In his interview with Stuart Rochester on the experience of American prisoners of war in Southeast Asia (mainly Vietnam):

LAMB: Here's another line in the book and it just--I was taking these out of context. `I couldn't quite put my finger on it, but the atmosphere was changing for the worse. By November, for starters, food rations at the Zoo were cut drastically and kitchen staff were no longer washing off the human fertilizer the Vietnamese used for their crops.' A lot of reference to human feces being in--in rice, you know, all over these cells. How much--explain the human fertilizer business.