I assessed 6 fractions of total inorganic P namely, Saloid-P (1 N NH4Cl), Al-P (0.5 N NH4F), Fe-P (0.1 M NaOH), Occluded-P (0.1 M NaOH), Ca-P (0.5 N H2SO4) and Residual P (HF digestion) following the selective sequential extraction procedure of Peterson and Corey (1966).
The Residual-P share in my paddy soil (clayey Inceptisol, circumneutral pH, high in Fe and Al) is quite large. Where do Residual-P and other fractions of P necessarily come from? What exactly do they consist of?
I'm afraid that I must tell you that the P 'fractions' defined by sequential extraction are unrelated to chemical speciation. That does not preclude you from characterising those fractions, e.g., using IR and NMR ...
Paul Milham sir, I am extremely glad to receive your reply!
Thanks for reminding me that soil P 'pools' can't be superimposed over soil P 'fractions'. Unfortunately, I'm posted at a very young research institute in India and don't have access to either FTIR or NMR.
I am still curious about what might be the dominant composition of the residual fraction of P because it contributes to almost 40% of the total P content in my paddy soil. The studied soil is clayey Inceptisol with a pH of 7.84, high in amorphous Fe and Al, rich in organic (1.72%) and inorganic C (1.39%), geogenically arsenic-contaminated, and has been regularly fertilized with NPK for the last four decades. The abundance of different P fractions that I found in my study follows: Residual-P>Ca-P>Fe-P>Al-P>Occluded-P>Saloid-P.
If we are using selective extractants to solubilize the elements (Fe, Al, Ca) majorly responsible for binding P to the soil, and if P is never an integral component of resistant aluminosilicate minerals, then where should this high amount of residual P be coming from when our soil is digested with HF? Is it coming from the P:
1. strongly associated with recalcitrant organic matter? However, the soil underwent 17 hours of shaking with 0.1 N NaOH for dissolving Fe-P and Occluded P, and NaOH should dissolve a considerable part of organic matter too.
2. structurally associated with or strongly adsorbed on well-crystalline Ca-, Fe-, or Al-compounds that our extractants are unable to dissolve?
When the probable mineral saturation states were studied in GWB software, it appeared that the discrete P-bearing minerals that could precipitate in our soil are Hydroxyapatite, Whitlockite, MnHPO4 and Strengite. Do these discrete minerals have some contribution to make in the residual pool? Or, do they get dissolved by the selective extractants that we are using?
3. so strongly occluded within Fe and Al oxyhydroxides or CaCO3 that 0.1 N NaOH has failed to solubilize?
4. associated with some other forms?
Until I get access to the relevant instruments to analytically address my doubts, I need the help of your profound expertise in soil chemistry in the form of theoretical insight.
Best regards,
Roy
Hi Roy,
A lot will be organic but with that history there is likely to also be lots attached to the hydrous Al and Fe oxides (extractable by acidic oxalate) but in this case you might need to first neutralise the carbonates.
As for the lack of resources, if you have good data this is an opportunity to approach an expert in a well-endowed institution to be a collaborator!
A synchrotron scientist might take the bait!
Regards, Paul.
Hi Paul Milham sir,
Thanks for your insightful reply, and I shall start looking for interested collaborators with synchrotron facilities!
Best regards,
Roy
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