52 



PRACTICE 25. 

 FIXATION OF PHOSPHORUS BY SOILS. 



Each group of students will use one of the following soils : 



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. 



Dissolve i gm. of double superphosphate in 500 cc. of water. Filter 

 and keep in a stoppered bottle. 



(a) Determine the phosphorus in 50 cc. portions of this solution by 

 concentrating to 25 cc. and following the directions given in Practice 21. 



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



used dissolve ppt. 



Ave .... 



(b) Dilute 50 cc. of the double superphosphate solution to 250 cc. and 

 percolate through 100 gm. of soil held in a percolator as in Practice 6. De- 

 termine the phosphorus in 100 cc. portions by concentrating to 25 cc. and 

 following the directions given in Practice 21, using only 15 cc. of the am- 

 monium molybdate solution. 



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



used dissolve ppt. 



Ave 



Percent phosphorus fixed by soil 



(c) Thoroughly mix 10 gm. of CaCO3 with 100 gm. of the same soil, 

 dilute 50 cc. of the double superphosphate solution to 250 cc., percolate 

 through soil and determine the phosphorus in 100 cc. portions of the per- 

 colate in the same manner as above. 



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



used dissolve ppt. 



Ave ...... 



Percent phosphorus fixed by soil 



Calculate the results obtained and with these record the results ob- 

 tained by three members of each group as indicated in the table on the 

 following page. 



Give the general reaction. Explain the effect of the CaCO3. How was 

 the fixation brought about in the non-calcareous soils? 



