P and K cycles

Fragment of a discussion from Course talk:APBI200

K cycle
In all the nutrient cycle diagrams in your class notes, inputs are show by blue lines, losses by red lines and transformations by black lines. Have a look at the K cycle diagram in lecture #25/#26 and see what you think.

Losses from the soil imply that a nutrient or compound is lost from the soil (e.g. via erosion, leaching or gaseous losses). In the case of K fixation, K is not longer available to plants - but it is not lost from the soil profile.
The arrow from K in soil solution labelled leaching indicates that K+ in solution can be lost via leaching. Note that cations held on exchanges sites are not readily leached.

SandraBrown (talk)00:52, 28 April 2020

Thanks for your reply! I had another look at lecture 25. Would the other main reason for lack of phosphate ions in soil solution be that minerals containing P have pretty low solubility and therefore little phosphorus can be released from the crystalline structure into soil solution? I also had a question about the nitrogen cycle diagram in lecture 26: Could you please explain why the arrow labelled fixation that points from fertilizers/industrial fixation to nitrates is blue? My understanding of any kind of fixation was that it makes nutrients unavailable to plants by adsorption to colloids etc, so wouldn't it be a loss and therefore colored in red? How is this kind of fixation considered an input?

OliviaSheena (talk)03:27, 28 April 2020

P cycle
Olivia, in your first post you identified 1 reason: P fixation; here you identify a 2nd reason - minerals containing P have v. low solubility. Both reasons contribute.

N cycle
Industrial N fixation on the diagram refers to the Haber-Bosch process by which atmospheric N2 is converted to NH3; this is how synthetic fertilizers are made. You could replace Fertilizer / Industrial Fertilizer with synthetic or chemical fertilizers (and they are an input to the soil)

The term fixation: does it always indicate that nutrients are made unavailable to plants? Hint think about inputs to the N cycle?

SandraBrown (talk)16:13, 28 April 2020

From looking at the N cycle again I would say fixation doesn't always render plant nutrients unavailable. I can see there is another input arrow for photochemical fixation (NHO3) that goes directly into the nitrate pool. My understanding is that atmospheric N compounds are fixed into NHO3 and when they enter the soil solution, they dissociate, releasing a proton and forming NO3- which is plant available. Is that correct? In more general terms, is any kind of N fixation that's labelled as an input described as the conversion of atmospheric N to an ionic, reactive form of N such as nitrite, carried out by soil organisms?

OliviaSheena (talk)18:03, 28 April 2020

What Sandra was hinting to is the process of ammonium (NH4+) fixation that traps (or fixes) ammonium ion in the inter-layer spacing of certain phyllosilicate minerals. Through that process of ammonium fixation, ammonium ions from soil solution (where they are available to plants) are made unavailable to plants.

MajaKrzic (talk)20:41, 28 April 2020