i;2 SOIL CONDITIONS AND PLANT GROWTH 



analytical operations are carried out in the usual manner, but special care must 

 be taken to free the solution from silica or organic matter " (The Soil). As a 

 rule only potash and phosphoric acid are determined, but where other bases 

 are wanted they are estimated in the usual way. 



Potash. 50-100 c.c. of the solution are evaporated to dryness after 

 addition of 0-5 gram of pure CaCO 3 if the original soil did not effervesce 

 when HC1 was added. Two courses are then open : 



(a) The residue is gently ignited over a Bunsen burner till it is completely 

 charred, it is then taken up with water several times till all the potassium 

 chloride is dissolved (Neubauer's method 1 (212)). To the clear filtrate 5 c.c. 

 of platinum chloride (containing '005 gram 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. perchloric acid (sp. gr. 1-12), 

 concentrate till 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 50-100 

 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 now digested for - hour on a sand bath with 

 50 c.c. of 10 per cent. H 2 SO 4 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 

 2 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 pre- 

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

 completely dissolves. Measure the excess by titration, using phenolphthalein 



N 

 as indicator, i cc. of alkali = -0003004 gms. P 2 O 5 . 2 



Available Potash and Phosphoric Acid. Dyer's directions are as follows : 

 200 grams dry soil are placed in a Winchester quart bottle with 2 litres of dis- 

 tilled water in which are dissolved 20 grams of pure citric acid. The soil is 

 allowed to remain in contact with the solution at ordinary temperatures for 

 seven days, and is shaken a number of times each day. The solution is then 

 filtered, and 500 c.c. taken for each determination ; this is evaporated to dry- 

 ness, and gently incinerated at a low temperature. The residue is dissolved 



1 The older method due to Tatlock is still sometimes used. It is described by Dyer 

 (91). 



2 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 modification is given here. The method is 

 applicable for the " available " P 2 O 5 , but in this case the residue from the citric acid extrac- 

 tion has first to be heated two hours at 120-160 to render the silica insoluble. The older 

 method is described by Dyer (91). 



