The fertility of the soil

Question

I came across this question in my book, it says:

The fertility of the soil increases when there is a lot of ....... in it.

The choices to fill the space were:

Nitrogen - nodular bacteria - fungi

I would exclude fungi as in my course I don't have anything says that fungi affect the fertility of the soil. Both nitrogen and nodular bacteria are good choices and I am confused between them. Nitrogen is an important nutrient, nodular bacteria are the nitrogent-fixing bacteria, so both are important to the firtility of the soil, but I chose nitrogen, as I thought it is more correct, but I found that the answer was "nodular bacteria" so why is this? Is there anything I don't understand?

Answer 1

From this paper from 1958 (page 2),

... gaseous nitrogen of the atmosphere represents a vast store of potential fertility. It is not directly available to plants. Nitrogen-fixing bacteria, how- ever, absorb this gas from the soil solution and convert it to cell protein. When the cells die, other microbes attack the protein and convert the nitro- genous constituents to ammonium, which then becomes an available nutrient. Similarly, microbial action on plant and animal residues releases the many combined nutrients from their unavailable forms.

From this, one of the criteria for soil to be fertile is the availability of nitrogen to be available to plants in a readily usable form. In a two stage process, one group of microbes (bacteria) fixes the nitrogen by converting into a protein, within their cells. When this group of microbes die, another group of microbes uses these proteins to produce ammonia, which is a nutrient that plants can use.

Also, as this article states, dead bacteria are critical to soil fertility, because nearly 40 percent of a the biomass of bacteria are converted to organic soil components. strong text

The Nitrogen Cycle

Answer 2

What does 'nitrogen' mean?

Nitrogen might mean gaseous $N_2$. It might also mean compounds, which contain nitrogen, such as nitrate, ammonia and ammonium.

$N_2$

$N_2$ cannot be metabolized by most plants because it is to inert. You need bacteria or plants (e.g. Legume family plants) to perform nitrogen fixation. During this processes the $N_2$ is converted into bio-available compounds such as ammonia. If these plants/bacteria are not present, the $N_2$ is useless and does not increase the fertility.

Nitrate and Ammonium/Ammonium

Ammonium and nitrate can directly be 'used' by plants and soil bacteria. Therefore, if we provide these compounds, we increase the fertility.

Summary

Generally, 'nitrogen' does not increase the fertility of soils but depends on how it is bound - elemental or in a bio-available form. Therefore, 'nitrogen' does not necessarily increase the fertility. In contrast, nitrogen fixing bacteria do increase the fertility of the soil (also long as $N_2$ concentrations in the atmosphere are as high as present).

Answer 3

The answer is Nitrogen. Pay attention that nitrogen must be mineralized before it is available for plants: https://cropaia.com/blog/nitrogen-in-plants/