See the following for a description of differences to be expected with the two methods described.

The question is which method is able to correlate with crop response best?

Otherwise, yes there are a number of insoluble forms of mixed calcium apatites incorporating Ca and Magnesium, and other anions such as halides and hydroxide. The sodium ion is not really involved, only divalent and trivalent cations.

http://www.fao.org/docrep/007/y5053e/y5053e0b.htm

Sometimes there was indications that both Ca and Mg are responsible for P fixation in high pH soils. 

Dear Portia,

Phosphorus remains broadly in two form: Labile (fraction adsorbed over mineral particles, Fe-Mn Oxides, Organic matters) and immobile (mineral fragments of parent rocks). As you are looking soil samples, therefore look organic carbon concentration also. More is the organic matter, higher will be P. This relation holds true for most of the soils.

In alkaline soils calcium is the dominant cation that will react with PO4. and in acidic soils Al is the dominant ion that will react with phosphate, followed by Fe and Mn.

If you want to study in detail the P abundance in your samples, I will suggest it is better to go for P-Speciation analysis, that will give you a clear understanding about the phenomenon responsible for the difference in two locations. You can refer the following paper:

Ruttenberg, K. C., N. O. Ogawa, F. Tamburini, R. A. Briggs, N. D. Colasacco, and E. Joyce. "Improved, high-throughput approach for phosphorus speciation in natural sediments via the SEDEX sequential extraction method." Limnology and Oceanography: Methods 7, no. 5 (2009): 319-333.

Dear Portia,

It is a pity that you do not provide the level of K.

In the first place the level of available soil P cannot be considered low. At the same time Ca is not high at all and neither is Na. If anything the level of Mg is on the high side in relation to the Ca level. 

There is much discussion on the issue, but it is usually assumed that Ca and Na are antagonistic to P where uptake of P by the plant is concerned. Mg on the other hand has an uptake enhancing effect on P.

Do these soils form surface crusts upon rainfall or do they have compact B horizons. Both are signs of too high Mg.

But now what is your real problem? Do these soils not respond to P application?

Kind regards,

Roelf

Dear Portia, I agree this the other contributors and would like to add  that Manganese can be related to Phosphorus availability issues.

In highly weathered  acid red soils particularly oxisols they are many times high in Manganese. On the surface of these the nodules of oxidated material is seen commonly.

These soils will liberate Manganese in soluble forms under water saturated conditions and pH under 5.7 in particular this low pH threshold is higher than Aluminum and Iron which cause interference with metabolism under acid soil conditions.

When these cations are soluble they interfere with Phosphate. In relation to the oxisol high Manganese issue the use of Silicon rich materials will help liberate precipatated Phosphorus and does help alleviate Manganese toxicity. 

As a general rule if acid soils are limed to 5.7 pH they will overcome the issues with Iron, Aluminum and Manganese in that order of progressio  for Manganese 5.7 pH Aluminum 5.1 and iron 4.5 pH. 

There is a Silicon soil test which will determine Silicon deficiency and when this is bolstered the defensive metabolisms of many plants are favored particularly plants like rice that are heavily dependent on Silicon. 

Hope you found some useful information from these comments. PRH

Prof. Acharya has rightly suggested that P Specification analysis will give the clear picture. 

Prof Acharya, After looking at your soil test more closely I believe Sodium is your biggest issue. The acidity is fine in relation to sodium the use of Calcium Sulfate would be suggested. Ideally the Calcium to Magnesium ratio should be greater than 3 and the Calcium reading greater than 1000. Calcium sulfate will be useful in mobilizing Sodium out of the system by it competitive reaction with sodium and will also not change the ideal pH you are currently enjoying.  Your available and total P are not that bad actually. Best use of P would be as a starter fertilizer. To refine the defining issues tissue analysis is suggested to complement the soil analysis. PRH

Mg fixes P in  soils. however you must carry out  soil anaiysis to determine the levels of Ca, Mg K , Na .If the levels of Ca is higher  than Mg there will be nutrient antagonism  and therebyreducing Mg content.In addition you need to conduct correlation studies testing different extractants such as Bray P1,Olsen  P, and Bray P2 to choose the best extractant for soil P extraction

/Solo una pregunta --ya que estoy de acuerdo con mis colegas en sus respuestas y discusion --que tipo de arcilla se tiene y dos la materia organica  tipo y cantidad

I agree with my colleagues in their approach and discussion. I have only two questions. What kind of clay you have? What is the content of organic matter and state of weathering? We must remember that both operate according to the pH, the state of crystallinity, and that kaolinite, amorphous clay and organic matter attach and retain phosphorus. Also the "active calcium" or calcium carbonate size 2 microns, attach and retain phosphorus.

Very interesting discussion . A soil having pH of 6.1 , will have the possibility of major fraction of p getting fixed by Fe, Al . i dont think , the processes and the soil condition s responsible for   P being  fixed by Ca will have any similarity to those when P being  fixed by Na or Mg . The latter conditions only prevail under  sodic condition where soil pH is usually more than 8.5 , and goes upto 11.0 under dominating influence of exchangeable Na. And , this is the reason , most of the times , sodic soils have much higher water soluble P due to better solubilisation of sodium phosphate ...

There are some very interesting answers, but if you responded to the question about growth effect of P fertiliser I didn't see your answer.  The thinking behind that question and the query about exchangeable K data, is pointing at other possible limitations, and to the fact that 'critical' Bray P values are highly variable between soils.  

Lastly, and something I did not see discussed is that a pH in 1 M KCl of 6.1 could correspond with a water pH around 7 and the soil could indeed contain free Ca/MgCO3, which throws your question into a different context.  If you add 1 M HCl does the soil fizz?  Solid phase carbonates could  cause the large difference between the two Bray tests.  In any event even at this pH the hydrous oxides are not without P binding activity.  

Dear Portia,

The data provided by you for P extractable in Bray's- P1 and -P2 indicate that  possibly the soil has some tricalcium phosphate which is partly extracted when you use Bray's-P2 extractant. A higher concentration of HCl in Bray's-P2 is capable of extracting more calcium phosphates. Bray's extractants are capable of  releasing P from Al- and Fe-phosphates minerals and also from phosphate esters present in soil organic matter. The presence of ammonium fluoride replaces some P retained by soil through ligand exchange on the surfaces of soil sesquioxides, oxyhydroxides and hydroxides of Fe and Al.

Further, I wish to point out that you have not mentioned where the reported values for Ca, Mg and Na are indicating total or exchangeable content of these bases in the soil. You have also not reported the organic C and clay content of the soil.

As suggested by other respondents, P speciation or analysis of different chemical pools of P in this soil would be necessary to have definite explanation for your observations. 

As pointed out by Dr.Hepperly and Dr.Acharya,in acid soils next to Al and Fe, Mn may have some role in P fixation.There is possibility of P retention on carbonates of Ca and Mg.But I am not sure of the magnitude of Mg role in relation to Ca which is much studied.

Soil pH equal to 6.1 which might give slightly higher pH in the soil water extract  (around 7.0 or so) does not indicate any possibility of sizable presence of free CaCO3 or MgCO3 in this soil. 

There is a plausible presence of chloro- or -fluoro-apatite, di- and tri-calcium phosphate minerals besides some amounts of Fe-, Al-, adsorbed-, organo-phosphates in this soil. 

Dear Dr.Srivastava,my comment is general and is not made for soils employed by Portia in her study.For soils with a pH above 6.5,the H2PO4 ion concentration is controlled by the solubility of calcium phosphates,more soluble than hydroxy apatite and less soluble than octacalcium phosphate,unless the soil contains more than 2% CaCO3 when it lies on or above the octacalcium phosphate line in solubility diagram(Ulrich and Khanna 1968.Geoderma 2:65).We know that CaCO3 precipitates in soils above pH 8.3.So solubility diagrams and solubilty product constants were mostly employed for calcium phosphates and P containing soil minerals in neutral ,alkaline and calcareous soils.In alluvial soil of Delhi (pH 7.3-7.5),in the absence of P application, the native P was octacalcium phosphate(OCP).With addition of phosphate fertilizer, the initial P product was dicalcium dihydrate and was slowly transformed into OCP with time.In red soils (pH 5.4- 6.8), hydroxyapatite (HA)and fluorapatite(FA) were the main native compounds while the added P resulted in formation of tricalcium phosphate(TCP),HA or FA.In black soils (pH 8.1-9.0),the native P occured asTCP and the added P was transformed in to OCP.In alluvial soils( pH 7.0-8.2),depending on soil conditions,TCP,HA and P compounds more insoluble than HA were the native phosphates.The added P accumulated as OCP,TCP or HA in alluvial soils.The following publication may referred if it can be accessed.

Native phosphate and reaction products of applied phosphate in soils of Andhra Pradesh by Lakshmi,G.;Subba Rao,I.V. and Subba Rao,A.1987.J.Indian Soc.Soil.Sci. 35:624-630

Dear Dr. Rao, Thanks for a detailed reply.

The concern about fixation of Phosphorus is a valid one. Phosphorus of all the major essential nutrients is the most expensive and one that is most limiting in many circumstances.

Phosphorus is most important for the early development of roots so critical for optimal crop development. The most cost effective use of Phosphate is as a seeding application by  placing the starter fertilizer high in Phosphorus a few centimeters to the side and a centimeters below the seed placement row in the seeding for field crops.

This placement stimulates the early rooting and the banding overcomes soil fixation and distance avoids salt issues.

 In terms of an organic solution the use of earthworm castings are rich in soluble Phosphorus and has an advantages of a wide spectrum of micronutrients and humic materials.

The use synthetic fertilizers in direct proximity of seeds is a potential issues when salts can become toxic.

Under tropical  red acid soil conditions quite notable for P fixation Silicate materials can help mobilize soil Phosphorus which is fixed as immobile soil Phosphorus.

The deep understanding of the nuances of soil properties are key to tailoring the right strategy to get the best response with the least amounts of materials for farmers trying to optimize their results and minimizing cost and negative effects. .  

The value of organic manures and composts including vermicompost  is well appreciated in terms of reducing or arresting P fixation,release of fixed P by various mechanisms and extended period of availabilty of P in organic material amended soils is well appreciated but not practiced  sincerely every where.Placement of P is well tested practice but still not practiced universally.Many farmers in India still broadcast and then incorporate P and other fertilizers.The use of silicate materials to mobilize P is not in practice(though advocated).Dr.Hepperly ,I appreciate you for high- lighting the issues connected with efficient use of P in agriculture( without inviting environmental contamination problems).Very sincere efforts should be made to popularize the known nutrient and manure management best practices for animal and human health ,economy  and safe environment.

I understand that the question is what other cations bind phosphorus? The answer would be the calcium, aluminum, iron and silicon can do, there are other compounds that can, in the case of allophane, kaolinite, organic matter and active calcium carbonate, iron oxides. But we can also have it in two ways: adsorbed, absorbed or occluded, finally the crystalline forms, among which are considered the Phosphorite (calcium phosphate), Apatite, Variscite Strengite, Vivianite, Gibbsite. In each case we can establish methods of fractionation of minerals; phosphate occluded or absorbed; and the adsorbate, in addition to the method of total phosphorus by fusion with sodium carbonate. I think the availability of phosphorus would be subject to the particular characteristics of each case and it is therefore necessary to establish the best method that allows us to quantify and determine the soluble phosphate and available to the plant.

Ca, Mg and possibly Fe and Al can bind P if you use Mehlich-3 extractant to determine the predominant cations {Ca+Mg VS. Fe+Al) in alkaline soils. Mehlich-3 is buffered at pH of 2.6. However, Bray-1 is not appropriate as an extractant for alkaline soils, since Bray-1 is at pH of 2.5 but is not buffered.