Many of our crops need less nitrogen fertilizer when grown after legumes, such as soybeans, alfalfa, or field peas. Legumes are plants that can take- or “fix”- nitrogen out of the air, where it exists in near limitless amounts, and add it to the soil. This ability to fix nitrogen is common in nature, in environments as diverse as fields, forests, lakes, deserts, and seas, and yet the demand for nitrogen by living things is often greater than the supply produced by nitrogen fixation. Why is this the case? We look at nitrogen in our water, forests, and cropland to get to the bottom of this fundamental question. Interviewees: Chris Filstrup, Nancy Rabalais, Mary Beth Adams