Odyssey
 What I think X represents in Aldo Leopold's Odyssey is nitrogen.  When it first talked about coming off of a rock, I was assuming that it has an element of some sort, I just didn't know which one.  The mystery became more transparent at the end of the first paragraph when Aldo stated when X was pulled up into the world of living things, it helps build a flower, then become an acorn, which, in turn, fattened a deer, and finally filled up an Indian, and it was then I was pretty positive the element was nitrogen because this process explains the biogeochemical cycle of nitrogen.  First off, the nitrogen cycle starts off with a plant taking nitrogen from the soil (the flower, which becomes an acorn).  Next in the nitrogen cycle, consumers get nitrogen from the food they eat (the deer eating the acorn, then the Indian eating the deer).  The one part of the nitrogen cycle not demonstrated in this example is the decomposition of a living thing releasing nitrogen back into the soil, but it is demonstrated later on the in chapter when Aldo states that for every atom that is lost in the sea (a living thing dying), the prairie pulls out another on the decaying rocks. 
What I think Y represents is the molecule H20, or better known as water.  This one was a little tougher to decipher than X, but once Aldo started talking about how erosion engineers built dams and terraces to hold it and army engineers constructed levees and wing-dams to flush it from rivers, that's what made me think that Aldo was referring to water molecules.  Then Odyssey talks about how they built a beaver pond and how Y landed in one of them.  The biogeochemical cycle this process explains is the water cycle.  This is described when it talks about where the water came from before it reached the pool how it made several trips through water plants, fish, and waterfowls, which, to me, made me think of runoff water from mountains and eventually landing in a lake, stream, river, or the ocean.  Although it doesn't really mention the process of evaporation, there is a mention of rain still pelting the fields and the only way for water to fall in the form of precipitation is if there is enough water for precipitation to occur.  Since there are so many different atoms and molecules in the human body, it is hard to dictate where each one has come from.  However, I am pretty positive I've obtained nitrogen when I ate spinach earlier today and consumed water when I was working out in the gym earlier today.  I think it is insane how and where all of these atoms and molecules come from, and how many are present in specific plant or animal.


Here is a representation of the nitrogen cycle.

Posted above is a representation of how the water cycle functions.