And as you gain experience, you'll notice subtle color differences. Look at the largest one there in the center of the image... now look at the one up, and over one cell to the right. See the smaller one is darker in color? The primary nutrient for a growing seedling is nitrogen - it's not the only necessary ingredient, but at this stage, it is the one most required for vegetative (leafy) growth. Nitrogen also happens to be the most easy to determine nutrient need by plant color/growth characteristics.

In seedlings of that maturity, when you see a pale color, it is a sign that the plant can use some nitrogen (as well as the other necessary nutrients.) It can be difficult to tell without a comparison, but you have one there when you look at the 2 right next to each other.

When the leaves are shading each other to a large extent they need more room. At that point I feel they are ready for the garden. They will be 6 to 8 inches. The roots will be confined but if you water for a few days they will adapt. Some people grow them much larger but then you need individual pots and more space like a greenhouse.

I used this on my starts this year and had good results.

also get those lights much closer, like this. you want the lights just above the leaves but not touching.

My 3 week tomato, pepper and eggplants in front of my daylight fluorescent stand. A few years ago I tried fertilizer as soon as the seed leaves unfolded and another without. 2 weeks later the fertilized plants were noticeably more developed. Soil had little nutrient. I use miracle grow or similar at 1/2 ml or 1/8 tsp per qt/ltr in all water. Could use normal indoor fertilizer rate weekly.

Hort article suggested that outdoor wind movement stimulated stem strength. They found improvement with moving tops of plants 2x daily. I use the light duster in background. Some people use fan.

Plants also need Calcium & Mag. Usually comes from lime in soil. Liquid fertilizer does not contain them since they precipitate. Make sure some is in potting soil.

"Plants also need Calcium & Mag. Usually comes from lime in soil. Liquid fertilizer does not contain them since they precipitate."

Liquid fertilizers do indeed contain both Ca and Mg - you just need to find the correct one that includes a full range of micronutrients or trace elements. Most of the Dyna Gro products will provide these.

By the way, are your summers warm enough for Tomatoes? I know that in northern Europe, they often use a greenhouse.

Barely :-)) The ones that have a short harvest interval do best here. That includes any cherry tomatoes and the very early bearing types, like Early Girl, Oregon Spring, Momotaro, Stupice, Sub-Arctic. Beefsteak types and many heirlooms do not fair so well.