less fruits, early die off. Continue to feed your plants while in pots and then plant them into well-amended soil.
To determine you soil nutrient needs you can have a soil test done. I find these tests to be much more accurate and informative then the home testing kits, but the home variety will give you some indication of what your soil needs. You can also check with your local Master Gardeners office for advice on local soil needs. The basic components for healthy soil are nitrogen, phosphorus, potassium, calcium, and trace minerals. For further information check out organic fertilizers .
As we feed our soil we feed ourselves.