Regardless of soil test methods , you use , intensity of color while testing for P is a measure of soil test value for P. But , soil color is no way determines the type of soil test method to be used for determination of P. Our normal understanding is Bray method holds good for acidic soils , while Olsen and Mehlich 3 are very effective for alkaline and calcareous soils. Many of the red soils have alkaline pH , so i do not think , Bray extraction is to be necessarily used for red soils. under such conditions, Olsen -P will hold good.  Abhishek has attached a very good PDF stating that Bray-P could be equally effective in  alkaline soils having dolomite than calcite , since calcite reacts faster than dolomite . But , most important is the coefficient of determination in a given soil, regardless of soil test method. 

Lots of good advice above.  pH has traditionally been the main determinant, e.g.: Bray for acidic soils and Olsen for alkaline.  More recently many people have wanted to apply a single extraction for P regardless of pH, and the availability of multi element capability in ICP has extended that desire to include elements other than P; hence Mehlich etc.  All these tests measure the easily available P, but they also extract some of the more slowly available fractions, and the extent of this process depends on a complex set of soil properties, i.e. it can vary widely between soils.  I realise that this may or may not be useful ..... but those are the realities. 

Interesting response from Paul . since soil test method for P is by and large guided by the soil pH , color of soil would aid the least in selecting the suitable soil test method for P. For example . red soils , it could be acidic or alkaline in nature .

Interesting theme!!

Further, red soils are visibly dominated with sesquioxides. There, iP availability/release is a function of Fe/Al-P compounds. And, Fe and Al complexation by fluorides and subsequent release of iP is the mechanism of Bray extraction.

Because of this I was motivated to use Bray extractant.

Can someone please throw light on this aspect?

Dear MK Dabi,

As previously Dr Srivastava pointed out red soils it could be acidic or alkanine. Lets see why, taking as example the great group of Rhodoxeralfs (Alfisols) which prevail within Mediterranean red soils.

Their red color is mainly attributed to hematite (Fe2O3) formed during rubification while their specific pedoenvironment in which formed is characterized by an association of long dry period and high internal drainage, which in many cases (like in Greece) prevails under the karstic conditions created by hard limestone. In this case the acidic conditions prevail in the formed agrilic horizon  and the Fe/Al oxides is the main sorbents of phosphorus.

However sometimes recarbonation process and accumulation of secondary carbonates in the argilic horizon changes the acidic nature of the soil and in this case a red soil may have pH values almost 8.0.

For more details see the attached:

 Accumulation of secondary carbonate evidence by ascending capillary in mediterranean argillic horizons (Cordoba, Andalusia, Spain) by Espejo et al.

In this case, perhaps the use of a more universal extractant like mehlich-3, a soil P test which not relate with soil pH, is more appropriate.

http://ovidsp.tx.ovid.com/sp-3.20.0b/ovidweb.cgi?QS2=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

Nice response Dr Dionisios, i agree with you . Very interesting .

Thanks, Drs. for your precious contributions.

I would like to understand P availability from sesquioxide-bound iP in red soils. What if a red soil is neutral in reaction? Do we still go for Olsen extractant while the extraction chemistry is unrelated?

Mr.Manoj,have you observed any colour/turbidity in Brays"s extracts of red soil?

Dr. Subba Rao, I have not observed any turbidity in Bray P extracts. It happens because of complexation followed by settling of amorphous clays (sesquioxides). 

The effect is so quick that the suspension settles immediately after shaking for just five minutes.

Mr.Manoj,as you are aware soil pH(predominant range in a particular area) is themain criterion in choosing an extractant, say Olsen or Bray  for P.To my knowledge red soils may vary predominantly in pH 5-7.5.I have not comeacross calcareous red soils(in surface soils or even upto 30cm).We recommend acid soil liming to a pH of around 6.0(some may advocate upto 6.5).The Al3plus or AlOH plus 2,Al(OH) plus activity will also come down by pH 6.0-6.5.If your red soils have pH predominantly aroud 6.5-7.5 range, I think there is no harm to select the Olsen's extractant in place of Bray's method.Melich 3 method is not thoroughly tested in India,though it is a good method for soils varying from acid to alkaline.Probably you need not worry for colour of red soil.Dr.Milham and Dr.Gaparatos have provided good additional useful information. 

Dr.Vocasek,good information.

Soil color sometimes excite the method to be followed for soil P estimation. However, the pH phenomenon to decide the method as indicated for Bray/Olsen is great.

Dr.Vocasek, I appreciate your comments.In surface soils, the problem of interference of calcium carbonate with P extraction arise in soils when pH exceeds 8.2-8.3.Scientists reported lack of correlation between Bray1 P and Mehlich P in calcareous soils.Olsen's P and Mehlich P are correlated in calcareous soils but the amount of P extracted by the former reagent will be less than the latter.So from neutral (6.5-7.5) to alkaline and calcareous soils both Olsen and Mehlich reagents can be used.As mentioned by me earlier, in India we have very limited information on soil test crop response correlation with Mehlich method.So we can not immediately go for Mehlich method. We can safely use Olsen method on such soils.

Sir,

My answer for the question is Yes.  Normally Black soils will have Neutral and alkaline pH. Similarly Red soils / Yellow soils will have Acidic to Neutral pH.  Extraction of P from acidic soils require Brays extractant and from Neutral to alkaline soils require Olsen reagent.

Simple. Availability of P depends on soil pH.

And Methods are in place for acidic and alkaline soil for this purpose.

So far as colour is concerned the thumb rule is that the red soils are acidic and black soils are alkaline or near alkaline. You can decide your method accordingly/

CAUTION

1.There are soils which are red yet calcareous (see research gate for papers)

2.There are alluvial soils, especially soils of the IndoGangetic Plains (IGP), which are exceptions. You need to check both colour and pH. Besides the alluvium (paddy) soils (river) in Assam and Tripura will behave differently in terms of pH (the former is near neutral and the other is acidic)...colour remaining same.

3.Colour of soils and  its classification may be in error. There are commonly referred black soils (Vertisols) which may be red in colour... even 2.5 YR... formed in red bolls...these are also Vertisols.

4. There are reports of black soils (generally considered calcareous) which  may be non-calcareous!

Regards

tapas

Bray's P-release mechanism targets sesquioxide-bound P. My clear question is, what if we have a red soil (sesquioxide-bound P rich) of pH 7 and we go for Bray P? What's wrong in that?

Logically you are right.

Give atry and compare with other P method.

But first check the soil pH.

Regards

tapas

I have a similar problem. What is the effect of applying Olsen extraction method  for P determination on acid soils (pH 4) which consists of 99% sand (dune soils), with little Fe en Al? I find results which seem to me too high considering current and past land use?

What about the color of the soil? Does it have to do something with selection of P extraction method?

In my question, it has nothing to do with the color of the soil. Sorry to confuse a little bit the focus of the original question. In ecological studies Olsen P is used to determine available P regardless the pH of the soil. So my question is more a side issue related Olse P method. What is the effect of applying Olsen extraction method  for P determination on acid soils (pH 4) which consists of 99% sand (dune soils), with little Fe en Al? I find results which seem to me too high considering current and past land use?