I have to determine different P fractions (readily plant-available P, labile inorganic
P (another plant-available fraction), sesquioxide- (Fe-oxide and Al-oxide) associated inorganic P and Ca-associated P) after sequential extraction in the compost prepared from poultry manure. Could it be possible to find these fraction by using ICP, ICP-MS or ICP-AES rather than following the laborious Molybdenum Blue method (Olsen and Sommers, 1982)?
We are analyzing Total phosphorus by ICP-OES in our Institute. The procedure is follows. But not sure about the analysis of Phosphorus fractions.
The sample (0.5-1 g) was digested using microwave digestion method (Anton Par micro wave system) for mineral analysis with 6 ml of nitric acid and 2 ml of hydrogen peroxide in inert polymeric microwave vessels. The vessels were sealed and heated in the microwave digestion system. The initial temperature was increased to 180°C and maintained at this temperature for 10 min. An interval of 10 min was given for ramp and then was decreased to 70°C for 8 min. The vessels were kept in the same condition for 10 min for venting. The resulting solutions were cooled and diluted to the known volume (25 ml) with de-ionized water. The minerals were determined by inductively coupled plasma-optical emission spectrometry (ICP-OES; Agilent 5100 SUV) using the 5.2 software. The certified reference material of ICP multi-element standard solution (10 mg/L; Merck) was used for calibration. The ranges of the calibration curves (5 points) were selected to match the expected concentrations of all the elements of the samples being analysed. The correlation coefficient (r) obtained for all cases was 0.9999.
In principal you can analyse the various fraction by icp-aes. However, you have to be aware that icp measures total soluble P, whereas the molybdenum blue method only detect ortho-phosphates. In compost the man problem might be organic P which will be extracted especially by alkaline solvents. At least you have to keep this in mind when you interpretate your reults and compare them to others.
As pointed out by Dieter Lohr,there are problems in estimating organic P fractions with ICP as ICP can not distinguish between soluble/extractable organic and inorganic P in the medium.In normal fractionation,say Hedley's fractionation scheme different P fractions are extracted by using reagents like 0.5M NaHCO3,0.1M NaOH and 1.0 M HCl etc.In the extracts, inorganic P(Pi) is directly estimated by colorimetric method,say Murphy and Riley method. Pi is estimated before and after digesting the organic P in each extract with H2SO4/H2O2(other recommended digestion methods also) and by difference the organic P(Po) fraction is obtained.The following two references provide the details of P fractionation in manure/compost using Hedley's fractionation and also estimation of organic P fractions both by conventional method (after digestion) and ICP method and their associated complications if any etc.
Phosphorus forms in manure and compost and their release during simulated rainfall . J.Environ Quality 29:(5)1462-1469,2000.
https://pubag.nal.usda.gov/pubag/downloadPDF.xhtml?id=19027&content=PDF
Phosphorus concentrations in sequentially fractionated soil samples as affected by digestion methods.Nascimento et al. Scientific Reports | 5:17967 | DOI:10.1038/srep17967 2015 Open access.
https://www.nature.com/articles/srep17967.pdf
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