water quality sensors

does someone knows phosporus and nitrogen (P and N) sensors for water quality inspection ??

There are some water quality sensors, but I couldn't find anything for measuring phosphorous and nitrogen in water, there are some for measuring them in soil

What do you really want? Nitrate and phosphate? Do you want something you dip to test and put away or something that constantly monitors? Nitrate and phosphate can be done by ISE but the probes are several thousand dollars and won't be suitable for constant monitor unless you're prepared to replace them on a weekly basis or maybe even more often than that depending on what type of water you're measuring.

My friends in the farming community regularly send soil samples for analysis to determine the fertiliser required - but I don't think there are accurate portable techniques.


Not cheap accurate portable techniques at least. You could use a mass-spectrometer, that'd be
accurate, and they make miniature ones to go on space probes to analyse stuff from other planets!
Very expensive. Good article on wikipedia: Miniature mass spectrometer - Wikipedia

I guess you want to measure Nitrates and Phosphates.
There are no cheap sensors for it.

What is the threshold and the range you have to measure?
Are we talking about waste water limits, Drinkable water?