30 MANUAL OF GENERAL AGRICULTURE. 



until the precipitate has come down. Pour off the clear 

 liquid at the top and transfer the rest to the phosphoric 

 acid tube. Suppose that the precipitate, which is 

 molybdo-phosphate of ammonium, when it has settled into 

 the neck of the tube forms a column one centimeter high. 

 This would indicate a content of .1 of 1% of phosphoric 

 acid in the soil. With a file mark the height of this col- 

 umn after making sure that the test is correct either by 

 comparison with tests of others or by repeated tests. 



(b) Test soil as follows: Take two grams of soil, 

 burn it on a red-hot iron for five minutes or until it is light 

 gray in color. Place in a test tube and add 3 or 4 c.c. 

 nitric acid, then heat until it just begins to boil ; add 2 or 

 3 c.c. of water and filter into a test tube. Allow 4 or 5 c.c. 

 of water to run through the soil into the filtrate and add 

 to the filtrate its own volume of ammonia molybdate, then 

 place the test tube in a beaker of hot water until the pre- 

 cipitate has come down. Pour off the clear liquid at the 

 top and transfer the rest to the phosphoric acid tube. If 

 the precipitate is above the standard phosphoric acid mark 

 we know that the soil is well supplied with phosphoric 

 acid, i. e., there is more than .1 of 1%. Use a millimeter 

 rule and calculate the exact per cent. 



29. DETERMINATION OF LIME. 



Materials : Whiting, sand treated with dilute hydro- 

 chloric acid, test tube, saturated solution of oxalate of am- 

 monia, ammonium chlorid, file, funnel the neck of which is 

 not more than one-eighth inch in diameter and closed at 

 the bottom by heating in a hot flame, filter paper, beaker, 

 millimeter rule. 



(a) First prepare a standard of comparison as fol- 

 lows: To exactly 25 grams of sand previously treated 

 with hydrochloric acid, add a quarter of a gram of whit- 

 ing, which will give a content of one per cent of carbonate 

 of lime in the soil. Thoroughly mix with the sand. Place 

 a gram of this sand in a test tube and add 1 c.c. chem- 

 ically pure hydrochloric acid. Heat until it just begins to 

 boil, then add ammonia water until a permanent precipi- 



