Corydoras type of bottom feeders like digging around. Yes, it is their nature. Correct me if I am wrong, most river and lakes have a layer of sand or gravel as the top substrate. So they are basically digging around in sand and gravel, not in dirt.
I guess you made sense on nitrogen was not fully used by the plants due to the lack of other nutrients. It is exactly the reason why not everyone is using CO2 injection and high intensity lights. Plants can definitely make use more of the nitrate if there is more CO2 and lights. Same case with other nutrients.
However as a fish keeper, I am just a little lazy on putting too much effort on the plants. If anyone who are willing to spend time and effort to calculate the exact amount of other nutrients they need to supplement the tank for the nitrate to disappear, feel free to do it. I will look up to them as professional aquatic "gardeners". To me, I use what is easier for myself. I will stick with weekly partial water change to keep my system clean and healthy.
If anyone who are willing to spend time and effort to calculate the exact amount of other nutrients they need to supplement the tank for the nitrate to disappear, feel free to do it. I will look up to them as professional aquatic "gardeners". 
To me, I use what is easier for myself. I will stick with weekly partial water change to keep my system clean and healthy.
