Garden writer David Beaulieu suggests using nitrogen and water to help kill a tree stump. He explains that the process begins with cutting the stump as closely to the ground as possible. The next step involves drilling holes a few inches deep into the stump in numerous places with a wide drill bit to apply the nitrogen and water.
First, fill the holes with water before adding a fertilizer with a high nitrogen content, such as cow manure or a commercial-grade fertilizer. Next, soak the ground around the stump, and then cover the area with a plastic tarp to retain the moisture. Beaulieu then says to apply an organic mulch, such as tree bark or hay, over the plastic tarp and water it thoroughly. The additional mulch helps retain the moisture and makes the tarp heavier so that it does not blow away or move.
Although this is the last part of the process, Beaulieu explains that removing the mulch and tarp for a moment and again thoroughly soaking the stump and the ground around it speeds up the process. He also mentions that a product called Stump Remover, along with kerosene and fire, as another way to kill a tree stump.