Hi Dimpho Elvis

This is why I am trying to make understand that exchangeable K alone can not be the indicator of K fertility of a soil. Some of exchangeable K extracted  by 1N NH4OAc is not able to go exchange with Ca and Mg of exchange site. This amount of exchangeable K is known as minimum exchangeable K (Kmin) and this amount varies with soil to soil. So, this Kmin of a specific soil should be should be determined before K recommendation. In my study I found that about 80% of exchangeable K is Kmin that means 80% of the exchangeable K (extracted by 1N NH4OAc ) is not available to plant. This amount of exch. K is the minimum requirement of this soil i.e., this is a permanent character of a soil.

Please check the PDF attachments.

Hi Aminul Islam,

Iif you soil pH is less tan 8.0 you  can to use 1M NH4OAc 7.0 pH, However your soils is alkaline you could to use this same method with a change in the pH solution 1M NH4OAc 8.2 pH.

Regards

Anacleto Sosa

In our laboratory we use ammonium lactate-acetate method of Egner-Riehm-Domingo, with an extraction time of 2 hours,  for  potassium determinations of all soils. 

Regards. 

I cannot help with respect to saline soils ,but in tropical soils of low or medium pH, responses  to K addition seem to be as well or better predicted by strong mineral acid extraction as by various ammonium salt extractants.  In Sarawak they used HCl but in the  PDF2 attached by Arvind Singh they used HNO3 

I agree with Dr Ian Baillie view. We also found in our experiment where addition of K has ameliorative effect in calcareous soil with ph above 7.5.

I agree with Dr Ian Baillie view. We also found in our experiment where addition of K has ameliorative effect in calcareous soil with ph above 7.5

In our situation we are finding that K calibration studies are, apart from the extractant,, markedly influenced by soil texture. In very sandy soils vertical K movement can be rapid and subsequent topsoil samples fail to give a true reflection of plant availability. Also, many clay soils appear to have appreciable reserves of K that are available in the medium term. Then, of course, there is the well established effect of K release/fixation on air drying prior to analysis..

  In Western Australia which has very sandy soils vertical K movement can be rapid [Rainfall dependent] and  the topsoil [frequently 0-10 cm] samples often do not give a true indication of plant available K. The Colwell K soil extraction procedure is used widely in Australia.

Very interesting discussion friends. I have some points to toss up.. Is it possible to arrive at some universal extractant(s) for estimating available K ( which can address some part of non-exchangeable K as well ), regardless of crops and soils mineralogy ..? then see calibration response using soil test-crop response relationship to suggest different soil test limits for different crops and  soils differing in their mineralogies..? 

My observation is that soil K assessment with the extractant(s) can not give real picture of the K status. There was no significant change in available K (NH4OAc extracted) due to Long-term omission or addition of K. Quantity-intensity (Q/I) relationship study give details about K fertility status of soil. For instance, K concentration or activity ratio, minimum exchangeable K (K min), labile K, specifically and non-specifically available K, potential buffering capacity of K etc. can be known from Q/I study.

Interesting  response Dr  Aminul , still we need to have an indices using water soluble -K, exchangeable -K , non-exchangeable _K , lattice -K relating crop response  .In the absence of such information , how shall we evaluate soil teat-based K response . You will wonder , in a smectite rich black clay soils, we found  24 different soil test based NH4OAc-K values in 12 months ( we did not take soil samples more than that , otherwise who knows , there will still be many more variations)  , which value truly represents the Q/I relationship   represents the crop response ability..? i agree with you about the extreme sensitivity of NH4OAc-K to even temperature at which soil samples are dried , moisture at which samples are taken ...

Good points Anoop! K soil test calibration is certainly not easy, especially as subsoil reserves below normal sampling depth often play an important role.

Kind regards.

Dr. Aminul- I guess your soil is a part of the Indo-Gangetic alluvium and such soils contains quite a good amount of biotite mica, which is the constant source of K both in soil solution and soil exchange sites. Some experiments in  north-western India IGP soils with rice-wheat  no crop response to added K is observed. even after 25 years' of cropping. In contrast crop response in deep black soils of penninsular India is observed for the first 3-4 years years' cotton cultivation because black soils contain very low amount of biotite. Similarly, in some  red ferruginous of the  southern  India in semi-arid climate crop response to added K is not very common as such soils are also enriched with biotite mica.

Your observation on the crop response to added K even when soils are rich in available K  could be explained with the expected presence of silt and clay vermiculite in your soils. Please check by calculating the percentage of exchangeable K with respect to CEC of the soils. It is expected to be ~ 5-10%, which indicates the presence of vermiculite and is the exclusive mineral to fix/absorb added K. If K is added as basal dose then it prevents the fixing of NH4 ions from getting fixed in the interlayer of vermiculite, which in turn helps in easy assimilation of N by the plants. To prove this you can set up an experiment in the laboratory. Many thanks for sharing your experience, which  will help stimulate good discussion and debate.

Thank you sir Dr. Dilip Kumar Pal for nice and elaborate discussion. For your kind information, my soil is not Indo-Gangetic flood plain alluvium soil. It is terrace soil and there is no scope to replenish the soil K through seasonal flooding or sedimentation. This type if soil is K deficient. The major clay minerals of this soils are mica, kaolinite, interstratified mica-vermiculite-smectite and kaolinite-smectite. Clay percentage about 40% and silt content about 52%. Percentage of exchangeable K with respect to CEC is very low (0.39% at 0-15 cm depth under long-term K omission while it is 0.58% with K fertilization, 0.63% at 15-30 cm depth under K omission while it is 0.78% with K fertilization).

However, my question is about the method of assessment for K fertility status of soil. potassium fertilizer recommendation for rice cultivation based on 1M ammonium acetate (NH4OAc) extractable K status often does not match with crop K requirement and thus gradual soil K depletion is taking place. Likewise, exchangeable K may not be a reliable indicator of K supplying power of a soil under intensive copping in the long run. In this situation, quantity-intensity relationships (Q/I) study can be used to predict K availability to plants in which K+ in the soil solution (an intensity factor

I) is related to the changes in exchangeable K+ (a quantity factor Q). Normally, the Q/I curve describing the exchange of K for calcium (Ca) and magnesium (Mg) is constructed by plotting the change in exchangeable K against the activity ratio of K to Ca and Mg. The Q/I curve represents relatively a permanent characteristic of a soil. This relationship is used to obtain various parameters, including labile K composed of non-specifically available K and specifically available K, K intensity or K activity or equilibrium K concentration ratio, change in solution K, equilibrium exchangeable K, magnitude of conversion of solution K to exchangeable K and potential buffering capacity of K. In correlation studies, these parameters alone or in combination have sometimes been shown to be better prediction of K+ uptake by plants than exchangeable K+ measured by 1M NH4OAc and help in understanding the  mechanism of K+ supply to plants. This type of relationship is supported by many researcher throughout the world have made fertilizer recommendations for different field practices and soil. 

As the other respondents pointed out, the issues concern two parts: The soil and the plant. Traditional soil analysis methods used extractions the soil samples (like ammonium acetate) and analysis of the soluble cations, whose concentration/activity serves as an index to the soil fertility. Two problems: It concerns only soil fertility and not plants, and it doesn't account for interaction with other soluble ions. The buffer power method provides a better tool, since it gives both soil ability to supply the nutrient, and also a plant fertility component of the concentration in the soil solution. Still, when it comes to saline soils, one has to account for interaction among ions, both in the soil component, and in the plant response to potassium concentration, while interacting with the other cations. I am afraid there is no short cut and a single solution like a universal extractant. Unlike in the past, we have the ability to account for all these effects simultaneously, using all the relevant components in the system by putting them in a model, including the specific response of the plant. This approach may provide a comprehensive treatment of the system as a whole.

Dear sir (Moshe Silberbush), in Q/I relationship study exchange of K ion evaluated or estimated with ratio of Ca and Mg ions in the soil.

Potassium concentration ratio (CR) can be calculated in the presence of Ca and Mg as follows:

CR = CKf/(Caf+Mgf)1/2

Where CKf, Caf and Mgf are concentration of K, Ca and Mg in equilibrium solutions

Soil fertility obviously consider the plant fertility. Otherwise, desired yield can not be achieved. This method evaluate some permanent characteristics of soil related to K status.  

Interesting point Dr Aminul. More important is have prediction equation that should relate with K-fertility vis-a-vis yield response at a given soil test value..?  

Good day folks,

I agree fully with the analytical issues (prognostic values of various soil tests), but we cannot ignore the fact that plants utilize K at depths considerably greater than normal sampling depth (15 t0 20cm) or the fact that high intensity sampling --- say 30 to 40 cores per sample --- is a must. This issue is, in my opinion, often ignored. Subsoil sampling is not practical at such an intensity at the farm level and, in my experience, where I come from is not often adhered to even for top soils Theory must match practice.

However, that is not to denigrate the very interesting and valuable research being done.

Regards.to all.

Dear Aminul,

(1) The neutral 1 M NH4OAc which extracts both solution and exchangeable K is the most common soil test method used to develop fertilizer K recommendations. This method does not measure plant-available non-exchangeable K nor the relationships among different pools of soil K. When exchangeable K is depleted to its critical level, further plant K uptake is regulated by the rate of K release from the non-exchageable pool. The inadequacy of the NH4OAc soil test has been clearly demonstrated in illitic and vermiculitic soils. NH4OAc extractable K alone was a poor indicator of K availability to cardamom in kaolinitic soils, unless it is integrated with soil K buffer power. Obviously, NH4OAc extractable K alone is not a reliable estimate of plant-available K.

(2) Richards and Bates (1988) found that Step K (loosely bound non-exchageable K released by repeated extraction with 1 M HNO3) was a more reliable index of K supplying power of Canadian soils than is NH4OAc extractable K.

(3) The sodium tetraphenylboron (NaBPh4) procedure was developed by Smith and Scott (1966) to release interlayer K in soil micas. The NaBPh4 can mimic the action of plant roots by depletion of soil-solution K as KBPh4 and cause further release of exchangeable and non-exchangeable K. The NaBPh4 method allows flexibility as a soil test because the extraction (incubation) time can be varied to alter the amount of K released. Wentworth and Rossi (1972) showed that although the extractability of K by NaBPh4 in some layer-silicate minerals was in the order vermiculite> illte> biotie> phlogopite> muscovite; extractable K in all minerals was well related to K released to barley in the greenhouse.

(4) Plant available K and dry matter yield were well related to NH4OAc extractable K only in soils with low non-exchangeable K contribution, but they were well related to NaBPh4 extractable K in all soils.

Dr.Kundu,I appreciate  your good and pertinent comments.In the recent past lot of developments took place with NaBPh4 method.There are procedures available in recent literature using this method to study K release kinetics,release of fast/rapid and slow release forms of nonexchangeable  K and an estimate of available/exchangeable K.Future lies in use of NaBPh4 method to estimate a part of nonexchangeable K which is plant-available in short to medium term(fast-release nonexchangeable K) in illitic or vermiculite soils.The slow release K may be  important in longterm cropping.The following two papers provide links  to previous work and also new developments with this method.

Rates of release of nonexchangeable potassium in NewZealand soils measured by modified sodium tetraphenyl-boron method. Carey ,P.L. and Metherell,A.K.New Zealand Journal of Agricultural Research,46:3,185-197 Link to this atrticle http://dx.doi.org/10.1080/00288233.2003.9513546

Plant-available potassium assessment with a modified sodium tetraphenylboron method Cox,A.E. et al 1999.Soil Science Society of America Journal 63:902-911.

Sir Annangi Subba Rao, thanks for your important information. I agreed with your comments and remarks. Modified sodium tetraphenyl-boron method is an important way to assess K fertility status in the long run. Thanks you again.

Hi Aminul,

I agree with what some of the researchers have suggested. The problems with the exchangeable soil K test is that it does not account for 1) subsoil K; 2) levels of non exchangeable (slowly available K); and 3) K fixation. Lack crop response could be the results of any of the three aspects I have mentioned. And there lies the insufficiency of the exchangeable soil K test.

 Based on the mineralogy of your soils it is most like that you are getting the contribution from non exchangeable K.

Very good point Dimpho. You remarks well accepted , but when  at a given soil test value , if there is response , soil test limit needs to be revisited in relation to type of soil mineralogy and extractant used..?

Modified sodium tetraphenyl-boron method involves heating in a digestion block on a hot plate at 150 deg C. Similarly use boiling nitric acid also requires heating. Both the methods extract non-exchangeable from the interlayers of micas through diffusion  process. In India,soils under semi-arid tropical (SAT) environment cover large geographical area of active agricultural practice and  are mostly calcareous, non-sodic to sodic in nature, and have low saturated hydraulic conductivity (< 20 mm hr ) in general . In such soils K release may not be a diffusion controlled process as the applicability of diffusion models to K release from micas depends on the  presumption that diffusion gradients are kept constant with time; and this condition is not likely to be fulfilled in SAT soils, where rainfall is low and hydraulic properties of soils is impaired. In SAT soils, K concentration in soil solutions can however 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. As a result, exchange coefficients are going to replace diffusion constants as the rate determining factors of K exchange , and therefore K release may not be a diffusion controlled process in SAT soils. However, to establish the equilibrium concentration of K is only applicable when the reaction periods are sufficiently long. In several K release studies on SAT soils , it was observed that the exchange systems of soils

were fairly close to equilibrium after the reaction period of 3 days. Thus the routine 1N NH OAc method, which does not allow equilibrium to establish, may not represent K value controlled by K-selectivity of mica minerals and also the actual soil K availability.  Therefore amidst the present 'hit and trial' approach, a fresh research initiative is warranted for a selective quantification of biotite mica and

Ca-zeolite (containing also K) in the common situation in soils containing mixtures of biotite, muscovite and Ca-zeolite to pinpoint their selective role in building up soil available K.

Hi Dimpho Elvis

This is why I am trying to make understand that exchangeable K alone can not be the indicator of K fertility of a soil. Some of exchangeable K extracted  by 1N NH4OAc is not able to go exchange with Ca and Mg of exchange site. This amount of exchangeable K is known as minimum exchangeable K (Kmin) and this amount varies with soil to soil. So, this Kmin of a specific soil should be should be determined before K recommendation. In my study I found that about 80% of exchangeable K is Kmin that means 80% of the exchangeable K (extracted by 1N NH4OAc ) is not available to plant. This amount of exch. K is the minimum requirement of this soil i.e., this is a permanent character of a soil.

Dear sir (Dr. A Islam),

Very interesting question as well as discussions.

Soil testing is the most accurate tool available to determine nutrient requirements for a crop. However, unique technique/method of soil test is not equally suitable for all soils. It depends on soil types, soil properties, climate, etc. For example, in case of saline soil, we should have to determine the available K from the extract of saturated paste, I think. So, we should consider the soil conditions, climate, etc. before determining fertilizer requirement or soil fertility status.

 Dr.Islam,you are correct that after attaining minimal exchangeable K level most of the K supply to plants comes from the non-exchangeable  K.Plant removal of K reduces the amount of exchangeable K in the soils until cerain minimal level chacteristic of the soil is reached(in green house exhaustive cropping minimal level of exchangeable K is reached in about 5 harvests of sudan grass),but further K removal results in no further reduction in exchangeable K.Initial K level in soil and % illite in soil determine the level of minmal exchangeable K.After attaining minimal exchangeable K most of the K release or replenishment comes from the nonexchangeable K.Soils with high illite content show high replenishment rates.At minimal exchangeable K the %contribution of nonexchangeabl K varied from 87.1 to 96.5% in green house exhaustive cropping.The following reference may be consulted.

Potassium replenishment capacity of illitic soils at their minimal exchangeable K in relation to clay mineralogy.Srinivasa Rao,Ch. and Khera,M.S. Z.Pflanzenernahr.Bodenk.,157:467-470 1994

Annangi Subba Rao, thanks for important information. This why I always saying that exchangeable K alone is not a reliable indicator of soil k fertility. Non-exchangeable K (NEK) plays a vital role for K fertility in the long run. So we should consider the status of NEK before K recommendation for crop. 

Dear MN Islam, 

Thank you for your thought and observation. You are right in your track. Soil test value always not give the clear picture of nutrient requirement of a crop. It depends so many factors of which some are you mentioned in your answer. We found good response with higher or lower nutrient dose than STB in some specific areas of Bangladesh. For example, K recommendation in saline soil based on soil test value will not work due to the influence of other factors. On the other hand K recommendation based on only exchangeable K without considering other forms of soil K specially non-exchangeable K, minimal K value of a soil, mineral and textural characteristics of soil will not give any conclusive result.

Interesting discussion Dr Islam . I feel , as long as sufficient pool of K is maintained in water soluble and exchangeable phase , non-exchangeable -K will not come into picture , its only when these two K- fractions are at sub-optimum levels ,  then only, non-exchangeable -K will jump for rescue of plant K-nutrition. In that case, i still believe,  ammonium acetate K has fair justice to throw sufficient light on plant available K test value(s).

Dr Anoop you are right and I respect your answer. I like to mention that exchangeable K and solution K seldom changed even under long-term K omission situation which means the release of K from other forms like non-exchangeable K or mineral K. So, long-term omission of K does not change exchangeable K but exhausted the other forms from soil which is not wise. We have to recognize which forms of K become mined from soil, because exchangeable and soluble K remain more or less same under both K fertilized and K omission soil. Potassium fertilization has negligible effect on these two fractions of K in soil.

Agreed , but it will be difficult to monitor K-nutrition exclusively  based on non-exchangeable -K. Soil mineralogy plays a pivotal role in such relation .  Illite /semctite soils resist much against depletion in water soluble/exchangeable-K than kaolinite soils. Very often , such relation is also dictated by potassium to calcium plus magnesium concentration ..

Anoop Sir

We can monitor all the fractions after every 3-5 years of cropping. In our research field we are conducting a long-term K experiment under rice-rice cropping system. After eight years of cropping (16 rice crops) we found that water soluble and exchangeable K fractions remain almost unchanged under both K fertilized and non-fertilized condition while non-exchangeable and mineral K fractions become exhausted due to K omission. On the other hand non-exchangeable and mineral K fractions improved due to K fertilization. 

Please check the K content of the irrigation water if used to grow crops,which do not respond to added K fertilizer.

I think a right time has come to take a serious note over the paradoxical situation of estimating avail K or exchangeable K by an universal extractant like 1N ammonium acetate of pH 7. There is also a dire need to understand what are K minerals that contribute in building up the water soluble and exchangeable K. It is now well known fact that among the soil  K bearing minerals, sand, silt and clay size  biotite micas are mainly responsible  for K release in soils  at ambient temperature  and  pressure.  No role of muscovite mica and K-feldspar  in releasing K in presence of biotite,  is also ascertained. It will be preferable to use these fine-grained  biotites  as K bearing minerals instead of using term ' illites'. Soil clay mica often has poorer crystallinity, lower K-content and higher water  content  than macroscopic micas. These clay-size micas are often referred to as' illites'. However, 'illite' is not be used as a specific mineral name  but as a term indicating that it is a mixed assemblage of minerals with mica as the main component. In addition, soil zeolites (like Ca-zeolites ,especially  in shrink-swell soils of Indian sub-continent and elsewhere) do contribute  to the soluble and exchangeable K build-up in soils. Under such circumstances use of  a mono-valent cation like  ammonium  may not be a prudent approach. Use of some appropriate divalent cation as exchanger  may be a right  choice. For SAT soils there is need to get values at equilibrium  ensuring the K values are obtained based on K-selectivity co-efficient of  biotite and zeolites.

Dear Colleagues, I have just raised this issue based on some interesting research results obtained by us at NBSS& LUP. Please respond, suggest and criticize as to   how best we  together can develop a suitable available K determining method  for SAT soils as a first step. Thanks to all in advance.

Dear Dr.Pal,if I remember well,the issue of use of more appropiate term for illite was raised by my Clay Mineralogy(course) professor Dr.V.A.K Sharma during my Ph.D days at IARI.I fully agree with you that fine- grained biotite mica contributes to K release.Again the biotite may be primary mineral occuring in fine sand and silt sized fractions in addition to clay.As it readily releases K in comparison to muscovite(also primary mineral feldspar),it may be partially weathered especially in clay size fraction.Shall we use the term' clay- sized   partially weathered biotite mica' in place of illite?Or all biotite mica in all size fractions with different degree of weathering?(because the biotite mica in silt and fine -sand fractions can also release K).If I remember well Dr.VAK Sharma used to distinguish between mica and partially weathered mica(illite).My recent survey of literature indicates the important role of wedge zone K.It is also called' intermediate K'  or ' fast release nonexchangeable K'.This fast release nonexchangeable K contributes to K nutrition substantially in soils having micaceous minerals(probably biotite mica).Again the wedge zone participates in K fixation(specifically held and fixed K can also overlap depending extractant used for estimating available  K).What  we need to specify is the K source mineral in different soils,the extractant which can extract the exchangeable K and the extractant which can estimate the fast release  nonexchange K.

Thanks Dr DK Pal and A Subba Rao for in-depth discussion about soil clay mineralogy and K status or availability (fixation and release). I fully agreed that we have to recognize a suitable extracting method for exchangeable K and also for non-exchangeable K fron soil. Because non-exchangeable K plays a vital role in supplying K for wetland rice.

My opinion about muscovite and biotite is that it would better mention their original name (muscovite or biotite) with the degree of weathering or grain size. Actually I am expecting more remarks from you DK Pal and Rao. 

Recently a paper is published in Soil & Tillage Research journal from my PhD work related to evaluated the K fertility status of rice soil under long-term cropping. Please find the paper as attached file..

http://dx.doi.org/10.1016/j.still.2017.02.002