Decaying plant and animal matter returns nitrogen to the soil in the forms of which plants can take in and use. nitrogen gas and nitrogen oxides amino acids and polypeptides simple sugars, fatty acids, and glycerol ammonia, nitrite ions, and nitrate ions

Decaying plants and animals matters return nitrogen to the soil in the forms of ammonia, nitrite ions and nitrate ions. This process is called ammonification. During this process, amino groups in decaying matters are converted to ammonium by micro organisms [primary autotrophic]. The ammonium produce is later converted to nitrite and nitrate ions by appropriate microbes.

It will fall off and tip over. 

Aphysical change is one that you can put in and remove like a ham sandwich. the stuff you put in there can be changem its not permanet. a chemical change is on that can't be change thats permanet. like when you make a cake and bake it. you can't get those ingredients back.