As I understand ,there is no proof that the dilute acid extractable P and K have come from labile organic matter in soil.P and K can be released from both organic and inorganic constituents by solvent action of the acid.As K is not a constituent of any plant organelle  and present cell sap ,it can be removed from soil organic matter by dilute organic acid.K held in exchangeable form can also be extracted by dilute acid.In case of P  dilute acid may not release it  from organic constituents.But it can remove P from inorganic constituents by solvent action.In Hedley et al(1982) fractionation procedure the labile organic and inorganic P are  extracted with 0.5 M NaHCO3.The inorganic labile P is estimated directly and the organic  labile P is estimated as the difference between total P(  after digestion)and inorganic P.Please refer to the paper of Hedley et al. for details .Hedley MJ,Stewart JWB,Chauhan,BS(1982)Changes in inorganic  soil phosphorus fractions induced by cultivation practies and by laboratory incubations .Soil Sci.Soc.Am.J.46:970-976ra

 How are the mobile forms defined? What are the amendments, what is their composition and how much applied?  

Zenon , very intelligently placed question . Why don't we see the role organic matter in neutralizing the difference in mobility of  P and K vis sieve function of soils  , since under un-manured  acidic soils , P  is far lesser mobile than K. But , moment soils are manured , the labile fraction of organic matter improves the sieve function soils , thereby , negating the higher mobility of K , and eventually both the cations are staked  at same mobility .

Zenon sir,

It is true as said by Anoop sir and Subba Rao sir.  That is K is more mobile than P in acid soils. And K is not the constituent part of plant cell. However it is present in sap which will come out very easily on hydration of organic material in soil. Particularly in Luvisol, the clay content in surface will be low. The organic matter will act as source for adsorption. This might be the reason for simillar behaviour of P and K in such soils.

Thanks

regards

Rajakumar

Dear colleagues,Can any body explain what is sieve function of soil?.How labile organic matter influences the sieve function of soil?How it similarly or dissimilarly influences the P and K extraction K by 0.2M HCl  in acid soil amended with farm manure and straw(soil pH buffered by these amendments)?I understand that the dilute acid extracted water soluble and exchangeable K but what is the mechanism of release( by0.2MHCl)of P from inorganic and organic phases?

Sieve function of soil is very nicely explained by Dr Paul Reed Hepperly from University of Puerto Rico  on the same   platform  ( ResearchGate)  ) under the question HOW CAN WE ELEVATE THE CROP RESPONSE EFFICINECY OF ORGANIC MANURES ( Editor : A.K.Srivastava), perhaps you also participated in the discussion . Please go through it , and if it  is still not clear , please read any latest textbook on Adsorption Chemistry. How different  soil colloids behave under the influence of organic manuring  ? or what kind of reorientation in behaviour of different soil colloids takes place  as result of change in organic matter content of the soil , all constitute  a part of sieve function of soil .  

I think I have to read more.

all organic forms of P undergo more or less rapid mineralisation and by this pass always through the soluble P pool of a soil. Any further conversion process from feeds the side specific P pools without any differenciation to from where the P originally came from!

Dr.Schnug,We have soil amended with two types of organic materials-farm yard manure and straw alone and in combination with NPK fertilizer.In surface soils the farm manure amended soil released more P than straw amended soil .The behavior is similar  in fertilizer P and organic sources combination also. 0.2 M HCl can extract P from two sources.-P mineralized from organic sources (it can be from labile organic pool)and from solvent nature of dilute acid primarily from Al and Fe bound P(plus some Ca-P,if it is present in this acid soil)

the enigma of agricultural experimentation: results are never comparabel, which keeps our business exceiting! Browse my papers, there are a number of papers dealing with the issue!

Subba Rao sir,

The acid does not differentiate the sources, either from inorganic or organic. It only reacts on labile form of P.   Besides, it acts on soil bound P which will become soluble on addition of acids. At the same time if Ca-bound P is present, the acid will suppress the release and allow the actual that correlates with available P and crop growth in acid soils.

Dr.Rajakumar,in my first reply I have explained what is labile organic P and what is labile inorganicP. The extraction method followed by Dr.zenon does not permit estimation of labile fractions of P.What is the soil bound P( except Al and Fe bound P ) your are  referring?Ca bound P may not be found in this soil .Even if present, how the acid soluble  Ca-P is suppressed in acid soil in acidic extract ant?

Hi!

Yes of course. Flexible connection of phosphorus and potassium exists in the form of: KH2PO4 or K2HPO4

Best wishes.

Subba Rao Sir,

In acid soils, the Ca will be inactive i.e. its activity is reduced or restricted.  In Brays & Kurtz extraction procedure, the acidified extractant is used  to avoid release of Ca-P. In EDTA titration for Ca, the pH is raised to 9.5-10 by adding NaOH which makes Ca active and keep in solution.  Hence in acid soils, Ca-P is not taken into account.

Dr.Rajakumar ,I always maintained that in acid soils Ca-P  will not be present and if present its contribution will be small in comparison to Al and Fe bound P in P extraction by dilute acids..Suppression of Ca activity by other ions, low availability of Ca in the solution in acid soils is different from solubility of Ca-P( if present especially in soils with pH>6.0).I do not want to comment on EDTA titration and your view.The following brief paper may clarify the things regarding P extraction in different soils by various extraction methods.Selection of appropriate phosphorus test for soils  . Elrashidi,M.A . www.nrcs.usda.gov/internet/FSE_ Documents//nrcs142p2_051918.pdf