Plants deficient in potassium shows chlorosis (loss of green colour) along the leaf margins or tips starting with the bottom leaves and progressing up the plant. Other symptoms are stunted plants with small branches and little vigour

High levels of potassium (above 10%) will leads to calcium, boron, manganese and magnesium deficiencies. pH will also be high, due to the excessive amounts of sodium and potassium on the soil particles or ‘colloids’. Potassium uptake will be restricted as sodium is more abundant than potassium.

A soil pH range of 6.2 to 6.8 is generally considered as ideal. Soil pH becomes self-adjusting when the important cations - calcium, magnesium, potassium and sodium are in proper equilibrium. Thus it would be better to say that pH is the result, not the cause, of nutrient interactions and levels in the soil.

Potassium

Potassium is essential for healthy tree growth, fruit size and fruit quality. Ideal soil saturation of potassium can vary from 4-7% depending on the crop type. Woody plants require extra K to produce new wood. Most crops have a strong demand for potassium; to assist the translocation of sugars and starches to fruit, promoting size, stomata regulation (guard cells) and respiration of plants, controlling cell water pressure and for fruit colour.

The majority of the local soil test reports indicates adequate or excess levels of potassium in soil. However, if potassium application becomes essential it can be supplied through potassium sulphate or manures, which contain modest levels of K and is handy in high pH sands. When the manure is decomposing the potassium is released slowly into the soil and is not easily leached like other potassium forms (Potassium nitrate, Potassium sulphate) Grape marc and wood ash contain good levels of potassium, but should be used sparingly or an imbalance with other cations can occur and lock-ups can result. Sul-po-mag (sulphur, potassium and magnesium) is a good product if all three elements are required.

Excess potassium will affect magnesium uptake and soils in the pH range between 7.5- 8.5 restrict potassium uptake via plant roots, even though it may be far in excess. Many local crops require spraying of potassium to size tomatoes, capsicums and cucumbers due to the soil pH level, in many circumstances even though potassium may be above 10% saturation.

When potassium and sodium are in excess (>10% combined saturation) then manganese, boron, magnesium, phosphorus and calcium can be restricted severely as well as potassium. Sodium saturation above potassium is not ideal, especially in hot conditions, as sodium is taken up more readily and can cause plant cell breakdown.

If soil potassium is above 7.5% herbicides may not perform as effectively. Weed pressure is usually higher with excess potassium in the soil.

Ideal potassium level improves colour, size and firmness of fruits and also new wood development.

Hi Anil,

Thanks for your answer.

Is there a need to take into account fixation of added fertilizer K in fertilizer recommendations?

Mr.Elephant,please go through the link in researchgate pertaining to your question.

https://www.researchgate.net/post/What_is_the_best_method_to_estimate_nonexchangeable_potassium_in_soil_

Hi Dr Rao,

Thank you for your response. I have been following that discussion.

What I was hoping to achieve here was the philosophy with which we approach soil K testing. Two issues are outstanding as far as I am concerned. 1. the contribution from the non-exchangeable K which the discussion you raised attempts to address. 2. the irreversible fixation of K which is hardly addressed.

If our recommendations are to be cost-effective and fertilizer application more efficient than these two issues need to be properly addressed.  

Dear Dimpho Elvis Elephant,

Wet and dry condition of soil will influences the exchangeable Soil K. Hence it is essential to assess the exchangeable soil K with field moist sample (sieved 2mm) with prior drying and processing. It will give you clear idea of exchangeable soil K. It is clearly mentioned  in the potassium paradox article. 

Dear Dr.Elephant,What is the source of information on irreversible fixation of potassium?As I understand, the K fixed by illitic minerals(partially weathered mica)is reversible and available to crop uptake.Scientists reported 60 to 90% of K supply from nonexchangeable sources under exhaustive cropping without manure or fertilizer K application. Under continuous cropping without K  addition externally,the exchangeable K in soil  reachs certain minimum value,the so called minimal exchangeable K. Under  those conditions, most of K supply to plants come from nonexchangeable source.But the crop plants may be stressed for their K requirement.However, there are reports of some what strong fixation of K by vermiculite mineral in vertisols. When inter-layer K is exhausted in illitic or vermiculite mineral- rich soils.,the fertilizer K requirement will increase but one need not saturate all the K fixation sites with one time K application.Even in long term experiments,application of lesser amount of K than the actual K requirement of crop,the crop is meeting its requirement with partial K supply from nonexchangeable source. The subsoil K supply,crop  residue K addition and irrigation (especially ground water)  water  supply all these need to be taken into consideration.

Hi Dr Rao,

I cant remember where I read about irreversible fixation. But you have touched on the issue I raised. The fertilizer K requirement is affected by degree of fixation to a point where for a given Exchangeable K the recommendations should be higher in high K fixing soils compared low K fixing soil. But is this accounted for in routine soil K test? And how can we include such factors?

Dr.Elephant,K fixation is related to the extent of K depletion in wedge and inter-layer sites/zones. So,one can establish a relation between extent of K fixation and fertilizer K requirement for a crop.But determination of K fixation in routine soil testing laboratory is not an easy job.Better is to estimate nonexchangeable in soils and relate it with crop response and then work out fertilizer K requirement.We discussed a lot on this aspect when we discussed my question. Even though the methods based on resin and sodium tetra phenyl boron appear to scientifically(based on mineralogy)  well defined and convincing, the procedures are time consuming,costly and may not be adaptable to routine soil testing.But with good calibration work either I M or 0.5 M HNO3 ( boiling nitric acid) can be tried.There are other methods based on HCl and H2SO4 acids with different strengths ,even without boiling.Though acid handling is  a little problem ,but those methods may less time consuming and less costly.With a little more time and cost,one can go for tetra phenyl boron method with recent modifications and improvements.

I do agree with you in that measurement of K-fixation can be laborious. But can't we develop an index for K-fixation which we can try and predict from routinely measured properties (using multiple linear regression)? 

Dr.Elephant, we can estimate K fixation ,even by short period incubation  period or predict  it from some routinely measured properties .Still it is difficult  to suggest it for use in developing K fertilizer recommendations.The following three publications may provide good information on the topic of your interest.

Potassium fixation in soils and recover of fertilizer potassium by the plants. Simonis A et al .poster presentation in a seminar and available online

natres.psu.ac.th/Link/SoilCongress/bdd/symp13b/1164-t.pdf

Potassium fixation and its significance for California crop production By Pettygrove S. et al.

Better Crops/vol.95,No.4 2011 Available online.

Can we predict K fixation in San Joaquin valley from soil texture and mineralogy?Murashkina,M and others.

https://www..cdfa.ca.gov/is/docs/Pettygrove-00.pdf

Hi Dr Rao,

Thanks for these articles. They are definitely a good read and an invaluable source.

Much Appreciation

Dimpho

Dr. Elephant

There is another approach for soil K test  by exchangeable resin or membrane. I did some work on it and I believe that it is still a useful method. You may notice that the availability of soil K can be assigned as the concept of intensity and potential and the membrane-K is a type of intensity ,,, and it is sensitive to soil environmental conditions, including temperature and moisture. All of these factors may lead the effect of K fertilizer are less compared to N fertilizer...