156 EXPERIMENTS WITH PLANTS 



hour or so, stirring it up from time to time. Next place it in the sink 

 and allow water to run through it for some time, until water, after being 

 thoroughly stirred up with it, no longer gives any acid reaction to litmus. 

 Spread the sand out to dry and when fairly dry take 25 grams for the 

 experiment, setting the rest aside for subsequent tests. The sand thus 

 treated has all the soluble plant- food washed out. We will now proceed 

 to add to it definite amounts of plant-food in order that we may see what 

 results these amounts give in our tests. A good conient of soluble^ 

 phosphoric acid in the soil is one-tenth of one per cent. We will there- 

 fore add this amount to the sand and make a test. To the 25 grams of 

 sand add 5 cc. of % per cent phosphoric acid (made by dissolving one 

 gram of solid phosphoric acid in 200 ec. of water) : this will give .025 

 gram in 25 grams of sand, or one-tenth of one per cent. Take two grams 

 of the sand which has been moistened with the acid, burn it for five 

 minutes on a red-hot shovel:^ place it in a test-tube and add 3 or 4 cc. 

 of pure nitric acid; heat until it just begins to boil; add 2 or 3 cc. of 

 water and filter. Allow 4 or 5 cc. of water to lun through the sand into 

 the filtered liquid, in order to wash out the acid, and add to the filtered 

 liquid an equal volume of a solution of molybdate of ammonia. (This is 

 made by adding 10 grams of ammonium molybdate to 25 cc. of distilled 

 water; then add 15 cc. of strong ammonia (chemically pure) and 150 

 grams nitric acid (chemically pure): keep warm and if a yellow precipi- 

 tate occurs pour oflf the clear liquid for use.) Warm until it feels rather 

 hot to the hand, then put it aside to settle. Pour off the clear liquid at 

 the top and transfer the rest to a small funnel,^ the neck of which is not 

 more than one-twelfth of an inch in diameter inside and whi<?h is sealed 

 tightly at the bottom with sealing-wax. On the outside of the neck 

 paste a paper scale ruled in millimeters (or in tenths of an inch). Let 



1 That is, soluble in the acids which are ordinarily used in making tests: the 

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

 will not appear in the tests and, being insoluble, is not directlj' available to the 

 plant. 



2 The burning is for the purpose of removing vegetable matter and humus, 

 and should, with ordinary soil samples, be carried on until the soil is light 

 gray in color. 



3 Still better are the tubes used vtith centrifugal apparatus. 



