352 SOIL CONDITIONS AND PLANT GEO WTM 



M. Salter/ and Continental methods by von 'Sigmond (266(2). 

 See also p. 344. 



Potash. — 50 to 100 c.c. of the solution are evaporated to dry- 

 ness, after addition of 0-5 grm. of pure CaCOa if the original soil did 

 not effervesce on addition of HCl. Two courses are then open : — 



(a) The residue is gently ignited over a Bunsen burner until 

 completely charred, and is then extracted with water until all the 

 potassium chloride has dissolved (Neubauer's method^ (212)). To 

 the clear filtrate 5 c.c. of platinum chloride (containing -005 grm. 

 Pt per c.c.) are added and the mixture slowly concentrated on the 

 water bath to a very small bulk. The potassium platino-chloride is 

 filtered off in a Gooch crucible, washed with 80 per cent, alcohol, 

 dried and weighed. 



or [b) Add 10 c.c. of 5 per cent, baryta solution, evaporate to 

 dryness, ignite and take up with water as in {a), add 2*5 c.c. per- 

 chloric acid (sp. gr. i"i2), concentrate until dense fumes are given 

 off, allow to cool, add 20 c.c. 95 per cent, alcohol, and stir. Decant 

 off the clear alcohol, add 40 c.c. alcohol containing 0*2 per cent, 

 perchloric acid, transfer to a tared filter paper, wash with 50 to 100 

 c.c. of 95 per cent, alcohol till the runnings are no longer acid, dry 

 at 100° and weigh as KCIO4. 



Phosphoric Acid. — The charred residue from which the potassium 



chloride has been removed is digested for half an hour on a sand 



bath with 50 c.c. of 10 per cent. H2SO4 and filtered; the filtrate is 



treated with 25 c.c. cone. NH4NO3 solution and warmed to 55° C. ; 



25 c.c. ammonium molybdate, previously warmed to 55° C, is added 



and the whole allowed to stand for two hours and filtered. Wash 



with 2 per cent. NaNOs till the washings are neutral, transfer the 



precipitate and filter paper to the beaker used for the precipitation, 



and add a known volume of standard alkali so that the precipitate 



completely dissolves. Measure the excess by titration, using phenol- 



N 

 phthalein as indicator, i c.c. of — alkali = "0003004 grm. P205.^ 



^ West Virginia Bui., 159, igi6. 



* The older method due to Tatlock is still sometimes used. It is described 

 by Dyer (91). 



3 This volumetric method was originally described in Bull. 46 (revised). 

 United States Division of Chemistry (Washington, 1898). A careful examination 

 has been made by Prescott and the conditions laid down under which it gives 

 satisfactory results {jfourn. Agric. Set., 19x4, 6, 111-120). Prescott's modifica- 

 tion is given here. The method is applicable for the "available" P2O5, but in 



