Practice exam #3 - QUESTION 4

Biological N2 fixation occurs when soil microbes transform atmospheric N2 gas into NH3 using the enzyme nitrogenase which is carried out by bacteria, cyanobacteria, and actinobacteria. This process adds nitrogen to the soil. NH4+ fixation occurs when ammonium is fixed within the interlayer space of certain kinds of phyllosilicate clay minerals that have an interlayer space the same size as the radius of NH4+. This results in NH4+ becoming unavailable to plants until the phyllosilicate clay mineral weathers. This is a transformation process of nitrogen within the soil. Phosphate fixation is when Fe, Al, and Mn compounds complex with certain forms of phosphate and crash out of solution. These phosphate will not be available until another chemical process breaks down the metal complex containing it. This is another transformation process within the soil, this time with phosphorus.

Blaise, with respect to P fixation consider both low and high pH.