Oh dear! The English grammar standards of undergraduates and candidates does not bode well for the future, does it? We had a thread about this a week or two ago.
Rhyscx got it right, among others.
Subject names should never be capitalised (unless you are writing in German, of course). Course names are, but you will nearly always be writing about the subject rather than the course in a PS, especially bearing in mind that you are applying to five different courses, each of which may have a different name longer that the obvious one (such as Geography with Geoscience). An exception arises in the case of a language, of course. But linguists have a special reason to make sure they do not make mistakes anyway.
You can argue that it makes no difference, or looks better, or is too pedantic to worry about, that no one will notice, or it makes the subject stand out, or emphasises its importance (all of which I have seen argued) but it is far better to just make sure everything in your PS is absolutely right and capitalising a subject is just plain wrong, grammatically. For most applicants it is the only thing that connects directly from you to the admissions tutor. Even science admissions tutors are interested in your precision and your PS may betray you.
I'm not saying that making the odd error is going to lose you an offer, but you may be making more errors (such as typos, spelling mistakes and missed words) than you are aware of and this is an easy one to get right. After all, none of you would write your PS liberally seeded with spelling errors, would you?
In the future, many of you will have reports or important selling documents to write which could well cost your employer money to have sub-edited to remove such indiscretions. In fact, part of my living is earned in doing just that. This will not endear you to them. It is much better to start getting it right early.