Currently we are using 1M boiling nitric acid but are looking in non destructive spectroscopic techniques

The old methods I have seen use cation exchange resins in the H+ form.  I have seen little evidence relating the test values to plant performance.  However, this is not a field I read actively in.

Quantity Intensity Relationship of K can used for nonechangeable K determination.

It is not in place right now to follow. It is a challenging task and thus requires a fresh research initiative.

The best approach of non exchangeable K-forms efficiently available for plants, are the amounts of water-soluble –K forms; particularly for high demanding plants in potassium during the critical periods, the estimations of the soil contents in water soluble –K, are precious data; furthermore, in high fertilized cultivations as greenhouses, the establishing of correlation data between the electrical conductivity of water soluble extract of the soils and the reciprocal contents of these extracts in water soluble amounts in K and N are efficiently valuable, for a long term period.

Would anyone please explain how boiling nitric acid  acts on K minerals like K-feldspars and micas? Any mineral acid will attack the mineral structure to release  K.How such enhanced K release amount  could be related to soil K-availability while K concentration in soil solutions can  be expected to approach equilibrium levels, which will depend primarily on the K selectivity of the mica mineral, and this remains as the determining factor for K equilibrium concentration of the solution phase?

 I thank Drs. Elephant,Paul,Das,Pal,and Nikolaos for their good responses.Dr.Paul,as for your information and also based my review on the subject several times(please see my publication list),the boiling1N nitric acid method is the most popular method which received world wide attention as a measure of nonexchangeable K in soil.An associated method which requires several extractions using the same reagent is' step K'.Dr.Paul,the resin method and NaTPB(sodium tetra phenyl boron) method were also much investigated methods for assessing the non-exch. K. Na-TPB is a costly reagent for routine use.Resin method is also costly and cumbersome.Dilute  salt solutions(0.01MCaCl2) and dilute acids(0.01M HCl) need several extractions for K release.The Q/I parameters,the labile K, the equilibrium potassium concentration and buffering power essentially measure the exchangeable K,solution K and the rate of K replenishment from exchange/solid phase against depletion in solution K.Water soluble K may serve as a good measure of available K in soils under regular K fertilization.When exchangeable K reaches certain minimum level due to continuous cropping without K fertilization, most of the K supply to plant comes from non -exchangeable K.Unless non-exchangeable K is estimated ,we can not know what is happening to soil potassium reserves.

Exhaustive study on the same soil is needed followed by x-ray diffraction studies to be conducted in order to find any change in the mineral structures, like mica. Q/I has to be performed in all the sequence of gradual depletion of K.

It is well known that mineral acid with highly acidic pH would affect structure of mica, which will be revealed better under scanning electron microscope.

I agree with you Dr. Pal Sir

Dear colleagues, I am providing here under one reference of Dr Donald L.Sparks of University of Delaware on potassium where merits and demerits of different methods were discussed.Bio-availability of potassium

www.udel.edu/soilchem/sparks99hss.pdf

Dr.(Ms)Debjani,good suggestions.Please check your last but one  sentence.Is the ratio nonexchangeableK/exchangeable K ,instead of exchangeable K/nonexchangeable K?

Dr.Pal, are there any studies linking the suggestions made by Dr.Das and your self?

Dear Dr. ASR,

I  am still hunting the basic principle of  nitric acid use;. What happens if we use  HCl or sulphuric acid at low to very low concentration? We need to know why nitric acid was preferred to other mineral acids.  In the original paper possibly it does not explain. I think we need to think out of  the box  regarding the use of this  extensively followed method.I am attaching one of my paper dealt not directly on this issue but provides enogh clues as to how this non-exchangeable K needs to researched further.

I FOund some literature regarding the extraction/estimation of non exchangeable k in the " hand book of soil science by Malcolm E Sumners" page 11-42 & 43. But no explanation given elaborately about using the extractant.

Dear Indranil,

                      I very much appreciate your immediate response on this issue. It is high time that we stop to remain slave to any method that lacks good science background. Simply finding good correlation with  nitric acid extracted K and crop response may not satisfy anymore the young talents like you all.For our soils we need to develop methods that suit  Indian tropical soils, especially the SAT soils, which occupy > 50% of the total geographical area of the country.

Thank You Dr. Pal Sir. I have gone through a paper "A comparison of extraction methods to asses potassium availability" authored by "Timtong Darunsontaya et al. 2010" where the authors species that the K release by using HNO3 was more compared to NaTPB extraction which according to them was due to destructive dissolution of silicate minerals using HNO3, while NaTPB extracts only structural k from interlayer sites in micaceous minerals.

 Dear Indranil,

                        K release value from both the methods you have mentioned are far from real soil environment. Acidity of nitric acid does not exit in soils and NaTPB  method requires high temperature, which also do not exit in soils.Soil K release phenomenon needs to be understood at normal and pressure situation i.e., at ambient temperature and pressure. So it calls for an intense discussion among the working scientists  to  come up with a pragmatic method for soils of India.It needs a fresh research initiative.

Yes I agree with you Sir. I feel sequential method may be helpful. Sequentially if possible the exchangeable pools of k needed to be gradually depleted so that the contribution from non exchangeable pool gradually takes place to meet the equilibrium. After each time extraction XRD or SEM may be used to find any structural changes takes place by subsequent extraction. A similar study using plant as a medium for exchangeable k uptake and the similar ways and means to be followed in order to find a real situation for non exchangeable pool determination.

I am very much appreciating in generally sequential extractions.

I think we are deviating  from the purpose of our discussion.We  want a good method at the same time simple and adoptable for the soil testing laboratories.We published a lot  of K release studies using dilute acids,organic acids,dilute salt solutions under short term,long term and exhaustive cropping experiments.Many times good relationships were found between crop K uptake  and nonexchangeable K (HNO3 K).Also, between K release parameters and HNO3 K.Please see the publication lists of Ch.Srinivasa Rao and myself.I can also provide other sources of information also on this aspect.No body can force us to use only the HNO3 method but let us suggest what alternative methods are there and what additional merits they have for use in soil testing laboratories.

It was mentioned earlier that  an alternative method is to be worked out by us. Thus,I think we were not deviating from this purpose. Instead we need to look for a  firm science based solution. So discussion is the utmost necessity in such endeavour.

I agree with Dr. Pal Sir

Dear colleagues,My only concern is from the lot of research work  already done on potassium especially on nonexchangeable K in relation to crop nutrient uptake and response of crops, can we suggest a method for soil testing purpose.I am furnishing below some reference which can be accessed on internet for the benefit viewers and followers of this question. 

Potassium in the soils of New Zealand -A review by J.H. Kirkman and others  New Zealand Journal of Agricultural Research 37:2,207-227, 1994 www.tandfoline.com/doi/pdf/10.1080/00288233.1994.9513059

Forms of K as a function of clay mineralogy and soil development.by K.Nabiollahy and others Clay Minerals (2006)41,739-749scientificfinding.gau.ac.ir/....gau.../2006_Nabiollahi_Clay_Minerals_pdf

Effect of clay minerals and physicochemical properties of potassium availability under soil aquic conditions.by A.Raheb and A.Heidari Journal of Soil Science and Plant Nutrition.(2012)12(4,)747-761

www.scielo.cl/pdf/jsspn/v12n4/aop2912.pdf

Dr Rao, if I may ask, how does nitric-k compare with exhaustive cropping using ryegrass?

We have tried to find correlation between routinely measured soil parameters and nitric-k in order to simplify this for routine testing but correlations were low.

See the attached paper by Miles and Farina  

Dr.Elephant, I appreciate your questions. I have not worked with rye grass as test crop in exhaustive cropping but in one study we used sorghum and in another study several  crops(one month cropping with each crop).Important point is we should intensively exploit the soil K reserves through cropping. We will get good positive relationships between cumulative K uptake and  a measure of nonexchangeable K.If the cumulative K uptake is less due to cropping, we may get good relationship with exchangeable K.Also if the exchangeable K content of the soil is high we may not get  good relationship with nonexchangeable K.I have seen the paper attached by you.Sugarcane is long duration crop.Its daily requirement of K may not be high and crop can get it by slow replenishment from nonexchangeable K reserves.So it is necessary to monitor the nonexchangeable K in African soils.Also irrigation water also contains some K and the crop can readily use it.This also needs to be quantified.Please go through the publication list of my colleague Dr.Ch.Srinivasa Rao in Researchgate to get details of our work on exhaustive cropping and long term field experiments.

Dear Dr.Elephant,I missed one point. It is difficult to predict the  nonexchangeable K from from parameters like extractable Si ,volume weight, organic carbon and exchangeable K.Soil texture  and mineralogy(especially biotite mica,vermiculite and illite) determine the nonexchangeable K.Standardization of a spectroscopic method is good attempt.

Dr Rao thank you for your answer. It seems to me like mineralogy influences the behavior of potassium more than other measured soil parameters

Dr.Elephant, your  are right. Nonexchangeable K is determined by mineralogy of soil.Any method like the NaTPB method which determines the interlayer K is the  best. Even micaecious mineral mapping may help to demarcate geographical areas endowed with good K reserve and poor areas.Then K poor areas like red and lateritic soils may need greater attention from potassium management point of view.

There is never a "best" method Annangi! You may call it the most suitable or else. The criteria to select a method is in my opinion if you have eufficient data reported with the same method to compare, interprete and discuss your own findings!

Dr.Schnug, Thank you for your frank opinion. Agree that the NaTPB is not well tested soil test method for nonexchangeable K estimation in soil testing laboratories. As I understand the NaTPB and H-resin methods are based on interlayer K precipitation and displacement,respectively. As they are based on mineral K which is in slow equilibrium with exchangeable K,I think they may better reflect the long term K supplying power of soils.We need a method well tested method  for the purpose. From the literature available from different parts of world,the HNO3 method is somewhat well tested but the limitation is its chemistry of extraction and its relationship with minerals like biotite mica,vermiculite and illite and  the nature and extent of release of K  from those minerals.But limited information is available on this aspect and I have given some references in my earlier reply.

The discussions held so far it appears that for Indian tropical soils a fresh research initiative becomes the only option left.

Dear Ms.Debjani,you are right.We  can not use Q/I  relations or release studies in soil testing laboratories. In soil testing good old method(developed more than 50 years back) based on dilute acids,weak acids,dilute salt solutions, water etc.are being used in developed and developing countries.For estimating nonexchangeable K also methods are available based on strong acids,weak acids,dilute salt solutions and resins. To displace interlayer K, one has to use H ions from strong acids.Among them,1MHNO3 and 3M H2SO4 are popular.0.5MHNO3 was also tested.With acids we have two problems in soil testing labs-handling strong acids safely and boiling the acid  soil suspensions. We want a reagent which can displace interlayer K and extract it.NaTPB can extract inter layer K but it precipitates K which has to be solubilized again. The reagent is also costly.I believe that a good review of the subject is needed before fresh attempts as suggested by Dr.Pal.

Can we find a way to initiate such debate and discussions to find a good science based solution? Any sponsor for such endeavour? I shall wait for an opportunity in this regard.

Dear Dr.Pal,good suggestion. Let us explore the opportunity for discussion.

An initiative through a project mode involving young researchers and Ph.D students located at SAUs and ICAR NRM institute may be a good possibility. Dept of S& T, GOI  generally encourages such project when we can show how the burden of K fertilizer import could be lessened.

If a known concentration of NaTPB used to extract interlayer K, we can use it in excess so that unreacted NaTPB can be titrated  with Zephiran chloride (ZCl) to get the actual concentration used for extracting K, hence K concentration can be calculated. To approximate the amount of NaTPB, data on nonex_K by using HNO3 may be taken into consideration.