Here's why:
* Normal Summoning: When you Normal Summon a monster, you can only do it once per turn.
* Miracle Fertilizer: This spell card requires you to discard a Plant monster to Special Summon a Plant monster from your Graveyard. This means you need a Plant monster in your hand to activate it.
* Chain Links: You cannot activate Miracle Fertilizer as a Chain Link to your Normal Summon. You can only activate effects that can be activated in the same Chain as a Normal Summon if they have the "Can be activated during either player's turn" text.
Example:
1. You Normal Summon a Plant monster.
2. You cannot activate Miracle Fertilizer in the same turn to Special Summon another Plant monster from your Graveyard because you already used your Normal Summon.
Important Note:
You can Normal Summon a monster and then activate Miracle Fertilizer on your next turn, as long as you have a Plant monster in your hand to discard.
Let me know if you have any other questions about the rules of Yu-Gi-Oh!