PRACTICE XXXII 



SOIL ANALYSIS ; DETERMINATION OF THE EASILY SOLUBLE 



PLANT FOOD 



Place 100 g. of air-dried soil in. a 2-liter bottle, add 1 liter of 

 a 1 per cent citric acid solution, let stand seven days, shaking several 

 times each da}*, filter off 800 cc., evaporate to dryness finally in a 

 small porcelain dish, gently ignite over a Bunsen burner, add 10 cc. 

 of nitric acid, evaporate to dryness, dissolve in nitric acid, filter, and 

 wash with hot water. Make up to exactly 500 cc., mix thoroughly, 

 transfer to a dry bottle, and label Solution B. 1 



Concentrate 200 cc. of Solution B to about 25 cc., and determine 

 phosphorus according to the directions given in Practice XX, using 

 15 cc. of the clear ammonium molybdate solution. 



cc. HXO, cc. KOH to nig. P % P P in 



used dissolve 2 million 



ppt. of soil 



Av 



2. Potassium. 



Evaporate 200 cc. of Solution B to complete dryness and deter- 

 mine potassium as indicated in 3, Practice XXXI. 



Weight of dish + K 2 PtCl 6 



Weight of dish 



Weight of K 2 PtCl 6 



Weight of K 



Per cent of K 



Av. per cent 



K in 2 million of soil 



1 In order to avoid any difficulty in removing the organic acid, the use of one fifth 

 normal nitric acid, as suggested by Peters and Averitt,* or of still weaker nitric 

 acid, as suggested by d'Sigmond.t is to be recommended for the extraction of easily 

 soluble plant food. 



* Bureau of Chemistry, Bulletin .9.9, p. 115 ; and Bulletin 105, p. 142. 

 t Journal American Chemical Society, Vol. 29 (1907), p. 929. 



69 



