A purely mechanical method can produce a novel, more sustainable fertilizer in a less polluting way. Scientists have optimized a production method that is an adaptation of an ancient technique: by milling two common ingredients, urea and gypsum, the scientists produce a new solid compound that slowly releases two chemical elements critical to soil fertilization, nitrogen, and calcium. The milling method is rapid, efficient, and clean — as is the fertilizer product, which has the potential to reduce the nitrogen pollution that fouls water systems and contributes to climate change.