On the picture there are 3 seedlings antingen normal and looking healthy. The 4th one, the one to the right looks like struggling and may not make it.

Thank you for your answers. I agree that the soil I used is not the best, I put too much organic part and it has the tendency to stay too wet. It will be a problem for the coming autumn...
For sure when I will repot then I will use a granular substrate, but from what I understood I need to wait for the end of the winter.
During the repot, can I partially cut the tap root? How much do you suggest to cut?

It really depends on how the fine feeder roots has developed. I suggest evaluate every seedling separately. Remove as much tap root as possible without removing more than 50% of the feeder roots.

Behind my house are a couple of pine tree 10 meters or more high. Every year they drop hundreds of pine cones and each one has dozens of seeds. Each spring of the thousand or so seeds twenty or thirty sprout. At least half don't survive the season and probably fewer than ten are still around the next spring. No more than three or four last that year. Of those, maybe one survives more than five years.

Why? Well, sometimes environmental issues - we just went without rain about two months and it would take a very long tap root to survive that. But, in reality, if most of the thousand seeds grew to be 10 meters high that area would be pretty crowded. If the tree can manage to have one survive and pass its genes on each year it can probably call that a success.

Seedlings mostly don't survive. You can increase odds so that three out of a dozen survive, but that takes skill and luck. If you pick trees that have survived three years you will probably have better luck.

Hello, as you can see from the picture, the tips of the needle are becoming slightly brown. Is this normal?

 

In authum pines shed old needles. That is natural. As long as the new needles is healthy there are nothing to worry about.

But it looks like all the needles have the brown tip, not only the old ones... don't you have the same opinion from the picture I posted? 

Circuled in blue is som excamples of needles produced this year that looks perfectly healthy to me.

 

On mature pines it is expected that they drop 3 year old needles and keep those produced in the two last seasons. I do not have much own experience with seedlings, so cannot tell you if a perfectly healthy seedling would keep two years growth as well.

All that I know is that you are aproaching authum, that pines shed needles this time of the year and that you have what you have. No reason to spend a lot of enery worrying about things you cannot change.