「知」 itself means "to know". When used in the verb 「知る」 it is read as "shiru". I have no examples about when in a compound for 「知」 though. :/

The thing is many kanji can have the same kun or on reading. For example, both 「二」 and 「日」 have a reading of "ni". The first character, 「二」 (two), has an on reading of "ni" and a kun reading of "futa". 「日」 (sun/day) has on readings of "ni", "nichi" and "jitsu" (according to WWWJDIC) and kun readings of "hi", "bi" and "ka".

EDIT: Zeromus_X beat me too it. But basically what I was trying to say is that many kanji can have similar readings and sounds, but it is through writing the kanji that we get the difference in meaning. Thus writing in only hiragana sometimes can suck since you have no hint at what the word could mean, though if you recognize a kanji in a word at least you can take a stab at the meaning if you are not able to read it properly.