I never knew this..

Guano consists of ammonia, along with uric, phosphoric, oxalic, and carbonic acids, as well as some earth salts and impurities. Guano also has a high concentration of nitrates.

It is estimated that there is only enough phosphorus from current resources to last about 30 years.[2] Currently vast volumes of phosphorus are needed to produce fertilizer, as it is an essential plant macronutrient. Guano is rich in phosphorus and is an effective phosphorus fertilizer.