<p>The genitive case is in fact used for several things besides possession. </p>
<p>A Dictionary of Contemporary American Usage identifies seven genitive types:</p>
<pre><code>Classifying or descriptive genitive ("the room's furnishings")
Possessive genitive ("Irene's coat")
Subjective and objective genitive ("God's creation")
Genitive of purpose ("He has written many children's books.")
Measures and other adverbial genitives ("At one time the genitive form of certain words could be used as an adverb. Most of our adverbs that end in an 's' (or 'z') sound, such as "nowadays," "since," "sometimes," "upwards," are survivals from this period.)
Survivals of "an old genitive of source" ("hen's eggs")
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<p>Partitive and appositive genitives (don't exist in English, but we express them with an "of" phrase, as in "some of us," "the state of Ohio," "the title of president")</p>
<p>Since there is so much confusion about the genitive, I think you could go with or without the "'s" and be safe.</p>