Is there a chance you can provide a photo? Houseplants that have not been repotted in "many, many years" will naturally be under considerable stress. Container potting soil does not last forever and breaks down and compresses over time and that can alter both good drainage and proper aeration. Plus, plants become rootbound if left for an extended time in the same sized container and that needs to be addressed by root pruning or routine up-potting as necessary.

Do you fertilize? If so, with what and how often?

Thank you for the reply. I am attaching some photos. I do fertilize with a multipurpose houseplant formula, but not that often. Maybe every other month at most?

It used to be much fuller. The second photo shows where the branch fell off this morning. The yellowing starts randomly in the middle of branches sometimes, and sometimes at the ends. Then they turn brown and fall off. :-(

By the size of it (its very small for a 30 year old tree), it has not been getting enough light and is probably extremely root bound in its current pot.

Its in dire need of a full repot (removal of all soil, untangling of roots and root pruning), however, going into fall is not the right time of year to do that.

Give it more light not less, water as it needs it and do not mist.

open the curtain all the way and place the tree directly in front of the window.

Get some Foliage pro 9-3-6 and fertilize with that on a regular schedule.

id also recommended giving the soil a full flush at next watering. Place the tree in the tub and water about 10x more than the tree would need. This will flush out salts built up in the soil. The pot has a drain hole, right?

Yes, the pot has adequate drainage. It actually did not seem root bound when I repotted it recently, which surprised me. It had been slowly growing and green for many years.

How often should I fertilize it?

I had read that these plants liked humidity and misting, and indirect light. Is that inaccurate?

Regarding the light...it had been in a south-facing window for a number of years, but the blinds were closed on hot days. The room can get very warm and dry in the summer, and it's near a heating baseboard in the winter. No drafts though, because it was in a corner. I was afraid it was getting too hot in the summer when the blinds were left open.

I will try doing a soil flush outside while the weather is still nice.

Thanks.

That is inaccurate, correct. You should not mist any houseplants. It has no real benefit at all and could end up causing fungus problems.

The trees do like high humidity. However, misting is not the trick. Actually raising and keeping the relative humidity up with a humidifier would be beneficial.

These are tropical pine trees that naturally live outside in full sun.

Though, an indoor tree such as yours wouldn't be able to handle that without acclimation only because it's accustomed to low light levels.

That said, the tree would still love as much light as you can give it. Please remember that glass windows filter the sun so no matter what, it's nowhere near as intense as outdoor direct sun.

to start, I would place it right up in front of a window that receives full unobstructed light from the sky. I would keep the drapes open all day.

keeping it by a baseboard heater all winter is also a problem for the tree.

You say it wasn't root bound when you repotted, but earlier you said you only added new soil to the bottom?

repotting: removing from pot, removing ALL soil from in and around roots, untangling and pruning problematic roots.

Fertilizing really depends on how you water. Do you water in sips? Do you water until you see water come out the drain hole? How do you check the soil to ensure it's time to water again?

You'll see the root damage part while repotting. Root that you see while giving bigger pot maybe damaged root that no longer attached with the trunk, so giving bigger pot and extra soil won't help. Repotting is needed for sick plant to investigate root problem.

Ok, I understand now about the misting versus humidity. Maybe I will start by moving it closer to the window in the room it's now in, which is away from a baseboard, but without moving it back into direct light right away. However, in the winter there will be a problem with drafts as it is near a pellet stove and exterior door. It is tough to find the perfect spot!

The roots did not seem tangled or mushy - just normal roots, but I did not shake out all the dirt. The new pot was deeper and wider, hence the need for additional soil at the bottom and sides. I don't know what I can do regarding another repotting now that fall is here. Do you think I will need to remove the plant, shake out the dirt, flush the roots, and repot again to stop the disease process even though the timing is not great?

My check of soil is not super accurate: a finger dipped into the topsoil. Which is better: small sips of water or more at one time?

Pine have very dense root, so sometimes the suffocated part are deep inside on big root that make many root branches no longer functioning, and make the healthy root difficult to access good soil and nutrients. I'm not in temperate climate so can't give any advice related to best repotting time, but when the plant condition keep decreasing day by day, i ussually do things necessary asap.

Do not underestimate the benefits of root pruning in a case like this. This plant's needs have been neglected for many years and it will take more than a repot to revive it. Root pruning can revive and rejuvenate a root system.

Be sure to check regularly for spider mites. Nothing I see points to an infestation of mites, but NIPs are a very favorite host of the common plant pest.

Is there a guideline for root pruning? I am afraid I do not know what and how much to cut back.

For sick plant, i did this way. Remove root ball from the pot. Shake gently so any soil that not holded by root hairs will drop. If you see dry or rotted root, trace it up and cut. Keep doing it till all problematic root gone. Doing it fast and keep the root wet. When it's done, replant, give water thoroughly, and give it a good place to heal without any interruption (hold, shake, etc) for some weeks. In severe cases, healthy root maybe only 25% or less left, but it's better than a 100% with more than 50% damage root. The 25% root will have plenty room to grow.

my 18-year-old Norfolk Pine if you branches are turning yellow and I never had that happen I’ve change the light several times so I really actually should report it to a bigger pot correct?

@Kristie D. Cox - you need to start a new thread and pictures so you will get the help your plant needs. I always read other posts for help and ideas but if I have trouble it easier for you to get that help if start a new thread with the growing conditions and pictures are always helpful. good luck to you.