Giving too much fertilizer or giving too little are both bad news. You have to know how much is needed and be prepared to give an appropriate amount. Here is a simple conversion that you can work out. I am not saying to use miracle grow, this example is to give a common baseline for starting seedlings properly.
1/4 teaspoon of 15-30-15 miracle grow diluted in water is enough to properly fertilize 50 seedlings though the 6 leaf stage. You can easily convert this to any other fertilizer by adjusting the amount in proportion. For example, a 1-3-2 seaweed based product would need roughly 4 teaspoons of the seaweed emulsion for 50 plants. Check your product, the conversion is fairly easy.
Please note that this presumes the seed start mix was unfertilized to start with. If you began with fertilized mix, then you can probably wait until the plants are 4 to 6 inches tall before giving anything extra. I say "probably" because different mixes have varying amounts of fertilizers included. You have to adjust based on what the plants are doing.
Here are a few tricks I use. Look at the whorl of leaves at the top of the plant. If they are lighter colored, slightly yellowish, then you have too much nitrogen. If they are dark green but the plant is not growing, then you don't have enough nitrogen. If the leaves are purple, then check the growing temperature and if needed add a phosphorus source with low nitrogen. Potassium is tough to judge but there is a rule of thumb that too much phosphorus blocks absorption of potassium and vice versa. You have to keep them in balance. For this reason, a fertilizer with roughly twice as much phosphorous as potassium is best at the seedling stage.
Micronutrients are another thing many plants are abused with. As a general rule, most commercial seed start mixes will have enough micronutrients to keep seedlings going until they are 8 or 10 inches tall. If you are starting with something like pine bark fines or commercial peat moss, then you won't have enough micronutrients and the mix will be mildly acidic unless you added lime. The rule of thumb I use is to add 1/2 teaspoon of micronutrients with NPK and 1/4 cup of dolomite lime to 20 gallons of homemade mix. You may have to adjust this up or down a bit to make it work properly, especially if you start with something like coconut coir.
The last thing I will add is re sunlight vs artificial lighting. Tomato plants thrive under sunlight. Artificial lights are always somewhat unbalanced in this regard. That is why I have a greenhouse. Even so, I have a light stand and all my seed are started under fluorescent and moved to the greenhouse when they just start to show the first true leaves.
DarJones
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