Phosphorus is filtered out and stored for use as a fertilizer.

FORBES: Magazine Article

Chemical and fertilizer manufacturers like Dow and Potash, for example, use gas as a fuel and a feedstock.

FORBES: Shale Boom Still Has Plenty Of Room To Run

After their cotton had been ginned, farmers would keep some of the seed for next year's planting, plow some into the ground as fertilizer, and use the rest to feed their cows.

NPR: Texas Cotton: 'Farmer Profits at Every Step'