MANUAL OF OENEBAL AGEICULTUKE. 29 



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28, DETERMINATION OF PHOSPHORIC ACID. 



Materials: Pint of pure sand, dilute hydrochloric 

 acid, stirring rod, phosphoric acid tube.* %% solution 

 of phosphoric acid, ammonium molybdate solution, funnel, 

 test tube, filter paper, pan, iron pan or iron disc, blue lit- 

 mus paper, beaker, file, millimeter rule. 



(a) First prepare a standard of comparison** by 

 taking a pint of pure sand and pouring on it about three 

 times its volume or three pints of dilute hydrochloric acid. 

 Allow to stand for an hour or more, stirring from time to 

 time with a strong stirring rod. Place in the sink and 

 allow water to run through it for several hours, until 

 water after being thoroughly stirred up with it, no longer 

 gives any acid reaction with litmus. Dry the sand and 

 take 25 grams for this experiment, saving the rest for sub- 

 sequent tests. A good content of soluble*** phosphoric acid 

 in the soil is one-tenth of one per cent. We will add 

 this amount to the sand and make a test. To the 

 25 grams of sand add 5 c.c. of a %% solution of phos- 

 phoric acid. This gives .025 gram in 25 grams of sand or 

 .1 of 1%. Take two grams of the sand which has been 

 moistened with the acid and burn for five minutes on a 

 red-hot iron for the purpose of removing the vegetable 

 matter or humus. Place in a test tube and add 3 or 4 c.c. 

 of pure nitric acid, foat until it just begins to boil, then 

 add 2 or 3 c.c. of tap water and filter into a test tube. 

 Wash out the acid by allowing 4 or 5 c.c. of water to run 

 through the sand into the filtered liquid and add to the 

 filtrate, its own volume of a solution of molybdate of am- 

 monia. Then place the test tube in a beaker of hot water 



* These tubes can be obtained of Justinian Cairne Co., 573 Mar- 

 ket St., San Francisco, California, at 40 cents each or $4.20 per 

 dozen. 



**A standard of comparison is best made by taking two grams 

 of a soil in which the phosphoric acid has been accurately determined, 

 instead of purifying sand, etc. 



***That is, soluble in the acids used in making tests. The soil 

 may contain a great deal more phosphoric acid in insoluble form, but 

 this will not appear in the test and is not directly available to the 

 plant. 



