Among phytoplankton the diatoms are strong competitors and contribute significantly to total global primary production. Aspects of their life history, notably their high sinking rates, make them important to the export flux of carbon into the ocean interior. Unlike the majority of other phytoplankton, they utilize silicic acid (=silicate) to construct their cell walls and are controlled by its availability and distribution. Here a simple model is developed to study the relationship between the diatoms and the ocean's silicon cycle. The ecological component of this model pits the slightly superior diatoms against all other algae, with both groups competing for phosphate while the diatoms additionally require silicic acid. The model agrees reasonably with observed distributions of nutrients and with their biogeochemical fluxes. While theoretically superior, the diatoms are held in check by the availability of silicic acid, allowing the persistence and numerical dominance of the other algae. The concentrations of both nutrients are homeostatically controlled by the phytoplankton, and resist perturbations. Analysis finds that primary production in the model is ultimately controlled by phosphate, with silicic acid abundance controlling the fraction of the total produced by diatoms. Sensitivity analyses using more ecologically detailed variants of the model find that these results are generally robust. The model's treatment of the silica pump hypothesis <Dugdale and Wilkerson, 1998> is also examined.