Dr Tarafdar , your question is dead right  about inching towards SSNM that exploits the spatial variability in soil fertility  and then superimposing the fertilizer  requirement . The biggest question comes , what should be the grid size for soil sampling , and secondly the how best spatial variability in soil fertility variogram , we can use as a decision support for fertilizer recommendation..Here are some of my work on these lines with citrus as test crop...

Pleas e find enclosed another very good review entitled SSNM-based rationale of fertilizer use in perennial crops : A review  published in Indian Journal of Agricultural Sciences 84 (1): 3–17, January 2014/Review Article ( Authors:A K SRIVASTAVA, S N DAS, S K MALHOTRA and KAUSHIK MAJUMDAR)..PDF enclosed for further reading...

Just an addendum to Dr Srivastava's excellent comment: SSNM is basically plant based approached. 

I appreciate the question and initial responses.If it is a plant based approach,how the site-specific nutrient management appraoch takes care of soil variabilty in a given narrow geograpical area or  a farmer field?How it is practiced like precision agriculture or precision nutrient management on individual farmer's field as pointed out by Dr.Tarafdar?

Thank you Dr Srivastava, Dr. Rao and Dr. Dey for your excellent observation. My view is SSNM represents both, it represents a recognition that future gains in productivity and input-use efficiency require soil and crop management technologies that are more knowledge intensive and tailored to the specific characteristics of individual fields.

There is need for soil testing programme to determine the nutrient status for crop requirement so that we can determine the soil critical levels for different nutrients and if it is deficient required the amount of fertilisers will added to the soilsor if it is sufficient there is no need to add more fertilisers. It should be noted that blanket application of fertilisers  without soil test in the specific site destroys soil equilibrium and it is likened to a medical doctor prescribing treatments to a patient without clinical test and use the same prescrisption to treat other patients with similar observations

Thanks for your excellent reply. I think beside this we must consider crop nutrient demand, spatial variability in indigenous nutrient supply as well as cropping system and crop management.

I agree with you Dr Tarafdar. If have to make an effective use of soil fertility variograms, we need to have crop nutrient requirement . And this information has to be soil, cultivar/ variety and desired yield specific....then only SSNM could be effectively use as a part  of precision nutrient management.We also have to  make distinction between nutrient uptake and nutrient removal..

While doing so, there is still diversified opinion about the size of grid to be used for soil sampling, what should be the optimum area of one grid size for soil sampling..since lesser area per grid sample will offer more precision in Soil fertility spatial variogram..

You are absolutely right Dr. Srivastava, the ultimate goal of SSNM is high yield, high fertilizer use efficiency, and providing a locally adopted nutrient best management practice tailored to the field and season specific requirements of a crop. It is a knowledge intensive approach that requires data management to predict soil nutrient supply and plant uptake in absolute terms.

Dr Tarafdar, we often debate , how such spatial soil fertility maps act as decision advisory for fertilizer recommendation for different crops..??? Can a spatial soil fertility map developed for a field ( there is no crop in the field) aid in fertilizer recommendation for different crops..??

Dr. Srivastava, a modified QUEFTS (Quantitative Evaluation of theFertility of Tropical Soils ) model (Janssen et al 1990; Witt et al.1999) is used for this purpose.

But this doesn't work for all crops grown under variety of soil types..??

In a situation of balanced nutrition, the QUEFTS assumes a linear relationship between grain yield and plant nutrient uptake or constant internal efficiencies until yield targets reach about 70-80% of the genetic yield potential. Definitely it has to be tested for all the crop species. I think, following steps are important for SSNM ; establishing a target yield, estimation of nutrient requirement of the crop using QUEFTS ,  estimation of indigenous nutrient supply and computation of fertilizer rates.

Dr Tarafdar , if these are accomplished , i think, the entire work on SSNM  is over..

Thank you all for your excellent feed back.

Please find enclosed an excellent publication on SSNM , real field application...(PDF enclosed)

On-farm trials of site-specific N management for maximum winter oilseed rape (Brassica napus L.) yield

ABSTRACT : Site-specific nitrogen (N) fertilizer strategy based on soil mineral N (Nmin) test is crucial for maintaining high crop yield and high N-use efficiency. A twoyear field experiment was conducted to develop a site-specific N fertilizer management for winter oilseed rape in the 2011–12 and 2012–13 seasons in Wuhan, central China. In contrast to fixed N fertilizer recommendation (FN), the use of the Nmin test could optimize the N fertilizer inputs in time to fulfill crop N uptake during different growth stages and achieve high seed yield. Despite annual variations in seed yields and N fertilizer recommendations, the N recovery efficiency of the site-specific N fertilizer (SN) treatment was higher than that of the FN treatment. Consequently, the soil-based N strategy matches crop N uptake and soil N supply and achieves high yield depending on the site-specific soil-crop conditions. Source : JOURNAL OF PLANT NUTRITION 2017, VOL. 40, NO. 9, 1300–1311,http://dx.doi.org/10.1080/01904167.2016.1264599