MOVEMENTS OF SALTS IN SOILS. 97 



RETENTION OF NITRATES BY CLEAN SAND. 



In February, 1902, a sample of "Sea Island sand" in the 

 collection of this Bureau, was rendered nitric-acid-free by re- 

 peatedly washing the dry sand in disulphonic acid and then 

 treated with a solution of potassium nitrate. A quantity of this 

 sand 50 grams was washed during 3 minutes without dry- 

 ing, 10 consecutive times in 100 c. c. of distilled water, and the 

 amounts of NO 3 determined in each case. After the last wash- 

 ing the sample was dried and the sand itself treated directly 

 with disulphonic acid and an examination made for nitric acid. 

 The results obtained are given in the next table : 



Amounts of nitric acid recovered by repeated washing and then 

 treating the washed sand with disulphonic acid. 



Recovered with 1st washing of three minntes 3.12100 mg. 



Recovered with 2nd washing of three minutes 32840 mg. 



Recovered with 3rd washing of three minutes 04515 mg. 



Recovered with 4th washing of three minutes 01736 mg. 



Recovered with 5th washing of three minutes 01380 mg. 



Recovered with 6th washing of three minutes 01280 mg. 



Recovered with 7th washing of three minutes 01109 mg. 



Recovered with 8th washing of three minutes 01100 mg. 



Recovered with 9th washing of three' minutes 01100 mg. 



Recovered with 10th washing of three minutes 01101 mg. 



Recovered with disulphonic acid after drying 76290 mg. 



Total recovered 4.34551 mg. 



Amount present 4 . 12500 nag. 



Thepe and the previous observations point strongly to the 

 retention of nitric acid in some manner by soils, and indicate 

 that the close and slowly moving layers of water which move 

 over the surfaces of soil grains and granules by capillarity are 

 able to wash them more thoroughly than is practicable by simple 

 agitation in water or by the percolation of water through them. 



The larger amount of nitric acid, recovered by the repeated 

 washing, may be due simply to the failure of agreement between 

 duplicate determinations on samples taken from the same bulk 

 lot ; it may be in part due to the very slight color which the 

 disulphonic acid imparts to distilled water after neutralization 

 with ammonia, this becoming additive in such a series. Or, 

 again, there are forms of organic matter in soils which develop, 

 in connection with disulphonic acid, a color resembling the- yel- 

 low of the standard color solution ; these, if present in this sand, 

 7 



