Watering the plant with  too little  or too much fertilizer will slow down  plant growth.

Fertilizers

Plants need nutrients to grow and fertilizers are a good source  of nutrients that may be lacking in the soil. However too much of anything is not a good thing.

Plants absorb water from the soil into the roots through a process called osmosis. This process will require the salt content inside the roots to be higher than the salt content of the soil. As a result, the water molecules will migrate from the soil into the roots in order to balance the salt level in the soil and in the roots.

Using too much fertilizer will increase the salt content of the soil. If the salt content of the soil becomes higher than the salt content of the roots, the water molecules will migrate out of the root into the soil causing root burn. This will cause the plant to grow more slowly, become dehydrated and eventually die.

Too little fertilizer will cause the plants not being able to grow at the optimal speed due to the lack of nutrients in the soil.

Therefore using the correct amounts of fertilizer recommended by the manufacturer is the safest way for the plants to grow healthily.
 

The hypothesis that watering  plants with  too little  or too much fertilizer will slow down  plant growth is proven to be true.

Using the correct amounts of fertilizer is important to optimize the yield of the garden or the farms. Sometimes continual use of fertilizer will cause nutrients and salt to accumulate in the soil over a period of time. Therefore, using fertilizers once a week instead of daily or having the soil tested may be recommended to improve the yield and protect the crops.

What would happen if the experiment was repeated using different types of plants like bamboo or beans?

The experiment can be repeated using different brands of fertilizer.