100 THE PRINCIPLES OF SOIL MANAGEMENT 



when dry is due to several causes, one of which is much 

 the most prominent. This is cementing materials. A 

 cementing material is any material which binds sur- 

 faces together. In a gravel or sand pit, masses of the 

 material are sometimes found united into a conglom- 

 erate rock. After a protracted dry spell, moist sur- 

 faces show a white incrustation in the surface layer, 

 which is due to the deposition of the salts in solution 

 when the moisture evaporated, and this acts as a bind- 

 ing material. This is one of the main reasons why a 

 fully dried soil is usually so much harder than one 

 slightly moist. The salts of many kinds which were in 

 solution in the moisture have been deposited. 



This composite of dissolved salts is the first of four 

 common cementing materials which occur in the soil. 

 It is generally a weak binding material. The second 

 material is lime. Some soils are very rich in this com- 

 pound. Particularly is this true of most glacial soils, 

 and in North Dakota and other sections of the country 

 extended areas of gravel beds occur, in which the upper 

 two or three feet are completely bound together by the 

 deposition of lime between and around the particles. 

 It has been leached out of the soil above as bicarbonate, 

 under the influence of carbonated water formed by the 

 decaying organic matter, but here, in the loose gravel, 

 by the escape of some of the carbon dioxid it was 

 deposited. This is the usual history of the process. 

 Cementation by lime carbonate is a very common and 

 very general process. The third cementing material 

 is the various forms of iron usually oxides in various 

 stages of hydration. They have come into solution by 



