NO3 analysis procedures are subject to different interferences. This may lead wrong measurements. I believe you should Ficcus on the UV/VIA method.

Also, soil extract may contain considerable amount of organic acids that interfere with NO3 methods

@Fernando José da Silva : Thank you sir

Nitrogen is the nutrient most often deficient for crop production and its use can result in substantial economic return for farmers. However, when N inputs to the soil system exceeds crop needs, there is a possibility that excessive amounts of nitrate ( NO3 - N ) may enter either ground or surface water. For more details consult https://extension.umn.edu

@ Sylvester Peter Antai : Thank you sir