Minecraft wiki entry for Kelp_farming, associated text: Based on testing, a 20x20 square of kelp (400) down results in a rough 50% gain over the course of 1 minute, or about 600 kelp. This does not mean that half of the kelp grows 1 block; the random nature of the growth creates a negative logarithmic curve of how many plants reach certain heights. (L shaped curve)