This effect targets the Plant-Type monster to be Special Summoned, and continues to target the monster after it is Special Summoned.
If "Miracle Fertilizer" Special Summons multiple monsters, then "Miracle Fertilizer" is destroyed when any of the Summoned monsters are destroyed, even if some of the other Summoned monsters are still on the field.
The effect "●WIND: You must pay 500 Life Points to activate a Spell Card." of "Clear World" only applies to when a Spell Card is activated from the hand, or when a face-down Spell Card is activated. You do not pay 500 Life Points to activate the effect of a Spell Card which is already face-up on the field, like "Miracle Fertilizer" or "Blaze Accelerator".
Previously Official Rulings
These TCG rulings were issued by Upper Deck Entertainment and have since been deemed unofficial by Konami. They were previously considered official and can still be correct, as long as they follow the official gameplay rules, or unless Konami has issued a ruling that says otherwise. References: 
↑Konami Judge Program Rulings are considered unverified, as their source(s) are not publicly viewable. They are not confirmed as applicable to the TCG until they have been officially published by Konami.