Feb. II, 1918 Effect of Season and Crop Growth on Soil Extract 331 



EXTRACTION PROCEDURE 



It has been pointed out in an earlier portion of this discussion that 

 several extraction procedures have been applied to the study of the water- 

 soluble material of soils. In Europe the methods of Schloesing {46, 47) 

 and Mitscherlich (42) are largely favored, though the work of the U. S. 

 Bureau of Soils (61) has some followers. This latter system of extraction 

 is practically the only one which has been used in this country. The 

 desire to have the result of this investigation somewhat comparable to 

 previous work was therefore an argument in favor of this general pro- 

 cedure. This would not have held had there been any closer method of 

 approximating the actual soil solution, for the medium v/hich nourishes 

 the plant should always be our final object. As before noted, it is not 

 known that this actual solution has ever been obtained. It then be- 

 comes necessary to use some arbitrary method of extraction, and from 

 the data at hand in beginning this work the general procedure of the 

 U. S. Bureau of Soils was chosen. 



In all the seasonal studies which are recorded later only fresh soil 

 was used for the analytical studies. The great change which soil under- 

 goes in drying as seen from the work of King (55), outweighs any con- 

 siderations of expediency. King himself took the contrary view, but a 

 study of his own figures shows how large and how variable were the 

 changes between different soils caused by drying the samples previous 

 to analysis. His adoption of this modification was influenced by a 

 desire to carry on work at several stations from one central laboratory. 

 In the present investigation the soils had been brought together, so no 

 such reason existed. It has been desired throughout to perform all 

 work on the fresh soils and to show what differences existed in them. 



The uniform procedure of extraction is as follows : 



Place the weighed portion of fresh soil in a large mortar and mix with a small 

 amount of distilled water till it becomes a thin homogenous paste. Then add the 

 rest of the distilled water, making in all five times the weight of the soil and mix 

 the whole for exactly 3 minutes. Then transfer to wide-mouthed bottles all the 

 solution and suspended soil that will pour off and allow it to stand for 40 minutes. 

 At the end of this period pour the solution without mixing into cylinders of a Briggs 

 filtering apparatus, connected to the pressure system, and apply compressed air at 

 approximately 100 pounds' pressure till filtration is complete. Discard the first 

 portion of the solution, about 50 c. c, to avoid any dilution by the moisture held in 

 the Pasteur-Chamberland candles and also any small absorption by the filter from the 

 solution. 



After filtration is complete wash the filter candles and cylinders in tap water, rinse in 

 distilled water, and pass 600 to 800 c. c. of distilled water through each candle to wash 

 out any remaining portions of solution. After each weekly set of filtrations is com- 

 pleted, thoroughly dry the washed candles in an air oven at ico° C. to prevent the 

 development of bacterial flora in the outer pores of the tubes. The efficiency of 

 filtration of the candles is gradually impaired by continual use, owing to the deposition 

 of clay and fine silt in the unglazed porcelain of which it is composed. This can be 

 partly remedied by igniting the dried tubes in a mufile furnace for }A hour or more. 



