Drop the soda, immediately. You should only use this at most, once a month. It’s a very harsh hair stripper. Long term use can damage your follicles.
Soda and acids will dry your hair out, they’re great at stripping away residue, but more importantly, they’re breaking down the oils in your hair, which you need for a healthy head and to keep moisture locked into the scalp (avoiding flaking) and help remove grease/dirt.
This is where your dry scalp and brittle hair is coming from.
You need oil in your hair to lock in moisture and keep it hydrated.
Seriously, people, stop using the baking soda, you dafties, there’s a reason it’s an excellent cleaning product for the harshest of germs and grime.
Try combing in some water before bed, drop of olive oil/hair serum oil in your palm and massaged into your hair (not roots), them comb through thoroughly.
You want to go to bed with a damp head, not soaking, just damp, can wrap it up if you want. If you notice the hair starting to dry very quickly (<5 mins), your hair is very thirsty and you’ve been abusing it something rotten - Keep this routine for a few days and keep combing daily until your natural oil starts to penetrate all the way down your hair strands. The hair at your shoulders should match the shade of hair above your earline when properly hydrated.
Once your hair matches up, try washing it with warm water only, rinse well. Add oil/serum after each wash, because you will strip some of the oil out and again, you need it or your lower strands will start to dry and brittle quite quickly.
For emergency care, before bed, massage some cornflour (half a teaspoon) into your roots (sides, top and back of scalp), and then some water massaged in, comb through to tip. Should be a lot better the next morning.
NOTE: Avoid a brush, use a comb, because the flour will gunk up the brushes, which I find are a nightmare to clean.
Hot hair-dry/straighteners in the morning should help dry the flour with it’s greasy payload and brush out, if you’re noticing it’s still somewhat heavy or dirty.
Water not only improves hairs bonds, it also allows grease to move down the hair strand and away from the scalp. In cases where the scalp is dry, the grease clogs the follicles which in turn the derpy follicles produce more, it becomes itchy and flakes, it’s a never ending cycle. Stripping away the grease with soda and acids doesn’t solve the issue, it makes it worse, as the follicles then produce even more to replace what you stole from them.
Let water steal the oil: Both follicle, oil and hair are much happier and in balance when it does.
And when water steals too much, add in some olive oil or hair serum oil to compensate, bind it with a comb in of more water.