@ Linkon, the attached file may be useful to you.

If you want to determine the content of Nitrogen and Phosphorus mixed (Ammophos (NH4)2HPO4) fertilizer, qualitative reactions are carried out. If there is a determination of N and P in the soil or plants, then determination methods are used.

The elemental analysis of fertilizers can be done using inductively coupled plasma atomic emission spectroscopy (ICP-AES), flame atomic absorption spectroscopy (FAAS), and Microwave Plasma Atomic Emission Spectroscopy (MP-AES). However, in your case, I am not sure, but if there are extraction procedures available then why not.

You need to measure the amount of nutrient is an water solution over time. J. C. Trafalgar's reference is one way that simulates what is happening in the soil over time since this is a slow release fertilizer material. Your HACH DR 2400 should be adequate for determining the concentration of nutrients in your water extracts using the appropriate procedures for color development.

As a side note, I've had difficulties measuring nitrate reliably with a Hach spectrophotometer. I assume it has to do with the cadmium reduction step. I haven't used the Hach chromotropic acid method for nitrate, but it might be more reliable. (No references on this; just my thoughts.)

Hi,

I don't understand if you want to measure total P in the solid or the amount available at equilibrium with the solution. Of course, as we are talking about mineral fertilizer at the end all should be solubilize (time and water:soild ratio depended).

If you want the solution, as mentioned above you can use a verity of ways (e.g., ICP and colorimetrically). But, if you want total P in the solid, you should digest the solid before (using concentrated acid).

Additionally, as you are using MINERAL P fertilizer your total P is total mineral P.

Hope it helps.