While first land plants were smaller and simpler than the vascular plants, and would have had a hard time triggering a glaciation based on their own photosynthetic uptake, they modified soil to release phosphate as the vascular plants do. This would have lead to algal blooms as phosphate reached the ocean, drawing down carbon dioxide and leading to glaciation.
Work Step by Step
First, look at the differences between the first land plants and the vascular plants--the latter are much more robust in their own photosynthetic uptake. But then consider other effects of land plants, such as their indirect fertilization of oceanic algae.