How often you fertilize depends on the age of your seedlings and their current growing medium/container.
Once you apply your first round of fertilizer, you could apply the diluted fertilizer weekly for plants in small starter trays, or twice a month for those in larger starter pots or cups (such as 16 oz plastic cups or 3″ pots.)
Be sure to go easy on the nitrogen, and withhold nitrogen rich fertilizer if plants are becoming too spindly or leggy (as long as the legginess is not caused by poor lighting!)
Can Too Much Nitrogen Hurt Plants? How Much Is Too Much?
Too much nitrogen can cause plants to grow foliage and branches far too quickly.
Lots of growth is good, but if a plant grows too quickly, it causes the plant to become very weak.
A weak plant will collapse easily under fruit load, during storms, or when under attack by pests or disease.
In addition, you may also notice that you’ll have fewer blooms and less fruit on average; an excess of nitrogen directs the plant’s energy into the foliage rather than the fruit.
If this occurs, cease fertilizing immediately; in a month or so, you may return to using a fertilizer with lower levels of nitrogen.
Proceed with Caution: Tips for Using Fertilizer Properly
If you use fertilizer incorrectly, you could seriously harm your plants.
There are different formulations of fertilizer and different strengths that are used for varying stages of growth among garden plants.
Here are some tips for avoiding fertilizer damage:
Only use a fertilizer that is beneficial for tomatoes. For example, you don’t want to use a nitrogen rich fertilizer that is designed for grass or bushes; as we’ve discussed, too much nitrogen is a bad thing.
In addition, don’t use “weed & feed” fertilizers. These will kill your plants, as they contain an herbicide. I strongly advise that those who grow food plants never purchase or use those products at all; it can contaminate your garden easily, via rain, wind, and wildlife.
Never add a “little extra” because a plant is behind or struggling. Only fertilize with the amount that is directed on the package. Otherwise, you may burn your plants, give them too much nitrogen, or cause fertilizer salts to build up in your soil faster than they can rinse out.
Always space out fertilizer applications; if you fertilize too often, you run into the problems above.
Opt for organic or natural fertilizers when available! Some natural sources of ready to use fertilizer are:
Fish Scraps (try planting offal under plants after a day of fishing!)