The service draws on the absolutely massive Google Books corpus. Google estimates they've scanned and OCR'd more than 10 percent of all the books ever published, and they use about a third of the total books in the tool.
Language and book publishing trends are tricky things to nail down. For example, just because a word comes into more frequent use does not necessarily mean that the concrete realities we attach to those words in today's language have become more important or popular. But they do provide fascinating data points to consider when assessing cultural trends.
I did a few searches related to the content of this blog:
Below is the meat index comparing usage of the words chicken, beef, pork, and turkey (1900-2008):
Here's an "industrial agriculture" vs. "organic agriculture" throwdown (1940-2008):
Below is the fast food index showing the rise of pizza, hamburger, and fast food (1940-2008):
Here's what I'll call the foodie index showing community garden, farmers market, and csa (1900-2008):
Below is the vintage food index showing use of victory garden, pickling, and canning (1900-2000):
Finally the ag index showing frequency of farm, farmer, and agriculture (1900-2008):