Thanks for your reply, Sir.. i understand the 'chemistry' point and i agree with that. but what i am thinking that can we make stand alone device that can do automatically, just put your device in soil and everything done and got our reading on display device. because i have to do the chemical process manually.
and according to my aim, i don't have to go to every place for just measuring the NPK values of soil, i want every reading on my display.