The extent to which plant populations are seed vs. establishment limited can be understood by quantifying the recruitment function, describing the number of seedlings that establish as a function of the number of seeds added. Here, we derive a general equation for the recruitment function based on a mechanistic model describing how the availability of safe sites (sites suitable for germination and establishment) interacts with the number and distribution of seeds added to a plot to determine the number of recruits. The parameters of this recruitment function have a direct biological interpretation that can provide insight into the processes limiting recruitment in plant populations.