Thank you for your great question! Growing plants can certainly be tricky- if you dont give them enough fertilizer they may not grow well, and if you give them too much fertilizer they also may not grow well!

Now to your question- a few different things can happen when you use too much fertilizer:

-Your plants can get what is called burn or fertilizer burn, which causes them to look dried out and unhealthy. What is happening here is not that the fertilizer is taking any nutrients from the soil, but actually the large amount of fertilizer in the soil is making if very hard for the plant to absorb water. If this does happen, you can try giving your plants extra water to wash out the excess fertilizer.

-If you use too much fertilizer you can change the pH of your soil, which means that you are changing the plants environment from one they like to grow in to one they might not like to grow in. Farmers often test the pH (environment) of their soil to make sure the plants can stay healthy in that environment.

-Adding too much of one nutrient can make it difficult for plants to absorb other nutrients. Plants need man different kinds of nutrients, such as nitrogen, phosphorus, potassium, calcium, iron, and more. You may have learned about these nutrients in science class, or recognize them as things that even humans need! Well just like you have to have a balanced diet, so do plants. For example, giving a plant too much nitrogen means it might not be able to get enough potassium. So balance is very important.

-It should be noted that adding too much fertilizer can be bad for the environment. If you add too many nutrients that the plants cannot use, the extra can be washed away into rivers and oceans which may not be good. This isnt so much a concern for most people with a few plants, but it is something that large farms need to think about.

I hope this helps answer your question! Have a great day!