132 HATCH EXPERIMENT STATION. [Jan. 



secutive hours in a boiling-water bath, shaking once each 

 hour. The stopper of the flask should carry a condensing 

 tube, to prevent evaporation. The material is filtered, and 

 the residue is washed with distilled water until free of acid. 

 The organic matter in filtrate is oxidized with nitric acid 

 and evaporated to dryness on the water bath, finishing on 

 sand bath to complete dryness. The material when cool 

 is taken up with hot water and a few cubic centimeters of 

 hydrochloric acid, and again evaporated to complete dryness. 

 It is taken up as before, filtered and washed thoroughly with 

 cold water, cooled and made up to 500 c.c. 



II. — Directions of the Association of Official Agricultural Chemists 

 for the Determination of Available Phosiriioric Acid in Soils, 

 Fifth Normal Hydrochloric Acid being used as the Solvent. 



1. Determination of Moisture. — Use the official method 

 described in Bulletin 46, page 48, Division of Chemistry, 

 United States Department of Agriculture. Calculate all 

 results to the water free basis. 



2. Determination of Phosphoric Acid Soluble in Fifth 

 Normal Hydrochloric Acid. — {a) Preliminary treatment : 

 Digest 20 grams of soil with 200 c.c. of fifth normal hydro- 

 chloric acid at 40° C. for five hours. Titrate 20 c.c. of the 

 clear filtrate against a standard caustic soda solution, using 

 phenolphthaline for the indicator. From this data calculate 

 the amount of hydrochloric acid necessary to be added, so 

 that the solution will be fifth normal after allowing for the 

 acid neutralized. (b) The determination : Weigh out 50 to 

 100 grams of soil into an Erlenmeyer flask, and add 10 c.c. 

 of acid, corrected for neutralization as directed under (a) 

 for every gram of soil used. The flask is corked with a 

 rubber stopper, which carries a thermometer. The flask is 

 then placed in a water bath previously heated to 40° C, 

 and the contents of the flask are thoroughly shaken every 

 half hour during the digestion. The solution is then filtered 

 through a ribbed filter of two thicknesses of paper, refilter- 

 ing the first portion, if cloudy. The filter should be large 

 enough to receive the entire contents of the flask. Before 

 filtering the contents, the flask should be well shaken. Four 



