PRACTICE XXVII 



FIXATION OF PHOSPHOKUS BY SOILS 



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

 following : 



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



2. Subsoil of gr.ay 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. 



Dissolve 1 g. of double superphosphate in 500 cc. of water, filter, 

 and keep in a stoppered bottle. 



1. Determine the phosphorus in 50-cc. portions of this solution 

 by concentrating to 25 cc. and following the directions given in 

 Practice XX, but using 35 cc. of the ammonium molybdate solution. 



cc. HNO 3 cc. KOH to mg. P mg. P per cc. 



used dissolve of solution 



ppt. 



Av 



2. Dilute 50 cc. of the double superphosphate solution to 300 cc. 

 and allow it to percolate through 50 g. of soil held in a percolator, 

 as in Practice XXVI. Determine the phosphorus in 100-cc. portions 

 of the percolate after concentrating to 25 cc., according to the direc- 

 tions given in Practice XX, using only 10 cc. of the ammonium 

 molybdate solution. 



cc. HNO 3 cc. KOH to mg. P mg. P per 6 cc. 



used dissolve of percolate 



Av 



Per cent phosphorus fixed by soil. 



58 



