If you’re a gardener, thinking about getting your soil in shape probably means thinking about compost. But composting can be a mysterious process. What can go into the pile? How do you balance carbon and nitrogen? And how do you do it in a city? Compost expert Cassandra Marketos joins Host Ira Flatow to break down the basics of what she calls “purposeful decomposition,” and give practical tips for the backyard composter. Read an excerpt from Marketos’ new book, “Compost after Reading.” Guest:  Cassandra Marketos is the author of “Compost After Reading: A Practical Manifesto for Purposeful Decomposition.” Transcripts for each episode are available within 1-3 days at sciencefriday.com. Subscribe to this podcast. Plus, to stay updated on all things science, sign up for Science Friday's newsletters.