hi Amelia, no depleted means that soils are depleted in nutrients! soils contain a lot of important nutrients for plants like nitrogen, like phosphorus. And when we grow plants in soil, plants take these nutrients from the soil; and if we do not replace what plant took; soils get depleted.

The question is, how can we provide soil the nutrients back. it is very easy! We need to put the dead plant material to soil. So, when plants die, their cells dissolve in soil and nutrients go back to the soil. This is like what happens with leaves of the trees in the forest. We do not fertilize trees yet they grow very big. Dead leaves falling from trees go back to soil and feed the soil; bacteria and fungi help in this process. 

So, if we want soils to be not depleted, we need to do more composting: https://www.treehugger.com/benefits-of-composting-5179483

I am glad to hear that you are learning about soil types. Soil in almost every region can be different from each other. and this is very important for plants; because they get all of their nutrients from the soil. Also, soils can have other chemical properties that determine the plants health, taste etc. For instance, some plants like growing in acidic soils, some doesn`t. Some cannot grow in sandy soils, some can. So, each plant has a soil type preference and soil choices/soil health is very important for agriculture.

Another things about soil is that soil contain microorganisms! 1 table spoon of soil can contain thousands of bacteria and fungi. and meant times, these bacteria and fungi are very important for plant health too. Bacteria and fungi can help plants to get nutrients from soil, check this out: https://kids.kiddle.co/Mycorrhiza#:~:text=Arbuscular%20mycorrhiza%20%28AM%29%20fungi%20are%20unique%20in%20hosting,1970s.%20Research%20later%20identified%20them%20as%20true%20bacteria.

Hi group, I am sorry that I was not able to respond before. I am a postdoctoral researcher from the University of Wisconsin-Madison. This means that I completed my doctoral degree and I am working as a researcher at the university. I will try to answer your questions. I am glad that an agronomist came to your school.