Probably because you didn't use a blank sample, generally it adjusts the negative values and resolve contamination risks.

See attached file

Khalid

NO3-N (in clear solution) would absorb UV at 210 nm.,  Be cautioned: some soluble organic molecules also absorb at this wavelength, so if possible use the second derivative mode of your UV-VIS spectrometer.

Nitrate, ammonium and phosphate can all by measured by UV-Vis spectrophotometer.  I don't have the text by Jones so cannot comment on the specific methods you're using. Methods that work include

Cataldo et al. (1975) Rapid colorimetric determination of nitrate in plant tissues by nitration of salicylic acid. Commun. Soil Science and Plant Analysis 6(1) 71-80

Baethgen WE, Alley MM (1989) A manual colorimetric procedure for measuring ammonium nitrogen in soil and plant Kjeldahl digests.  Commun. Soil Sci. Plant Anal. 20 (9&10) 961-969

Murphy J, Riley J (1962) A modified single solution method for the determination of phosphate in natural waters. Anal. Chim. Acta. 27: 31-

For nitrate you can also use the method of Miranda et al:

Katrina M. Miranda, Michael G. Espey, and David A. Wink (2001) A Rapid, Simple Spectrophotometric Method for Simultaneous Detection of Nitrate and Nitrite. NITRIC OXIDE: Biology and Chemistry. Vol. 5, No. 1, pp. 62–71 

Before using these methods you need to check they work with your samples. As mentioned above, you need to run multiple blanks and also ensure that your standards are prepared in the same matrix as your samples.  It's also a good idea to test accuracy and precision. 

Test accuracy by creating a standard curve by serial dilution of a sample and/or via spike and recovery tests.  Both tests will show if the analysis is affected by the sample matrix. 

Test precision by repeated analysis of the same sample.  It’s best to do separate precision tests for the analytical method (replicate analyses of the same extract) and for the entire extraction and analysis procedure (extract the same sample several times and carry each extract through the analysis procedure). 

You can also try the dual wavelength UV method described in this paper:

Article Assessing the Potential for Ion Selective Electrodes and Dua...

Thank you for all of this information. I will go through all of the suggested papers and hopefully I can come up with something that will work. I appreciate your input.

hello,I try visible methode for measuring nitrate with sulfaniluc acide and N-(1-naphtyle ethylendiamine hydrochloric acide and zink but I didnt get the same adsorbtion number with the same concentration

Hello ,you can measure notrate at 220 and 275 nm . If your adsorbance in 275 nm is bigger than 10% of adsorbance at 225  nm this method isnt useful . But for measuring nitrate with this methode you can calculate the concentration according to:

Measure at 220nm - measure at 275 nm = your adsorbance 

no you cant measure nitrate

without cadmium reduction column