PRACTICE XXI 



DETERMINATION OF TOTAL PHOSPHORUS IN SOILS 



Each group of students will work upon one stock soil, such as the 

 following : 



1. Surface of gray silt loam. 5. Surface of black clay loam. 



2. Subsoil of gray silt loam. 6. Subsoil of black clay loam. 



3. Surface of brown silt loam. 7. Sandy soil. 



4. Subsoil of brown silt loam. 8. Peaty soil. 



Weigh 10 g. of sodium peroxid into a crucible of spun iron or of 

 porcelain of about 100-cc. capacity and thoroughly mix with it 5 g. 

 of soil. If the soil is peaty, burn off the greater part of the organic 

 matter before mixing with the sodium peroxid. If the soil is very 

 low in organic matter, add about 0.2 g. of pulverized sugar to hasten 

 the reaction. Heat the mixture carefully by applying a flame directly 

 upon the surface of the charge and upon the sides of the crucible 

 until the reaction is over, and keep at a low red heat for 15 minutes. 

 Do not allow fusion to take place. By means of a large funnel and 

 a stream of hot water transfer the charge to a 500-cc. measuring flask. 

 Acidify with hydrochloric acid, add several cc. of nitric acid, and boil. 

 If the reaction has taken place properly, there will be no particles 

 of undecomposed soil in the bottom of the flask. Let cool, make up 

 to the mark, and shake thoroughly. Allow the silica to settle and 

 draw off 200 cc. of the clear solution into a beaker. 



Add ammonium hydroxid until almost neutral, heat to boiling, 

 and add ammonium hydroxid slowly in slight excess. Keep boiling 

 hot for about one minute, stir, filter through a 15-cm. filter, and 

 wash at once with hot water until free of chlorids. Return the pre- 

 cipitate to the beaker by means of a stream of hot water, holding 

 the funnel over the beaker, and dissolve in hot nitric acid, pouring 

 the acid upon the filter to dissolve any precipitate remaining. Evapo- 

 rate solution and washings to complete dryness on a water bath, take 

 up with dilute nitric acid, heating if necessary, and filter out any 

 insoluble silica. Evaporate filtrate and washings to about 25 cc. and 

 determine phosphorus according to the directions given in Prac- 

 tice XX, using 15 cc. of the clear ammonium molybdate solution. 



cc. HNO 3 cc. KOH to mg. P % P P in 2 mil- 



used dissolve ppt. lion of soil 



Av 



With the above results record those obtained by three members of 

 each group, as indicated upon the following page. 



42 



