Why aren't all research articles on PubMed? I was recently doing some research on PubMed and I noticed that, in my field of research (plant science), some articles are referenced and others are not, even for two articles in the same journal...

Does everyone know what is (are) the condition(s) for a paper to be listed in PubMed?
Because PubMed is about biomedical literature and life science journals. Things are blurry, however, because of two factors:


Obviously, there is no strict definition of what is (or is not) biomedicine.
PubMed does not select “article by article”, but has a list of indexed journals, from which all papers are indexed. Yet, a single journal's scope can be very varied.


Combine the two, and you get the idea that there is a lot of variability in whether an article on a given topic is published. For example, my own research field is materials chemistry, and about two-thirds of my papers appear to be indexed in PubMed: probably because those were published in generic chemistry journals, which also happen to publish some papers of relevant to life science.