P and K cycles

P and K cycles

Hello! I have a few questions about P and K nutrient cycles. 1) Is the reason phosphates are so low in soil because they are easily fixed? 2) In the potassium cycle, are primary minerals (once the crystalline structure has been weathered) considered an input since they make up the majority of K+ released into soil solution? Or are fertilizers and residues the only two inputs since those have the blue arrows on potassium cycle slide? 3) Is fixation considered a loss even thought the K+ is still technically in the soil? 4) Could you please explain the arrow pointing from residues to soil solution that says "leaching" on the K cycle slide? I always associated leaching with the loss of a nutrient, so I'm confused how that arrow applies here.

OliviaSheena (talk)00:38, 28 April 2020

P cycle
You are partially right Olivia. Have a look at lecture #25 - you will find: 1) details on P fixation (i.e., different ways that phosphates are fixed in soils) and 2) additional reasons why phosphate ions in soil solution are low.

SandraBrown (talk)00:44, 28 April 2020

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