Arkaprava Roy Have you estimated total P? By which method? In any fractionation scheme the different extractants will not be able to extract all the fractions as expected and some portion remains unextracted and we call it residual by difference method.In some fractionations I have seen a substantial portion remains unextracted, It may be one form , say apatite P or other forms of insoluble P depending on the nature of soil especially acidic or alkaline soil. Organic P may not come in the fractions you have extracted.You need to digest the extracted fractions to get the organic P component.

In the sequential extraction procedure outlined by Peterson and Corey (1966), different fractions of phosphorus (P) can be extracted based on their association with various chemical compounds or phases. Here is an overview of the fractions you mentioned and their associated chemical compounds:

1. Saloid P: This fraction represents the water-soluble or readily exchangeable P, which is usually associated with phosphates in the soil solution or loosely bound to adsorption sites on soil particles.

2. Al-P: Al-P refers to the fraction of P associated with aluminum (Al) compounds in the soil. It can be extracted by using strong acid solutions that dissolve Al-bound P like aluminum phosphate minerals or organic complexes.

3. Fe-P: The Fe-P fraction represents the P association with iron (Fe) compounds in the soil. It can be extracted using strong reducing agents that dissolve Fe-bound P, which may include iron phosphate minerals or organic complexes.

4. Occluded P: Occluded P refers to P that is physically enclosed or trapped within mineral or soil aggregates. It generally requires more aggressive extraction techniques, such as strong acids or digestion with concentrated sulfuric acid and hydrogen peroxide, to release the occluded P.

5. Ca-P: The Ca-P fraction represents P associated with calcium (Ca) compounds in the soil. It can be extracted by using solutions that can dissolve calcium phosphate minerals or remove P from the surfaces of calcium carbonate minerals.

6. Residual P: The residual P fraction represents the P that is strongly bound or chemically fixed in persistent compounds, such as secondary minerals, recalcitrant organic matter, or insoluble P compounds. This fraction is the least extractable and often remains in the soil after other fractions are extracted. It requires more aggressive extraction methods, such as digestion with strong acids or fusion with alkali.

It's worth noting that the specific chemical compounds or associations within each fraction can vary depending on soil properties, environmental conditions, and the extraction methods used. Therefore, it's always important to consider the details of the specific extraction procedure followed to accurately interpret the results obtained.

The concept of residual phosphorus (P) refers to the portion of phosphorus that remains in the soil after commonly used extraction methods have been applied to extract the readily available forms of P, such as Al-P, Fe-P, occluded P, and Ca-P. Residual P is usually considered to be less readily available to plants compared to the other forms.

Residual P can consist of both organic and inorganic phosphorus compounds. Organic P refers to phosphorus that is bound within organic matter in the soil, such as humus, plant residues, or microbial biomass. These organic forms of P are typically more resistant to extraction and mineralization, meaning they are less easily accessible to plant roots.

On the other hand, there can also be difficultly extractable inorganic P in the residual fraction. This includes inorganic P forms that exhibit low solubility and are less accessible to plants. These forms can include certain phosphates, such as crystalline or poorly crystalline phosphates, calcium phosphates, iron phosphates, and aluminum phosphates.

Precise identification and quantification of the different forms of residual P often require specialized extraction techniques and analytical methods. Some of these techniques include sequential extraction methods, which aim to target specific forms of P based on their solubility or association with soil minerals.

Understanding the composition of residual P in your soil can be important for effective nutrient management. It helps in assessing the potential availability of P to plants, estimating the need for additional P fertilization, and evaluating the long-term sustainability of P management practices. Consulting with a soil scientist or agronomist can provide further insights based on your specific soil conditions and agricultural goals.

Arkaprava Roy You consult the paper Fractionation of inorganic and organic Phosphorus in virgin and cultivated soils.Soil.Sci.Soc.Am.J.49:127-130.The paper is available on Researchgate.net or on internet.The authors reported organic,inorganic fractions and residual P.They also explained the significance of the different fractions with possible changes under cultivation.

Annangi Subba Rao Sir, Thank you so much for your answer. I feel humbled to have received your reaction to my query.

For estimating P, I digested my soil sample with HF+HClO4 and the same acid mixture was used for digesting the residual fraction. Since the SOC content of my soil is quite high (1.72%), I too suspect the majority of the P in the residual fraction to be organic-P. Thanks for providing the insight that whatever portions of different fractions remain unextracted due to higher stability, those collectively form the residual fraction.

Godwin Sabah Thank you so much for the elaborate and comprehensive answer to my query! This is going to help me a lot.

I was wondering, do Al-P, Fe-P and Ca-P — apart from the discrete Al-P (eg. Variscite), Fe-P (eg. Strengite) and Ca-P (eg. Brushite) MINERALS — also include the STRONGLY ADSORBED P on the surface of Al-oxyhydroxides, Fe-oxyhydroxides and Ca-carbonate?

Yes, in addition to the discrete minerals such as variscite, strengite, and brushite, there is also a form of phosphorus (P) that is strongly adsorbed onto the surfaces of certain minerals.

When it comes to aluminum (Al), iron (Fe), and calcium (Ca), their oxyhydroxide minerals, such as aluminum oxyhydroxides (e.g., gibbsite), iron oxyhydroxides (e.g., goethite), and calcium carbonate (e.g., calcite), can act as sorbents for phosphorus.

This adsorbed form of phosphorus on the mineral surfaces plays an important role in regulating the availability and mobility of phosphorus in soils, sediments, and aquatic environments. It can act as a reservoir and slowly release phosphorus, impacting nutrient cycling and availability to plants and other organisms.

These adsorbed forms of phosphorus typically occur as amorphous or poorly crystalline compounds. They form strong bonds with the mineral surfaces through chemical interactions, such as surface complexation, ligand exchange, and electrostatic attraction. The extent of phosphorus adsorption depends on various factors, including pH, solution chemistry, mineralogy, and the specific properties of the phosphorus compound.

Understanding the behavior of strongly adsorbed phosphorus on mineral surfaces is crucial for managing nutrient availability, water quality, and ecosystem health, particularly in agricultural and aquatic systems.

Arkaprava Roy From the procedures followed by you for total and residual P, one can understand that the residual P estimated also includes the resistant organic P (possibly humin associated P) and unextracted part of humic P(as 0.1N NaOH and 1M HCl extractants can not extract all the humic P).The estimation of occluded P is also problematic.Please go through the following article which may clarify your doubts regarding the residual P estimation.You may have to follow a different procedure to get the residual P.

Comparison of methods to estimate occluded and resistant soil phosphorus.Soil Sci. Soc.Am.J 62:338-342 (1998)

Annangi Subba Rao Sir, Thank you! My regards.

Godwin Sabah, thanks again!

I was wondering, which gets extracted first, STRONGLY adsorbed P? Or, FRESHLY and LOOSELY PRECIPITATED discrete amorphous P minerals?

The extraction order of STRONGLY adsorbed P and FRESHLY and LOOSELY PRECIPITATED discrete amorphous P minerals can vary depending on the specific conditions and methods being used for extraction.

In general, strongly adsorbed P refers to phosphate ions that are tightly bound to soil particles or surfaces through electrostatic or chemical interactions. These are typically more difficult to extract compared to loosely precipitated P minerals.

On the other hand, freshly and loosely precipitated discrete amorphous P minerals refer to phosphate compounds that have recently formed and are present as discrete particles or precipitates in the soil. These minerals are often more labile and can be more readily extracted compared to strongly adsorbed P.

However, it is important to note that the extraction process for both strongly adsorbed P and loosely precipitated P minerals can vary depending on the specific extraction method or protocol being used. Different extraction techniques, such as chemical extractions, may target different forms of P and have different selectivity.

It is recommended to consult scientific literature or relevant research to determine the most appropriate extraction method for your specific study or analysis of P in soil samples.

Arkaprava Roy Please read the following article.It makes many things clear.It is a must read paper especially for those working on P.

How understanding soil chemistry can lead to better phosphate fertilizer practice:a 68 year journey (so far) By N.J.Barrow Plant and Soil 476,117-131(2022).Open access article.One will know many new things from this article.

Thank you so much, Annangi Subba Rao Sir. This is going to be a great help in deciphering the enigma of phosphorus in soil.

My regards to you, Sir.

Arkaprava Roy If you are in initial phase of your work, you may skip the Peterson and Corey method and go for Hedley fractionation / or its modified latest version scheme who is now being widely used .You may also estimate organic P in your soil samples as the samples contain high amount of organic matter.The following reference may be consulted for organic P estimation.There is a new method for organic P estimation based on NMR but facilities may a limitation for you.

Critical evaluation of methods for determination of total organic phosphorus in tropical soils . Soil Sci.Soc.Am.J.54:1261-1266(1990)