P and K cycles

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?

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?

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.