Determination of organic P

The chemical nature of half of the soil organic P remains unknown, but there are three main groups: inositol phosphates, phospholipids and nucleic acids. There are also phosphoproteins and sugar phosphates (Dalal, 1977). There appears to be no direct methods for the determination of organic phosphorus in soils, but the indirect procedures of extraction and ignition may be used to fractionate the soil phosphorus approximately into inorganic, organic and total P. One of the main difficulties is choosing an extractant for inorganic P that will not simultaneously cause partial hydrolysis of the organic P fraction. One scheme of fractionation of extractable soil P has been presented by Williams (1950), and a method based on this is given in Method 11.1 below. This may be summarized as follows:

a = inorganic P (acetic acid extractable) b = inorganic P (alkali extractable) c = inorganic + organic P (alkali extractable) a + b = inorganic extractable P d = c - b = organic alkali extractable P a + b + d = total extractable P.

If required, the remaining P not extracted by either extractant is obtained by subtracting (a + c) from the total P, which is determined by a separate P determination using a safer Na2CO3 fusion (Jackson, 1958) or the recommended, but more hazardous perchloric acid digestion technique (Olsen and Dean, 1965). An alternative ignition technique is given by Olsen and Dean (1965) whereby ignition converts organic P to inorganic P, and extraction of duplicate soil samples is carried out using concentrated HCl before and after ignition.

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