Fertility
There are three sources of the fertility buff.
Several patches are blessed with this at the start of the game. Fertility Boost is a starting ability on the Forest Giant and on the Ocean Giant Monsoon is unlocked with a Desert Ambassador.
Each level of Fertility Boost and Monsoon increase the chance of rolling a specific higher level Aspect when it is applied to that patch. For example if you have Level 2 Leaf Aspect you have a 75% chance of Potent Leaf Aspect and 25% chance of Greater Leaf Aspect, if at least Level 2 Fertility has been applied to that patch the probabilities are roughly reversed (75% Greater/25% Potent.) However, if there is only a Level 1 Fertility buff on the patch there is no change to the chances as Level 1 Fertility only increases the chance of Potent Aspects.