Vikki, very nice question . Yes , potassium exchange capacity (KEC)  as  exchangeable potassium on CEC , could be effectively taken as one of the indices of K-availability in soil, in addition to NH4OAc-K  .  Potassium exchange capacity of 3.10 cmol means , it is the potentially this much K is expected to be available for uptake by the plant . How much of it will be  taken up by the plant,  will surely depend upon microenvironment of the rhizosphere including the mineralogy and moisture availability .Regarding your sub-part of question about KEC  value as per months of a year , we have seen a huge variation ( Maximum being in those months receiving maximum rainfall and minimum during the drier months, so positively related to soil moisture  )  in NH4OAc-K  in smectite- rich black clay soils, depending upon the months of the year , irrespective of K-fertilization. In such soils , KEC infact  described  the K-availability , better than NH4OAc-K

Vikki, very nice question . Yes , potassium exchange capacity (KEC)  as  exchangeable potassium on CEC , could be effectively taken as one of the indices of K-availability in soil, in addition to NH4OAc-K  .  Potassium exchange capacity of 3.10 cmol means , it is the potentially this much K is expected to be available for uptake by the plant . How much of it will be  taken up by the plant,  will surely depend upon microenvironment of the rhizosphere including the mineralogy and moisture availability .Regarding your sub-part of question about KEC  value as per months of a year , we have seen a huge variation ( Maximum being in those months receiving maximum rainfall and minimum during the drier months, so positively related to soil moisture  )  in NH4OAc-K  in smectite- rich black clay soils, depending upon the months of the year , irrespective of K-fertilization. In such soils , KEC infact  described  the K-availability , better than NH4OAc-K

Mr.Wong,The term "available potassium" conventionally refers to exchangeable+water soluble K.The exchangeable K constitutes the major portion of available K,except in saline or sodic soils.We call it simply exchangeable K ,not potassium exchange capacity.Exchangeable K we estimate by ammonium acetate method either by shaking and filtration( Schollenberger and Simon 1945) or shaking and centrifugation(Knudsen et al.1982) methods.When exchangeable potassium is depleted by crop plants it will be replenished from the nonexchangeable K reserves in soil .The amount of nonexchangeable potassium in soil depends on the amount of micaceous minerals in a particular soil.Even the amount of exchangeable K depends on the mineralogy of soil,it will be more in smectitic clay dominant soils followed by illitic, kaolinitic mineral dominant soils.The presence of more amount of humus in soils increases the cation exchange capacity and also help to hold K along with other cations.

1) The value of 3.10 cmol/kg will represent only a proportion of potassium available (K held on exchange sites of soil) for plants. There is always a certain proportion of potassium present in the soil solution which is available for plant uptake. Therefore, theoretically 'available potassium' is the sum of exchangeable K and soil solution K. In practical, depending upon the clay mineralogy, even the non-exchangeable K can play a significant role in K nutrition of plants (Romheld and Kirby, 2010)

2) Soil K test are most likely to show lower K values in July as compared to May. (We observed huge variation between soil K levels measured in April and September's soil samples, however no appreciable differnnces were observed between May and July ). One of the main reason for this reduction will be plant uptake. Further, the extent of such reduction is dependent on various soil parameters such as the soil texture, clay mineralogy, soil moisture. The fluctuations in soil moisture can lead to scrolling up or collapse of clay lattice structure, resulting in release or entrapment of K ions. For more  details on temporal variation in soil K test results and the mechanism involved for such variations, you can refer to following book chapter and articles:

Sparks, D.L. and Huang, P.M. (1985) Physical Chemistry of Soil Potassium. In: Munson, R.D., Ed., Potassium in

Agriculture, ASA, CSSA and SSSA, Madison, 201-265.

Murrell, T.S. (2011) Why Are Soil Test Potassium Levels So Variable over Time in the Corn Belt?

http://fyi.uwex.edu/discoveryfarms/files/2011/06/murrell-k-variability.pdf

Mallarino, A.P., Barbagelata, P.A., Clover, M.W., Villavicencio, C.X. and Thompson, L.B. (2011) Factors Determining

High Temporal Soil-Test Potassium Variation and Soil Sampling and Testing Alternatives. Proceedings of the

North Central Region Soil and Plant Analyst Workshop, Bettendorf, IA, 23-24 February 2011, 3-14.

Romheld, V. and Kirby, E.A. 2010. Research on potassium in agriculture: needs and prospects. Plant and Soil. 335: 155-180.

Dear Vikky:

there are many different methods to calculate the ammount of one nutrient in the soil. Each method gives different results more or less useful depending on the purpose. For agronomic purposes is better know the available nutrient for the plant tan the total content of that element in the soil. Changeable K data is better tan total K. but there are other methods to determine the availability of nutrients.

On this topic you can browse my publications and many others related.

The straight away answer to the question is "No", because the soil test value is an index of K availability potential of the soil.

I Agree with Dr, Rashid

As mentioned by me in my reply that the use of the term potassium exchange capacity in this context is not appropriate .What Mr .Wong was referring to is exchangeable potassium which is plant-, available.If one desires to estimate potassium exchange capacity of soil,  he has to first saturate the exchange  sites of the soil with potassium ,remove the excess potassium salt  added to saturate(can be KCl)and replace it with another cation like Ca  and then estimate K in the extract/ leach ate and express it as  potassium exchange capacity.It should be similar to cation exchange capacity. Exchangeable K can change with season based on crop uptake or temperature  and moisture regime changes but potassium exchange capacity of soil may not change.

For calcareous, alkaline soils having mixed clay mineralogy, amonium acetate extraction is the best.

Abdul Rasid's comment is true but you might like a bit more information.  No soil test is strongly predictive of plant uptake, except perhaps nitrate in environments where it will not leach or denitrify.  Returning to the K question, regardless of questions about different K pools, you need to calculate whether the plants have removed an amount of K from the soil that will make a measurable decrease in the concentration of (say) exchangeable K.  For this sum need the amount of K removed,  the exchangeable K concentration and the soil bulk density.

        This is one of the most challenging question to soil scientists. The concept of "Plant available nutrients" lack adequate scientific foundation. In its absence, we have little choice, but to rely on one or other extraction techniques (mostly chemical, some are instrumental, some are resin-extraction) and correlating it with plant growth, especially yield parameters.

      For potassium, make-up and state of soil minerals could be deterministic aspect of predicting (or, linking) crop yield. In mica (illite in clay) dominated soils of Punjab (India), ammonium acetate extraction is routinely used for estimation of available-K, although in many such soils boiling nitric acid method could be a better option. Across the border, in the soils of Punjab (Pakistan) other chemical extraction methods are followed, although their soils are remarkably similar to the soils of Punjab (India).

       Hope you take care of the intricacies and challenges of K-availability in your research program and come-up with a long-awaited breakthrough in this area!

Siddhartha S Mukhopadhyay  

Although feedback on the question put forward by Vikki Wong has calmed down in the meantime, allow me to add some general thoughts:

Virtually all soil K tests that are in use today around the globe (and there are many different ones!) measure a certain pool size (capacity). It is good practice to put the term ‘plant available K’ always in quotes, or to avoid it altogether, as otherwise it suggests that there is a single fraction (number) that describes plant-available soil K, while there is no such thing! This is obvious from the fact that different extractants give different numbers, although they all claim to indicate the same.

In this regard Abdul Rashid correctly stated that the soil test value is an index (!) of K availability potential of the soil. Let’s pick up Paul Milham’s comment and calculate to what extent a typical removal figure (a 6 t wheat crop accumulates around 100 kg K2O per hectar) is going to change the soil test reading of 3.1 cmol/kg (= 1.209 g K/kg soil). Assuming a depth of the rooting zone of 30 cm, and a soil density of 1.5 kg/L, we have around 4.5 million kg soil per hectar. This gives 6,529 kg/ha K2O. Removal of 100 kg K2O would lower that to 6,429 kg/ha or by 2% only! So the soil test reading can be expected to come down from 3.1 to 3.05 cmol/kg. In view of the uncertainties in sampling, sample handling and analysing, that theoretical change is close to nothing.

The value as such should not interpreted. While NH4OAc-extractable K might be considered the maximum potential available for agricultural crops, it depends on several factors (soil factors: soil structure [deliberately destroyed in standard soil test procedures], soil moisture, buffering capacity; plant factors: root surface area, root architecture, root morphology, surface charge, transpiration affecting mass flow). Soil moisture is particularly important, as it has far reaching consequences for the diffusion of potassium toward the root surface. It’s a well-known phenomenon that the same soil, in a dry year, shows response to K application, while the same soil shows no response in a wet year. So, we would need to consider these many factors when converting the K potential into the K available for uptake in a given situation – to determine the K intensity.

From a plant nutrition perspective plant-available is what is taken up by the plant. The confusion all started when soil scientists proposed to come up with a single – and simple – soil extractant to describe a complex property like ‘plant-available soil K’.

Ion exchange resin, when used in situ, integrate many of the soil factors affecting K flux in the soil, particularly soil moisture. But does it really help? Typically, even in advanced agriculture, soil test results are considered valid for several years and could be used for 3-5 years. When sampling the soil in situ in a dry year, how to account for better K diffusion flux in a wet year? The concept of ‘buffering’ has been used for phosphorus to some extent, and was shown to significantly improve the quality of soil test results. I’m not aware of similar work on K.

I feel that we have already a good understanding of the various factors affecting soil K dynamics. But incorporating these in a comprehensive soil test procedure, considering capacity and intensity factors, would yield such a cumbersome procedure that no extension service would ever think of applying the same. So, in my view it’s important to always be aware of the limitations of the concept, and to use the exchangeable soil potassium fraction as an index of the K delivering capacity, and not as a correct measure for plant-available soil K.

Dear Dr.Gerendas,I appreciate your good and analytical reply.For over hundred yours we believed in extraction procedures and we are continuing with that approach.I raised the issue in another question"How best we estimate plant-available soil nutrients?"in the same forum.Whatever knowledge we gained through the concepts like nutrient potential ,buffering capacity,nutrient movement in soil-plant system ,rhizosphere  nutrient changes and transformations,resins and sensors etc.we did not bring in big way to soil testing.It is undisputed that plant absorbs K from soil solution.Soil solution K is very small(a few kgs) so it has to get replenishment  from exchangeable K.How much plant draws from exchangeable depends on its requirement.Depending on change in solution K and exchangeable K due K uptake , the non- exchangeable K  replenish the exchangeable K.So the changes in exchangeable K are masked.Exchangeable K may also get replenishment from irrigation water and addition of roots and stubble.Roots bring nutrients from lower layers and deposit in the surface layer.Also the the changes  in ex.K depends on soil mineralogy.In micaceous / illitic alluvial soils the change may not be conspicuous as the  non exchangeable K replenishes the ex.K.The changes in kaolinitic red and lateritic soils  are more  visible as they have very low micaceous minerals.In smectitic swell-shrink soils with high cation exchange capacity ,the ex.K content will be high and changes again depend on size of uptake and associated micaceous minerals.Also the soil exchangeable K is maintained at a certain minimum level at expense of nonexchangeable. So it is also desired to measure non -exchangeable K in soil. So there is no doubt that plant draws K from exchangeable pool but the changes are perceptible or not depends on several  factors as mentioned above.Dr.Gerendas calculated changes for 5 cm soil depth but plant can draw from 100 cm depth.We are not testing changes in 100 cm depth.I have seen a paper on K buffering in contrasting soils having similar amounts of exchangeable K. Beegle,D.B.and Baker,D.E.(1987)Diffferential potassium buffer buffer behaviour of individual soils related to potassium corrective treatments.Commun.in  Soil Sci.Plant Anal.,18(4),371-385.