I think it is a wonderful idea to give your plants nitrogen. Do it in the form of a good meal of NPK fertilizer with essential minerals at an appropriate rate consistent with the size of the plant.

Don't forget the plants you raise are your children, that each one is an individual and requires a well-balanced diet for proper growth. Anything less and you are compromising your plant and you certainly don't want to be guilty of using gavage on your babies. Nitrogen in greater amounts than your plant can assimilate will result in a lopsided fertilization program, poor growth characteristics (perhaps leggy growth, perhaps dense leaves shading out and killing earlier leaves and not feeding surgars to early fruit, perhaps interfering with the eventual transition to fruiting ... changing the plant's hormonal activity is not like a light switch, you will need to arrest the growth and lose all your gains and then some more and the plant will actually fall behind in the transition), and potentially damage, in the form of fertilizer burn which can stunt. IMO best to let the tomato kids ... be kids. Allow them to strike a balance in their lives while being adolescents, not too much, not too little, but just right. Most fertilizers overdo the nitrogen, so be cautious.

Cheers

PC

Nitogren is a required nutrient, it is the also the 'most' required in terms of quantity. Nitrogen will help you grow a healthy plant, tomatoes take up nutrients in about a 4-1-3 npk ratio throughout life. There is no need to cut nitrogen at any stage.

What you are describing is standard practice. But not just nitrogen and top growth isn't the primary concern; root development is.

As PC said above you "Do it in the form of a good meal of NPK fertilizer with essential minerals at an appropriate rate consistent with the size of the plant." Feeding young transplants with a diluted form of liquid or small amounts of low-dose type of fertilizer is a requirement for good plant health and growth.

Dave

I always just sprinkle a little slow-release lawn fertilizer on my tomato beds when I set the plants out (early March). My soil already has plenty of P and K. It lasts for a month or so, and encourages plant development. By the time the plants are flowering, that extra nitrogen is gone and the plants can concentrate on fruiting. I'm getting rained on by tomatoes right now.

The advice to avoid excessive N is mostly about avoiding too much nitrogen at a more advanced stage when the plant should be flowering and fruiting. Even then, it's not about avoiding all N--just avoiding too much N. Depending on your soil, a fertilizer that is for general plant or lawn use can be too much N at that stage.

Seedlings OTOH need everything, just in more dilute doses because they're small and the smaller trays/pots they're in don't leave much room for error if you overfertilize.

Like most others, I fertilize a little when transplanting. As they grow, I watch the top growing tips. If you look down from above (with smaller plants of course,) you can get an idea of N needs. When you see a slightly paling color in the growing tips (with a tinge of yellow,) but all the rest of the plant is a nice, rich green color, your plants would not mind a shot of nitrogen. If you see that the yellowing is also beginning to happen in the tops, and the lower growing tips (side branch tips,) your plants need nitrogen.

I ride the edge of nitrogen deficiency. I want the plants to have everything they can get for 2 weeks after transplanting, but once they start flowering, I want my plants to have what they need, but not more than they need.

What do I hope to cause in my plants? The desire to send out roots searching for nutrients; the ability to grow without hindrance from having too little N; and, for them to not stay in vegetative mode.

Tomatoes grow fast enough during flowering that the need to stay in vegetative mode is fairly short. By 2' - 2 1/2' or so - depending on variety and such conditions as temperature and irrigation - tomatoes should be flowering. Indeterminate varieties have no real limit, they will continue to grow and set trusses during flowering, fruit set, and harvest. So, to my thinking, growing them as annuals means giving them what they need to produce whatever greenery necessary for photosynthesis to set and ripen fruit, but not so much that they see no end to their growth cycle. I want them to produce seed, not to produce lush greenery.

VERY slight yellowing in the growing tips means that they can use N. Yellowing in the lower tips means they need N. Yellowing and dropping off of bottom leaves, combined with unanswered yellowing of the growing tips means they were N deficient, and the grower should have responded earlier.

Keep in mind though, that color changes due to N needs should not be confused with natural color variations and maturity differences, as well as not conflating N colors with other nutrient, (and pH) needs and deficiencies.