Feb. II, 1918 Effect of Season and Crop Growth on Soil Extract 335 



rigorous than the similar treatment which surface soil received in the 

 field. Even so it is seen to have produced several striking changes. 

 The nitrates and calcium are slightly decreased, but the greatest differ- 

 ence is observed in the total solids. Here we see a significant increase. 

 This is further shown by the volatile solids to be entirely due to a gain 

 in soluble organic matter. Such an increase in soluble organic materials 

 would suggest that the efifect of air-drying on the soil minerals would 

 largely be of a secondary nature. Any cultivation or stirring into 

 the soil mass of this air-dried portion of the soil would result in placing 

 this soluble material in circulation in the soil solution. It would thus 

 become available as food for the biological life of the soil, and may be one 

 of the factors tending to produce the large liberation of nutrients after 

 cultivation. 



In the final treatment of the two soil samples carbon-dioxid gas was 

 frequently passed through the jars containing the soil during the 24- 

 hour period. This resulted in a striking liberation of soluble mineral 

 compounds. There was also a slight increase in the production of ni- 

 trates in comparison to the untreated soil, but it is very noticeable that 

 the phosphates remained practically constant. This is significant. It 

 will be pointed out later that phosphates are the only one of the plant 

 foods with which the solution of each individual soil appears to be satu- 

 rated. The amount of nonvolatile solids liberated by this treatment is 

 over 200 p. p. m., and shows that the efifect of the carbon dioxid e^Jtends to 

 other compounds than those indicated. From this study it seems prob- 

 able that the effect of the carbon dioxid present in the soil air and dis- 

 solved in the soil solution is the most important influence that renders the 

 soil minerals soluble. 



NATURE OF THE WATER EXTRACT 



The above series of results establish the accuracy of the extraction pro- 

 cedure which has been employed. They do not throw any light on the 

 nature of the solution obtained. All of the minerals present in the soil 

 possess a small but appreciable solubility. Any soil extract will there- 

 fore contain not only the material which is actually in solution or very 

 readily soluble but also an extra amount derived from the soil minerals 

 themselves. This latter portion of the soil extract may in some cases 

 be different for the same soil. It is a well-known principle that when a 

 compound is treated with a solution containing a common ion its solu- 

 bility will be repressed. If we now turn to the graphs showing the nutri- 

 ents extracted from the same soils, cropped and uncropped (fig. 3-6), it 

 will be observed that there are striking differences, which will be dis- 

 cussed later. In the cropped soil there is much less soluble material, 

 and the solution used to extract it will more nearl}^ approach distilled 

 water than in its duplicate, where nothing has been removed by the 



