Dr.Surendran,for how long the soils are under cropping(crop/cropping system) and fertilizer use?What is the extractant used for estimating available P and what ratings are being followed to group soils into low ,medium and high?What are the recommended rates of P application to  the crops being grown?

Dr Rao,

1. The soil are continuously under different crops ( viz., Mixed cropping system ; mono crop of coconut, arecanut, paddy, banana, vegetables, pepper; crop rotation with different set of crops) 

2. Fertilizer use also varies from low input to high input ( Both organic and inorganic ). But interesting thing is continuously using factamphos in majority of the cases 

3. Extractant used is Bray I for acid soils and Olsen for alkaline and neutral soils. But I have tried with different extractant also 

4. Ratings are less than 11,11-22 and more than 22 kg / ha

5. Recommended rate is as per POP of KAU, but actual application may be differing.

I hope I have answered most of your questions.   The thing is that none of the attempts I have made lead to a solid conclusion, so what are all the other options I can look for

Thanks

 Dr.Surendran,thank you for the information provided by you.It appears to me that Olsen method is being used predominantly in Kerala, though acid soils occur there.Bray's reagent extracts more P than Olsen's and also the critical limits are different for Bray's method.P utilization by a crop in the year/season of application may be 15-20/25%.More than 75% of annual/seasonal application may be remaining in soil every year of P application.Over the years a lot of P can accumulate in soil and can saturate the anion exchange/P fixation sites.Once the fixation sites get saturated, the pool in available/labile  form and soil solution will increase.How- ever in soils with pH

Dear Sir,

Thanks for your detailed reply. Yes it is true, In State Planning Board coordinated study of multi-institutional project, also the results were more or less similar and in that project our analysis of nearly 3500+ (500 samples from other projects) soil samples with GPS coordinates also was similar. I have mentioned about that only. But the high value what I mentioned is more than 100 kg / ha or even more. Hence only I have tried with different possible way by doing the analysis based on crops, soils, districts, different extractants etc. But still the answer is High P. So only I am trying to attribute with some reasons for that. As you have suggested, already the Department of Agriculture have gone for skipping of P fertilizers based on soil test. Once again thanks for your detailed reply.  

Dr. Surendran compliments for a practical question. I am enjoying the intellectual input given by experts and carefully go through it. best wishes.

I have noticed that Legumes (clover red & white and Lotus) all thrive on high iron soil types that annually go through a Ferric/ferrous cycle. The legumes are able to extract P from the soil with no problem, conversely the grass species suffered from P deficiency and were not able to compete with the legumes for space in the pasture.  This observation raises the question of Available P tests, and how we need to factor in soil Al, Fe, Ca (when high) with specific correlation to various plant species.  

Dr. Surendran, I would like to add more with Dr. Rao answer. It seems your soil is very rich in total P content. Have you ever estimated the total P content of your soil as well as microbial biomass and activity? Some of the other reasons may be your soil may have high phosphatases and phytase activities as well as more organic acid concentration which are responsible to mobilizing  huge amount of plant unavailable  organic and inorganic P accumulated in the soil. As it seems your field is receiving always P fertilization therefore extra P accumulating in the plant available form has hardly used by the  crops and it is accumulating. 

I appreciate the points of Drs.Barlow and Tarafdar.The question raised by Dr.Barlow needs discussion based on published work and further research work.

Dr Surendran , is there any effort to mobilise such a high level of plant available  P through some legumes , or disocontinuation of P addition to faciliitate P to be exhausted  out to a manageable limits  . But , surely , its an interesting situation ..?

In continuation to the discussion, Could any one share the procedures about P fixing capacity of Soils, P fractionation , Adsorption and Desorption studies , P mobilization etc

Nice discussion....

Abstract : The transformation of applied P in soils from different horizons were examined after treatments with different P rates (0,20, 60 and 180 mg P L-1) and incubation periods (one, three, seven, 14 and 28 days) underlaboratory conditions. Phosphorus was extracted on each treatment combination to determine theamount of P fixed. Phosphorus recovered increased with added P. Phosphorus fixed increased with added P and depth throughout the incubation period. The soil showed a high P fixing capacity throughout the different horizons with P fixed varying between 57-99% in the upper horizons and 90-100% in the lower horizons. More than 90% of added P was fixed after one day of incubation. This therefore reduces the threat of eutrophication. Phosphorus fixed  correlated positively with clay and pH, and negatively with organic carbon and available P. Clay had a significant (p PAPR > TRP = control. The relative agronomic effectiveness of the PAPR was 63% of SSP. Although half the applied TRP was transformed to other forms than acid extractable apatite, this did not liberate P to the AEM. Less than 10% of the SSP was absorbed by the AEM.

In the nodule-rich upper slope soils, initial availability of added P was higher, but decreased more rapidly than in lower slope soils. We attributed this to an initial ‘concentration’ of fertilizer in the smaller volume of soil fines followed by slow sorption into the nodules. At the upper slope, much of the TRP and PAPR were transformed to less available forms, while at the lower slope more TRP remained as untransformed apatite. Source ; (Nutrient Cycling in Agroecosystems September 1998, Volume 52, Issue 1, pp 45–54)...PDF enclosed 

Thanks for sharing the information and papers. I have tried the sorption studies and found that P fixing capacity is 99 % upto the high concentration I tried. Now I am repeating the experiment with much higher concentration and checking for the fixation. 

As you have suggested, already the Department of Agriculture have gone for skipping of P fertilizers based on soil test or to apply the maintenance dose of 25 % of recommended fertilizer application . Yet I have not tested for Mobilization of P by legumes, but I may try it in near future. 

But still I am unable to arrive at a conclusion, about how these much of high P levels  might have reached?

 Interesting facts Anoop Ji. Clay and pH could be responsible for P-fixation in such soil. In the nodule-rich upper slope soils, initial availability of added P was higher, but decreased more rapidly than in lower slope soils. 

Yes Abishek I agree with you, However my question is Generally under laterite soils, availability of P will be reduced by Fixation of P. This fixation will be further enhanced in acidic condition, since it gets fixed by free Fe and AL oxides. But in contrast, most of the soil analysis for available P showed, huge values of available P. I am looking at the possible reasons for such by analyzing the OC, C fractions, Fe and AL oxides, other nutrient parameters etc. I am working on it and try to find a reason for this , Actually this was the question I have posted