If you deal with well ...
Published by SHLOMO Kramer, Retired Consultant at Shlomo Kramer Irrigation & Fertilization
If you deal with well irrigated soil you can try and use soil solution extraction ceramic cups to sample the soil solution and check the nitrate and other nitrogen component. The best is to do it in a well equipped lab. There ares some field analysing kits that give mainly qualitative results.
If the soil is dry the only practical and reliable way is tacking soil sample to the laboratory.