I found out about soft rush several years ago and for a long time I thought these rushes with a more compact flower heads were soft rush just coming in to flower. How wrong I was as I eventually discovered they are a separate species called compact rush (Juncus conglomeratus). It just goes to show that one should not rush into snap decision in nature identification but it is something I do all too often!
In my defence, however, compact rush and soft rush share a liking for the same environmental conditions, are both very common and both produce their florescence at the same time, June to August. Indeed my field guide describes it as being 'very similar to soft rush'. It does have its differences though, it is generally shorter than soft rush and has a stem with ridges rather than a smooth stem and, of course, the flower head is more 'compact'. No excuse really for any confusion.
This is one of six species that can dominate lowland damp grasslands according to Wikipedia but it does not say which are the other five! One is certainly soft rush so just four more to think of now!