After life...

In chapter 5 of Brave New World Lenina and Henry fly near the Crematorium where Lenina takes notice and asks Henry, "Why do the smoke-stacks have those things like balconies around them?"(Huxley, 73).
Henry then explains that it is phosphorus recovery, and then he goes into detail of what it is.
When Henry talked about the P₂O₅ leaving the chimney when someone was cremated, it made me think.
Did they cremate someone when they died (of natural causes, etc.)? Or was there a special age where people just had to 'retire', like in The Giver? And how did they die anyways?
Then when Henry explained that their purpose after life was to make plants grow, Lenina agreed, but stated, "'But queer that Alphas and Betas wont make any more plants grow than those nasty Gammas and Deltas and Epsilons down there'"(Huxley, 74).
If Gammas, Deltas, and Epsilons are considered different to the eyes of the Alphas and Betas, how are they 'physico-chemically equal'?

-Alyssa Kocon