54 



PRACTICE 26. 

 DETERMINATION OF PLANT FOOD IN SOILS. 



Each student will select a soil in which he is especially interested. 



(a) Nitrogen. 



Determine according to directions given in Practice 12. 



(b) .Dry Matter. 



Weigh out 5 gm. of the air-dried soil in a small porcelain dish, dry at 

 100 for 5 hours, cool in a desiccator, and weigh. 



Weight of vessel plus dry matter 



Weight of vessel 



Weight of dry matter 



Percent dry matter 



Average 



(c) Insoluble matter. 



Place 10 gm. of the air-dried soil in a 200 cc. Erlenmeyer flask, add 

 100 cc. of hydrochloric acid (sp. gr. 1.115), close with a rubber stopper in 

 which is a glass tube 18 inches long, and digest for ten hours on the water- 

 bath, at the temperature of boiling water, shaking once every hour. Dilute, 

 filter through a 15 cm. filter, wash free of chlorids with distilled water, 

 and evaporate the filtrate and washings to dryness with 5 cc. of nitric acid 

 to destroy organic matter. Take up with about 10 cc. of hydrochloric acid 

 and about 25 cc. of water, again run to dryness and heat three hours in an 

 air-oven at 110. Add about 15 cc. of hydrochloric acid, heat 30 minutes 

 on the water-bath, dilute, filter through a 9 cm. filter and wash with dis- 

 tilled water. Ignite the filter and contents with the main portion of the 

 insoluble matter. 



Weight of crucible plus insoluble matter 



Weight of crucible 



Weight of insoluble matter 



Percent of insoluble matter 



Average 



Make the filtrate and washings up to exactly 500 cc., mix thoroughly 

 and put in dry stoppered bottle. Label it Solution A. 



(d) Phosphorus. 



Evaporate 200 cc. of Solution A to about 20 cc. and determine phospho- 

 rus according to the directions given in Practice 21, using 15 cc. of the 

 clear ammonium molybdate solution. 



cc. HN0 3 cc. KOH to mg. P #P 



used dissolve ppt. 



Ave 



(e) Potassium, Sodium, and Iron. 



Evaporate 100 cc. of Solution A to complete dryness. Add about 25 

 cc. of hot water and 50 cc. of a saturated barium hydroxid solution, let 

 stand 30 minutes on the water-bath, filter through an n cm. filter, and 



