What is a major reservoir for ammonia?

The major reservoir for Ammonia is the natural soil, where it is present in abundance.

Explanation:

In soil, ammonia is found in quite abundance, where it is changed into nitrate through a process commonly knows as Nitrification. In the nitrification process, first, nitrite is formed which is followed by its conversion into nitrate by a bacteria known as Nitrobacteria. Finally, this nitrate is consumed by the plants, which are then eaten up by the animals.

the answer is true when observing or determining population size, a direct observation is use, in which each organism is counted in a population or in an area. one benefit of this measure is that you get the exact population count but it will take a long time to  determine  the  size  of a  population.