The correct answer is - For each plant type, either add fertilizer to all the plants or don't add fertilizer to any of the plants.
The best way to conduct the experiment with the different types of plants and the effects of the fertilizer is to add fertilizer to certain number of all types of plants, and for the rest of the plants to not add any fertilizer.
In this manner, in each plant type can be easily seen the effect that the fertilizer will have on their growth, as there will be plants from all three types that have and don't have fertilizer. In this way, a comparison can be made, both between the plants of the same type, and between the plants of different types.