Teaching mathematics in real-word contexts is a strategy to make math relevant for undergraduate students. However, very little research exists on students' methods of integrating contextual and mathematical knowledge through writing. This study uses an interdisciplinary writing assignment to investigate the relationship between students' written portrayal of context and mathematical argumentation. A qualitative analysis of 41 essays found that writing contextual details was associated with the use of multiple data sources and with the mathematical support of an argument. Writing about contextual details allows a student to create a sense of cohesion between mathematics and the thesis of the essay. The study suggests that pre-writing activities connecting context and mathematics may help students integrate interdisciplinary knowledge.