Erel Hatan : Not quite an easy task, I guess....

For (semi-)quantitative measurement I'd search for "Neßler" or "Nessler" Reagens / reagent (which interferes but with amines), a sensitive and quantitative analysis I had in mind could be done by the >> 'Berthelot-reaction', after which there is formed a blue Indophenol the concentration of which afterwards can be estimated/calculated by colorimetric anlaysis.

(cf. e.g.: SPINELLI et al,2017: An LC-MS Approach to Quantitative Measurement of Ammonia Isotopologues. OPEN ACCESS -

https://www.nature.com/articles/s41598-017-09993-6

It seems there are some kits available: look for (e.g., out of some possible sources like SIGMA-Aldrich: Reflectoquant® Ammonium; MERCK, etc., etc.):

(Millipore/Supelco)

Ammonium-test

Spectroquant

1.14752.0001

1.14752.0002

(which consists of a determination/assay with a color-chart and a sliding comparator).

Look also for 'Ammonium' in >> Tests in Water- and Food-Analytics.

Finally - Ultimately - your test-method will depend - as always - on your real sample and how you HAVE TO treat or possibly CAN TREAT your sample (intravital vs. texture destroying methods) as there might be available also other special techniques, like IR-spectroscopy with combustion methods combined with microscopy:

BUSIGNY et al, 2004:

Quantitative analysis of ammonium in biotite using infrared spectroscopy

American Mineralogist, Volume 89, pages 1625–1630, 2004

(e.g. cf. https://www.ipgp.fr/~cartigny/2004-Am.Min-Busignyetal.pdf]

Further readings perhaps:

ALI & LOVATT, 1995:

Evaluating Analytical Procedures for Quantifying Ammonium in Leaf Tissue.

https://journals.ashs.org/jashs/view/journals/jashs/120/5/article-p871.xml

J. AMER. SOC. HORT. SCI. 120(5):871-876. 1995., also - in RG - eventually visit: HUSTED et al, 2001:

Article A critical experimental evaluation of methods for determinat...

and more (commercial items, literature e.g. by GOOGLE-search for >> quantitative measurement assay NH4 NO3 ammonium PLANTS SEARCH< [Search for research, journals, people, etc.] insert your keywords-search phrase ==> THEN: Choose: 'PUBLICATIONS/JOURNALS' and / or 'ANSWERS' <

[so how I'd start personally: only on request (:-) )]