Practice exam #3 - QUESTION 4

Biological N₂ fixation is the biological process of converting N₂ in the atmosphere into ammonia (NH₄⁺). It is one of the processes that convert N into plant-available form.

NH₄ fixation is not a process of making N available to plants. Instead, it is one of the ways that available N is lost from the soil solution. Ammonium ions (NH₄⁺) can be captured by phyllosilicate clays in their inner layers. Strength of the inner layers holding together is strong enough to prevent the ammonium from leaving, making the ions unavailable to plants.

Phosphate fixation is similar to NH₄ fixation in the sense that they both make available nutrients unavailable. But different from the former, this fixation is a process where available phosphate ions in the soil solution are absorbed/reacted with metal (Fe, Al, Mn) compounds, clay minerals, and/or calcium compounds into plant-unavailable forms.

Please let me know if something in my explanation is off.

Eakin, consider Jacob's point that NH4+ fixed by phyllosilicate clays and fixed phosphate can be released by weathering (albeit slowly).