70 THE PHYSICAL PROPERTIES OF SOILS 



FOURTEENTH EXERCISE 



THE POWER OF SOILS TO RETAIN FER- 

 TILIZERS APPLIED IN SOLUTION 



All soils have the power of removing from sohition 

 certain salts which may be dissolved in percolating 

 waters. This is accomplished by direct absorption in 

 some cases, while in others it involves a preliminary 

 chemical change. The extent to which different fer- 

 tilizer constituents are retained has a direct bearing 

 upon the theory and practice of manuring. 



Prepare two glass percolator tubes and place a piece 

 of fine copper gauze in the bottom of each. Make up 

 to about 15 per cent of moisture a sufficient quantity 

 of sandy loam to fill both tubes. Carefully fill both 

 percolators to the same depth, compacting the soil 

 gently and uniformly by means of a wooden rammer. 



Suspend both percolators, and pour into one a solu- 

 tion of n-ioo sodium phosphate and into the other a 

 solution of n-ioo sodium nitrate, and keep adding the 

 solution until it begins to percolate from the bottom. 



Collect the filtrates separately and test each for the 

 salts applied and compare with the original solutions. 



A very rough quantitative test for phosphorus and 

 nitrogen will serve a useful purpose, but if the appara- 

 tus is available exact quantitative determinations 

 should be made on both the original solution and the 

 filtrates. 



What practical bearing has the experiment upon the 

 time and manner of applying commercial fertilizers to 

 soils in the field? 



