Question about gathering rate improvements

This may seem like a stupid question, but I have been unable to find an answer anywhere. When you research the next level of a resource gathering rate upgrade, does the new bonus stack with the old one, or does it replace the the old one? For example, according to the wiki, the Seed Drill improvement increases the gather rate at mills by 15%, and the Artificial Fertilizer upgrade increases the gather rate by 30%. Thus, my question is, once I've researched both, is the gather rate going to be increased by 30% or by 45% above the base rate? Is the 30% boost from Artificial Fertilizer IN ADDITION to the 15% from Seed Drill, or does it REPLACE the 15% boost?