322 



NATURE AND PROPERTIES OF SOILS 



Schulze 1 extracted a rich soil by slowly leaching one kilo 

 with pure water, one liter of water passing through in twenty- 

 four hours. The extract for each twenty-four hours was 

 analyzed every day for a period of six days. The total amounts 

 dissolved during each period were as follows : 



Table LXXII 



It will be noticed that the dissolved matter, both organic 

 and inorganic, fell off markedly after the first extraction. 

 Later extractions were doubtless supplied largely from the 

 substances held by absorption, which gradually diffused into 

 the water extract as the tendency to maintain equilibrium of 

 the solution overcame the absorptive action. With the re- 

 moval of the absorbed substances the equilibrium between 

 the absorption and solution surfaces and the surrounding so- 

 lution is disturbed, diffusion and solution are increased, and 

 more material gradually passes from the soil into the solution. 

 In this way, a more or less uniform and continuous extraction 

 is mantained. 



In spite of the obvious defects of the water extraction 

 method the work of Hoagland, Burd and Stewart 2 seems to 

 indicate that such data, if obtained over an extended period, 



1 Schulze, F., fiber den Phosphorsaure-Gehalt des Wasser-Auszugs der 

 Ackererde; Landw. Vers. Stat., Band 6, Seite 409-412, 1864. 



2 Burd, J. S., Water Extractions of Soils as a Criteria of their Crop- 



