Google on water quality sonde, there are some out there, probably not to the extent you would like to see.  They are expensive, require calibration from time to time, but potentially useful for some circumstances.  Advantage of on site measurements is don't have to preserve samples, worry about keeping cool, etc.  Global, Insitu and YSI are a few of the companies.  I do not keep fully up to speed on this.  My experience likes with most of these mentioned via flame emission and atomic absorptions spectrophy, the old fashioned way.  It was time consuming and fortunately at the University of Missouri, Professor Pickett, an expert in these analyses in the 1970s took some time to train me properly how to use his equipment, and when we had the funds, we obtained our own.  I carried his work forward by training several students.  But I agree with you, progress with sensors based equipment would be helpful as long as those using sensors continue to mix standards, calibrate equipment at proper frequency, use standardized methods of extraction when appropriate, etc.

There is much interest in this field, to my knowledge starting with ion-selective electrodes in the 1960s. Development of devices with selective responses to different chemical species has continued across a wide range of sensing techniques.  However, the large intrinsic spatial variation in soil properties has made it difficult to avoid the need to take very many measurements in situ, or obtain a representative sample from the field and take it to a laboratory.

The greatest gains have been with IR spectroscopy, but progress has been slow.  The search is on for devices that might be mounted on the front of a tractor and control fertiliser application on the move. 

Gaurav you point out a very important question/suggestion. Some development in this regard is very much required.  

i think the asoption-atomic was almost considered as the one of some significant methods

Thats really a tough ask , there are certain nutrient sensors for in-situ  measurement of nutrients like nitrates, phopshates etc , but again laboratory analysis is considered must to have well defined calibration , and most important of all, we need to bring them into solution to analyse them . This is where our major limitation comes , unlike water analysis..This  is one of the reasons , why proximal sensing of different nutrients started gaining popularity amongst  researchers ...

Here is some more information...

Accurate measurements of soil macronutrients (i.e., nitrogen, phosphorus, and potassium) are needed for efficient agricultural production, including site-specific crop management (SSCM), where fertilizer nutrient application rates are adjusted spatially based on local requirements. Rapid, non-destructive quantification of soil properties, including nutrient levels, has been possible with optical diffuse

reflectance sensing. Another approach, electrochemical sensing based on ion-selective electrodes or ionselective field effect transistors, has been recognized as useful in real-time analysis because of its simplicity, portability, rapid response, and ability to directly measure the analyte with a wide range ofsensitivity. Current sensor developments and related technologies that are applicable to the

measurement of soil macronutrients for SSCM are comprehensively reviewed. Examples of optical andelectrochemical sensors applied in soil analyses are given, while advantages and obstacles to theiradoption are discussed. It is proposed that on-the-go vehicle-based sensing systems have potential forefficiently and rapidly characterizing variability of soil macronutrients within a field. Source : Journal of Environmnetal Engineering .DOI: 10.1039/b906634a..PDF enclosed 

Sir,

your question is pertinent to current technology era. Already we have come across all kind of ion selective electrodes (electrochemical principle) but they gave us approximate values rather than accurate values. It may overcome with sensor based modified new entrants. We should learn a lot about solution chemistry to understand the competition between nutrient ions as well as to design a sensor enabled analyser. 

Nice response Barbara , these are some issues , in-situ determination will impose some  serious interventions , and results will vitiate from the original soil test values..

Some relevant information found, please review...!! 

Dear Gaurav,

I agree with Dr. Paul on the use of ion selective electrodes for measurement of concentration of different nutrients in soil and water.

(1) Ion selective electrodes (also known as specific ion electrodes) are used in analytical chemistry, where measurements of ionic concentration in an aqueous solution are required. Glass membranes are made from an ion-exchange type of glass (silicate or chalcogenide). This type of ion selective electrode has good selectivity, but only for several single-charged cations; mainly H+, Na+, and Ag+.

(2) Electrodes specific for each alkali metal ion, Li+, Na+, K+, Rb+ and Cs+ have been developed. The principle on which these electrodes are based is that the alkali metal ion is encapsulated in a molecular cavity whose size is matched to the size of the ion. For example, an electrode based on Valinomycin may be used for the determination of potassium ion (K+) concentration.

(3) Use of ion selective electrodes has been found to be quite promising for direct measurement of nutrient ions in water. However, interference of many other ions adversely affects the measurement and the accuracy of measurement in soils.