Re: Why do plants die when you over water them?

There are at least two reasons why overwatering can harm plants. One reason is 
a lack of oxygen in a waterlogged soil damages or kills plant roots. Plant 
roots require oxygen for cellular respiration. You could say plant 
roots "suffocate" without oxygen.

Another reason is that anaerobic microbes in a waterlogged soil produce toxic 
waste products that can harm plant roots. Thus, even if the plant roots have 
sufficient oxygen that they obtain internally from aboveground, the roots could 
still be damaged by the microbial waste products. 

Microbes in waterlogged soils also promote conversion of some mineral nutrients 
from nontoxic to toxic forms. For example, in waterlogged soils, nitrate is 
converted to ammonium, sulfate to sulfide and manganese ion +4 to manganese ion 
+2.