Dear Mohamad,

in our daily work the Yara N-Tester ist a very usefully tool: http://yara.com/products_services/fertilizers/support_services/support_tools/  It shows the remaining N-demand [kg ha-1], considering the culture, type and current EC.

We tried a second tool for some months, the "Nitrachek". It is cheaper, but you have to gain sap from plants' parts and the measurement is more complex. Furthermore it worked only, when the soils were not dried completely. You get the actual nitrate-content in the sap, and it's a little bit circumstancial zu calculate the next n-fertilizer apply. Manual attached.

read my review , it would be helpful :https://www.researchgate.net/publication/262188361_A_Review_of_Optical_Methods_for_Assessing_Nitrogen_Contents_During_Rice_Growth

Article A Review of Optical Methods for Assessing Nitrogen Contents ...

Thank you so much dear Harald

Actually I am looking for a satellite sensors. Some researcher work on multispectral remote sensing but I want to know which bands are useful in this case. Do you think this project  can be useful for industrial because all of the goal of this project is to reduce the cost of sampling and labours and optimizing fertilizers like nitrogen 

The NIR bands used in some of the spectral ground based instruments may be a start for this - the GIS  databases  may be useful to research, and the soil chemistry department at North Carolina State University (NCSU, USA) has some experience in remote sensing for N and P, I think that major issues may be satellite resolution and time elements, and of course it will work best if there are unequivocal ground truthing measures to correlate against..

NCSU has begun very good work on resolving these issues with UAV tech, that will put the technology of remote sensing nutrient levels in the arsenal of the progressive farmer who is equipped to do precision agriculture. It is obvious that resolution and real-time sensing parameters are improved for this.

Because flying UAV tech currently has energy limitations, groundbased robotics based UV spectral sensing may be a superior method that can have the energy density, to support a more robust sensor package and other farm chores associated with pest control on the same vehicle.

Thank you so much

Dear Mr. Nelson

Do you know which sensor and  bands  can be useful for using Satellite or UAV in this case? In addition, I want to find the best vegetation indices because of this I have to use a aerial vehicles like satellite or UAV. 

Thank you so much dear Mehdi

That would be very useful to me