[am_al_59]

The purpose of measuring N P K Ca Mg S is to determine the amount of material consumed by different types of plants in different periods.
The initial solution, which is prepared as a nutritious solution, includes highly accurate and measured doses of substances such as
Potassium Nitrate, Magnesium Sulphate, Calcium Nitrate, Potassium Monophosphate, Ammonium Nitrate, and other materials that supply the micro-nutrients required by the plants.
The resulting solution is very clear and after two to three days of water treatment, the water dissolved in the water (drainage water) can easily be measured by photometer.
The problem occurs when the initial solution of humic acid and fulvic acid is added. In this case, the water released is very dark and it is not possible to use this water in photometry.
I have searched a lot, but the easiest way is to use photometer and chemical reagents to measure the concentration of the material.
Due to the fact that the error of 10-15% is acceptable in test conditions, it is thought to isolate humic and fulvic acids to clear water without changing the concentration of other materials.

[Borek]

In principle AAS sounds better to me.

[Arkcon]

As was stated already, AAS or ICP-MS, if available, would give better results.  Let's work with what you've provided for us.

The purpose of measuring N P K Ca Mg S is to determine the amount of material consumed by different types of plants in different periods. The initial solution, which is prepared as a nutritious solution, includes highly accurate and measured doses of substances such as Potassium Nitrate, Magnesium Sulphate, Calcium Nitrate, Potassium Monophosphate, Ammonium Nitrate, and other materials that supply the micro-nutrients required by the plants.

This is a clear setup of the solution, and thoroughly describes the composition.  So thank you for that.

The resulting solution is very clear and after two to three days of water treatment, the water dissolved in the water (drainage water) can easily be measured by photometer.

What is the water treatment, and how is it tested with the photometer?  Is there a chemical reaction to generate a chormophore (a colored indicator, even in the UV or near IR.)  This process is not clear: the ions listed give a clear solution.  Offhand, only copper salts or iron salts, present as a trace in nutrient solutions have any visible light absorbance.  And you can't differentiate the small UV absorbance of some salts vs others.  It seems you'd need at least six wet chemical tests for your six elemental compositions you need, and you don't mention those, and I'm not even sure of six separate colormetric elemental reactions.  You've glossed over a great deal of explanation, here.

The problem occurs when the initial solution of humic acid and fulvic acid is added. In this case, the water released is very dark and it is not possible to use this water in photometry.

Yes, those are dark, this bit is clear.

I have searched a lot, but the easiest way is to use photometer and chemical reagents to measure the concentration of the material.

I'm unsure of this method, or its ease, as I described above.

Due to the fact that the error of 10-15% is acceptable in test conditions, it is thought to isolate humic and fulvic acids to clear water without changing the concentration of other materials.

Now this might be possible, some floculating reagents could precipitate the large organic acids.  There my well be some loss of analyte ions.  Its up to you to determine:

• what floculants you can use
• how well they remove most (not likely all) colored organic colloids
• how much ions the floculanted organics take with them (you will have to do a number of trials, with a known solution, and see if your results remain within your specification