Units for N and P input (and output)

I know the documentation talks about kg/ha/yr for nitrogen and phosphorus input. Consistent with the NPK labelling, I’m treating N as elemental nitrogen. This makes sense as it could be present in the ammonium form or nitrate form, but key is it being converted to soluble nitrate in the soil. P, however, in NPK labelling is the mass of P2O5. I’m assuming it doesn’t matter whether I convert to elemental P weight in the input, so long as I’m consistent in how I then interpret the exported mass of P. Is this right? There isn’t anything in the model that requires the input to be either one of elemental P or pentoxide P?

Thanks for the post @gtmaskall,

This is a bit in the weeds for me to take on but I’ll reach out to our science leads. Might be a little slow in getting a response as we head into the holiday.



Hi @gtmaskall -

Correct, there is not anything in the model that requires the input to be elemental P or pentoxide P. Practically speaking, the model could take any compound that you care to model, as long as it moves with water.

That said, P often moves more with sediment than water. In a more complex model, this would probably involve separate calculations, but our simple NDR model is very similar to our SDR model, so there are a lot of assumptions made there, but it allows us to lump N with P for nutrient movement.

~ Stacie


I’d been very vaguely mulling this over. So I guess if you’ve got something that moves in the water, it can have a subsurface component so you could model it as “N like”. If something just moves with water (like sediment) then it’s “P like”?

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