1899.] PUBLIC DOCUMENT — No. 33. 133 



hundred to 600 o.c. of Hh> filtrate (at 20° C.) are evaporated 

 to dryness after adding 1 to ^ c.c. of nitric acid. If there 

 is any appreciable amount of organic matter present, the 

 residue is to be carefully charred. Moisten the residue 

 with hydrochloric acid and add 50 to 100 c.c. of distilled 

 water, and then digest. Filter, neutralize with ammonia, 

 add 5 c.c. of strong nitric acid and 15 grams of nitrate of 

 ammonia in solution. Complete the determination accord- 

 ing to one of the official methods given for the determination 

 of phosphoric acid, or use the Goss method as given in Cir- 

 cular No. 4 to accompany Bulletin No. 4G. 



777. — Determination of the Available Phosphoric Acid in Soils by 

 Means of a One Per Cent. Solution of Citric Acid {Dr. B. 

 Deyer) . 



Preliminary Treatment. — Twenty grams of soil are di- 

 gested with 200 c.c. of a one per cent, citric acid solution 

 for five hours, at ordinary temperature (18° to 21° C). 

 The material is filtered and solution is titrated against a 

 standard alkali solution, to determine the amount of acid 

 neutralized by alkalies in the soil. For the estimation of 

 the "available" potash and phosphoric acid, one per cent, 

 citric acid solution has been employed, digesting 100 grams 

 of air-dried soil with 500 c.c. of the solvent, as directed in 

 the preliminary test, corrected for neutralization, for five 

 hours at room temperature. The filtered solution is evapo- 

 rated to dryness, charred, and the residue abstracted with 

 dilute hydrochloric acid and water. The filtrate from this 

 operation is treated for the determination of phosphoric acid 

 as directed in one of the official methods. 



IV. — Determination of the Available Phosphoric Acid in Soils by 

 Means of a Neutral Solution of Citrate of Am no mi it. 

 Ten grams of the soil are digested for one-half hour, at 

 65° C, with 500 c.c. of strictly neutral solution of citrate 

 of ammonia, specific gravity 1.09. The flask carries a 

 rubber stopper, and is thoroughly agitated every five min- 

 utes. At the expiration of thirty minutes, remove flask 

 from bath and filter as rapidly as possible. Wash thor- 

 oughly with water at 65° C. Evaporate the solution to 



