Dr. Hue, organic P content in the soil generally varying from 20 to 80% of total P, except some forest soil of high leaf fall area where you may find more than 90% P in organic form. The organic P declines sharply down the soil profile, and its content is higher in clayey soil than in coarse-textured soils. Organic P is located mostly in the fulvic and humic acid fractions of the soil humus. There is a strong relationship between phosphatases activities in the soil and amount of organic P mineralized from the organic to plant available inorganic forms. Phosphatases enzymes in the soil are contributed by plants ( acid phosphatase and phytase) and microorganisms ( acid phosphatase, alkaline phosphatase and phytase). With increase in organic matter content in the soil microbial activity is increasing as well as you may get more phosphatase and phytase activities, which helps to breakdown the C-O-P ester bond of organic P to plant available inorganic (phosphate) form.

Phosphorus is a key component of energy metabolism and function as a structural element. The production of phosphorus in nucleic acids to total organically bound P differs between tissues and cells: it is high in meristems and low in storage tissues. Phytates are presumably involved in the regulation of ester synthesis during grain filling and tuber growth. The function of phytate in seed germination is quite obvious. In general P requirement for optimum growth is in the range of  0.3 to 0.5% of the plant dry matter.

Interesting question indeed...

organic phosphorus is quant itatively an important phosphorus fraction in many soils, therc has been little investigation of its significance in plant nutrition. In cxpcrimcntal work on thc availability of soil phosphorus to plants, it has often been assumed that only thc inorganic fraction was opcrative, 'Here the possible significance of the organic fraction has been recognized, only speculation regarding its importance has generally been permissible because of the laek of a method for estimating the respective ('o11lrihut ions of the inorganic and organic fractions.The amounts of soil organic phosphorus mineralized during the plant growth should be  used by some as an index of organic phosphorus availability to plants. The amounts mineralized under actual conditions of plant growth may perhaps provide a more realistic estimate. Therefore, the  change in content of soil organic  phosphorus under conditions of plant growth and to obtain an estimate of phosphorus in plants derived from that source would surely reveal some interesting facts. 

In the absence of morc direct evidence, the accumulated knowledge of the behavior of soil organic phosphorus and nitrogcn might be linked to form a basis for inferring the importance of soil organic phosphorus in plant nutrition. Nitrogen and phosphorus are absorbed by plants in approximately the same ratio in which they occur in and are liberated from the soil organic matter. Nitrogen liberated in inorganic form from the soil organic matter may sel've as the"sole source of supply to nonleguminous plants. It appears, therefore, that the miIlCralized organic phosphorus might likewise be sufficient to servc as the sole source of supply to plants under these conditions. The question of the importance of organic phosphorus- cannot be settled in this way, however, since the behavior of inorganic phosphorus is by no means analogous to that of inorganic nitrogen in the soil. 

You have touched on perhaps the next big question which relates to the rate of mineralisation of organic matter and the release of the non-CHO constituents.  That is, you are asking about fluxes and their control.  Just one amendment to your statement, i.e. a substantial proportion of P in most manures and composts is inorganic. 

 good answer J. C. Tarafdar

Good amount of manure P which is present as inorganic P may be readily available to plants(as mentioned by Dr.Milham). Immediately after release of P by mineralization,a part of the phosphate ions may  be re - adsorbed  byAl,Fe oxides/hydrous oxides or  calcium carbonate or precipitated  byAl,Fe or Ca ions depending on soil pH.So estimation of mineralized P  may not be accurate.One may have to use radioactive 32P for this purpose.If the amount of mineralized P is small then estimation of small amount of P poses problem.

In surface horizons of most soils, P also occurs in organic forms in amounts that can vary widely (from 20% to 90% of the total P). Mineralisation / immobilisation processes occur with organic P in a manner generally analogous to nitrogen (N) and sulphur (S) but have been much less studied. A difficulty in the measurement of mineralised P is that it immediately reacts with inorganic soil constituents (i.e. PO4 released during mineralisation will be adsorbed by the soil rather than remaining in solution as mineralised N would. Thus, it is difficult to determine how much P has been released). Incubation experiments have shown that as the organic P decreases it is matched by an equivalent increase in acid extractable P (adsorbed and soluble P).

The microbial biomass represents a small but labile pool of soil P, mainly in the form of RNA and DNA, polyphosphates and phospholipids, with minor amounts of other P containing cell biochemicals like adenosine phosphates. Measurement of the microbial biomass P is an essential prerequisite for assessing its role in the P cycle, and its effect on plant nutrition. Until very recently, estimates of microbial P have had to be made from measurements of microbial biomass and literature values for P contents of laboratory cultured microorganisms. However, the P content of organisms can vary widely depending on growth conditions. New methods of measuring microbial P have been developed, based on the widely used chloroform-fumigation procedure for measuring soil microbial biomass; the P released from microbial cells following lysis with chloroform is extracted from the soil with NaHCO3.

Although difficult to measure, estimates of the comparative efficiencies of P uptake from soil by microorganisms and plant roots have indicated that microorganisms, and in particular bacteria, have very efficient P uptake mechanisms (Table 1.1). Competition for the small amounts of P in the soil solution is most intense in the rhizosphere, where substrates from root exudates, sloughed off root cells, tissues and mucigels sustain a larger and more active microbial population than in the bulk soil. Source :THE SCIENCE OF PHOSPHORUS NUTRITION: FORMS IN THE SOIL, PLANT UPTAKE, AND PLANT RESPONSE.

I suggest an experiment with labeled P32, so you can track the p present in the original waste (waste) and its subsequent distribution in the microbial biomass, the soil, the plant, as well as the p that is fixed in the soil

NMR spectroscopy may be useful in characterising forms in the original waste, provided that the Fe content is small.  Synchrotron techniques are probably better in the amended soil.  These techniques complement the suggested radioisotope approach  

Phosphorus rank in importance with N and K as major plant nutrients.

Phosphorus compounds in soil can be placed into the following three classes:

1. organic compound of the soil humus,

2. inorganic compounds in which the P is combined with Ca, Mg, Fe, Al and with clay minerals,

3. organic and inorganic P compounds associated with the cells of living matter. Microorganisms are involved in transformations of phosphorus between organic and mineral forms.

From 15 to 80% of the phosphorus in soils occurs in organic forms, the exact amount

being dependent upon the nature of the soil and its composition. The higher percentages are typical of peats and uncultivated forest soils.

From the standpoint of plant nutrion, phosphorus is adsorbed by plants largely as the

negatively charged primary and secondary orthophosphate ions (H2PO4- and HPO42-) which are present in the soil solution. Small quantities of soluble organic P compounds are also present in water extracts of soil.

ALL OF THE RESEARCHERS HAVE GIVEN VERY GOOD POINTS ON THE ISSUE.LABEL P studies do carry answer to queries of Dr Hue.

The balance description concerning P-total, P-available & P-organic, determined by soil physical & chemical properties as well by the process of soil organic matter decomposition, is very valuable datum in order to evaluate P-plant's nutrition.

We know that soil is a complex system where chemical analysis gives wonderful answers, but that alone is not enough to explain the question. The conclusions of the chemical analysis become more reliable and with better prospects for practical use, if the data come at least from a laboratory experiment, established in dynamically conditions (eg incubation experiment,, pot experiment); even better results we obtain with field experiment, but more reliable results are coming from a combination of the two processes. The field experiment alone, is not enough often to give answers to the problem, because there are indeterminate factors (eg climatic) that are not controlled. Then the combination of the laboratory experiment and the field conditions study, produces soil samples, where the application of modern chemical analyzes, gives wonderful conclusions.

Organic P is located mostly in the fulvic and humic acid fractions of the soil humus. There is a strong relationship between phosphatases activities in the soil and amount of organic P mineralized from the organic to plant available inorganic forms. Phosphorus' primary role in a plant is to store and transfer energy produced by photosynthesis for use in growth and reproductive processes. Soil P cycles in a variety forms in the soil. Adequate P levels promote root growth and winter hardiness, stimulate tillering, and hasten maturity. Provides essential nutrients for plants phosphorus as it is decomposed by microbes. Feeds and provides habitats for diverse soil organisms, including those that help fight plant pests and diseases.

Dear Nguyen Hue . See the following useful RG links: Article An Integrative Systems Perspective on Plant Phosphate Research

Article From soil to plant, the journey of P through trophic relatio...

Article Utilization of soil organic phosphorus as a strategic approa...

Kindly see also the following useful RG links: Article Utilization of soil residual phosphorus and internal reuse o...

http://www.scielo.cl/scielo.php?script=sci_arttext&pid=S0718-95162012000300015

Also check please the following useful RG links: Article Organic Soil Phosphorus Considerably Contributes to Plant Nu...

Chapter Bio-Organo-Phos: A Sustainable Approach for Managing Phospho...

Article Phosphorus Dynamics: From Soil to Plant

The following RG links are also very useful: Article Changes in phosphorus fractions associated with soil chemica...

Article Complex Forms of Soil Organic Phosphorus-A Major Component o...

Visit kindly the following useful links: Article Utilization of soil organic phosphorus as a strategic approa...

https://scielo.conicyt.cl/pdf/jsspn/v13n4/aop7913.pdf

https://www.publish.csiro.au/sr/CrossrefCites/S96047

https://repository.si.edu/bitstream/handle/10088/8909/stri_Eur_J_Soil_Sci_61_48-57.pdf?isAllowed=y&sequence=1

Organic P is located mostly in the fulvic and humic acid fractions of the soil humus. There is a strong relationship between phosphatases activities in the soil and amount of organic P mineralized from the organic to plant available inorganic forms.