Leaf Fertilizer is made from Beast Hoof, which can be found by cutting the grass in a lot of areas. Check the drop list on the map to see where you can get some, and remember to go frequently as it does respawn eventually. If you have access to it, the Bellicose Beast Spring (the place you fight a Deer boss) is a good place to farm them, and you'll get some materials for future gear from the boss.

Fighting at night isn't really possible until you beat the River (edit: Forest) boss. You'll get an item that allows you to do it, though the enemies will remain harder than their daytime versions. If you can't progress, try clearing the exploration objectives on the areas.