This method is based on techniques described by Norrish and Hutton (1964), Norrish and Chappell (1967) and Brown and Kanaris-Sotiriov (1969). Dry-powder pellets of finely divided (<0.05 mm) soil are used. Quantitative analysis by XRF should have measurement accuracy as good as 10% (Vempati et al. 1996), which is superior to the ASPAC ‘pooled’ data (Figure 9.1) that have a median robust CV of 13%. Note that Method 17A1 provides an alternative multi-elemental XRF option that uses modern instrumentation. Both total P and total S could be included in the multi-element suite.
Boric Acid
Technical grade boric acid (H3BO3).
Silica
This may be purchased. Alternatively, prepare as described by Hewitt (1966).
Phosphorus Standard
1 mL contains 10 mg of P.
Weigh 4.3936 g potassium dihydrogen phosphate (KH2PO4; previously dried at 130°C for 2 h), dissolve, and dilute to 100 mL with deionised water in a volumetric flask. Add 2 drops of chloroform (CHCl3) to suppress biological activity. When stored in a sealed, chemically inert container at ≈4°C, this solution should remain stable for at least 2–3 months.
Preparation of Solid, Standard Pellets
Add volumes of P Standard to weighed quantities of silica, as outlined in Table 9.3. Weigh the required silica into clean beakers. Distribute the P Standard dropwise over the surface of the silica so that all the solution is adsorbed on the silica without wetting any part of the beaker. Dry the impregnated silica (65°C), grind to <0.05 mm and press into a pellet as described later. These pellets have an extended life. When soils contain concentrations >0.5% P, add appropriate quantities of powdered KH2PO4 to adjusted weights of silica.
Table 9.3. Volumes of P standard and weights of sÕica for preparatŠn of pellets of known P content.
Wt. of sÕica (g) |
Vol. of P standard (mL) |
Equivalent soÕ concentratŠn (%P) |
4.8902 |
2.50 |
0.50 |
4.9451 |
1.25 |
0.25 |
4.9780 |
0.50 |
0.10 |
4.9890 |
0.25 |
0.05 |
4.9945 |
0.125 |
0.025 |
4.9978 |
0.050 |
0.01 |
Preparation of Pellets
Oven dry (65°C) ≈10 g of air-dry soil (<2 mm), add 0.5 g H3BO3 to serve as a binder, place into a clean 100 g capacity ring and pluck head, and grind in a ‘shatterbox’ for a minimum of 2 min, or until soil particle size is <0.05 mm (see Note 1). Pellet about 2 g of soil plus H3BO3 mix (<0.05 mm) into a 45 mm dia disc with a H3BO3 backing, using a hydraulic press of around 25 tonne total force.
Analysis
Follow manufacturer’s recommendations with respect to instrument set-up. This will typically be a sequential/simultaneous XRF spectrometer, probably with fixed channel monochromators optimised for P. When feasible, make the nett count (i.e. peak-background) equal to the concentration of the standard pellet (or related to it by some power of 10), so that elemental concentration may be recorded directly from the display.
Report total P (%). An oven-dry basis can be assumed.
1. Soil sample residues in the grinding head of the ‘shatterbox’ can be a serious source of contamination, especially when components become worn. For routine applications, these residues can be overcome between samples by scrubbing and washing with water then drying. Alternatively, the residues can be minimised by grinding a small quantity of acid-washed silica sand between each unknown sample.