352 SOIL CONDITIONS AND PLANT GROWTH 



M. Salter, 1 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 CaCO 3 if the original soil did 

 not effervesce on addition of HC1. 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 2 (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. 1*12), 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 o - 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 KC1O 4 . 



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. H^SC^ and filtered; the filtrate is 

 treated with 25 c.c. cone. NH 4 NO 3 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. NaNO 3 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. P 2 O 5 . 3 



10 



1 West Virginia Bui., 159, 1916. 



2 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 (Journ. Agric. Sci., 1914,6, 111-120). Prescott's modifica- 

 tion is given here. The method is applicable for the " available" P 2 O 5 , but in 



