This is absolutely true. Back before the (mostly German) petrochemical/pharmaceutical industries developed cost-efficient ways of processing hydrocarbons, guano was a major source of chemical stocks. 

Before the widespread employment of synthetic fertilizers (indirectly converting hydrocarbons to calories), guano was a major agricultural input and the ability to boost food production (and thus feed population, and thus raise armies) gave a significant advantage to powers which controlled it (and thus naval powers vs. land powers generally).

Also as a pretext, the Guano Islands Act allowed America to claim outposts that were vital for fueling/restocking purposes to extend naval power far enough to defend the Asian/Pacific possessions inherited from victory in the Spanish-American War.