The fertility of the soil
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.
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The Nitrogen Cycle