Dear Colleagues! Tell me a fairly simple, reproducible and effective method for determining phosphorus in soil with an estimate on a spectrophotometer. The task is to teach students to estimate the amount of phosphorus in various soils. I will be grateful for your help!
Dear Sergey,
I suggest you choose an extraction method that is used in the region and suited to the soil type, e.g. the extractant could be 0.5 M NaHCO3 (look up 'Olsen') for alkaline soils or soils containing CaCO3. Alternatively the extractant could be acidic for acidic soils (look up Mehlich 3 or Bray-1 ). Colorimetry would normally be using molybdate with reduction using ascorbic acid (see Murphy and Riley), and measurement of the intensity of the blue colour at 880 nm in a spectrophotometer or by comparison with a colour chart.
Regards, Paul.
I should add that the previous extraction methods apply to 'available' P not total P which is usually poorly related to plant requirements or environmental risk.
Dear Sergey,
To the best of my knowledge, the most commonly employed methods to measure soil available P (Olsen, Mehlich-3, and Bray-1) correspond to a specific extractor type and pH value, and their reliability depends on the soil type. The Olsen method is used for soils with mildly acid, neutral or alkaline pH, and it has wide international acceptance as an indicator of soil P fertility. Conversely, the Mehlich-3 method was developed for soils with acid-to-neutral pH and the Bray-1 method is indicated for soils with pH below 6.8.
As dear Paul explained well, the Olsen method is based on the extraction of phosphate from the soil by 0.5 N sodium bicarbonate solution adjusted to pH 8.5. In the process of extraction, hydroxide, and bicarbonate competitively desorb phosphate from soil particles and secondary absorption is minimized because of high pH. The orthophosphate ion reacts with ammonium molybdate and antimony potassium tartrate under acidic conditions to form a complex. This complex is reduced with ascorbic acid to form a blue complex that absorbs light at 880 nm.
If you are attempting to teach a method to your students, I would say that I have learned this method simply as a student.
Best regards.
I agree with you that this is very important, because often total phosphorus does not provide an objective assessment of the coverage of plant needs in a nutrient element.
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