27 





SOIL 



HARDPAN 



CEHENTED 

 GRAVEL, 

 SAND. 

 AND CLAY 



Fig. 4. Hardpan layer with 

 compact material beneath. Break- 

 ing the hardpan will be of little 

 benefit because of the cemented 

 nature of the substratum. 



is loose soil very similar to that above the hardpan (Fig. 3), and if 

 the pan is broken by dynamite or other means, irrigation waters and 

 plant roots will readily work down into the underlying soil mass. In 

 such cases the hardpan is not a 

 serious factor as it ordinarily re- 

 cements very slowly. In some cases 

 the hardpan is underlaid by a 

 compact, semi-cemented layer of 

 soil, sand and gravel that is prac- 

 tically impenetrable to water or to 

 plant roots. (See Fig. 4.) With 

 such soils, dynamiting the hardpan 

 is of little or no value as there 

 is no good soil beneath for the 

 roots to penetrate and no oppor- 

 tunity for drainage or aeration 

 through the substratum. 



There is another class of hard- 

 pan that has been formed at the 

 same time that the soil was formed. 

 This occurs where variations in the 

 soil - forming activities caused a 

 layer of soil to be deposited, then a 

 layer of material that cemented to 

 a hardpan, then another layer of 

 soil, another layer of hardpan, and 

 so on. (Fig. 5.) These hardpan 

 layers are hard to handle, as blast- 

 ing is not satisfactory unless each 

 of the layers is broken. The hard- 

 pan layers do not, however, exist as 

 continuous sheets because in the 

 process of formation of the soil, 

 portions were washed away, the 

 space being filled with other soil 

 material. This, together with the fact that the hardpan is often 

 cracked and sometimes rather soft, gives opportunity for irrigation 

 water and plant roots to penetrate to considerable depths. 

 , Alkali. Wherever the drainage conditions are poor and there is 

 a larger amount of water passing from the surface by evaporation 

 than passes down through the soil mass, there is the possibility of an 

 accumulation of soluble material or "alkali" on the surface. The 



SOIL 



HARDPAN 



SOIL 



HARDPAN 



SOIL 



HARDPAN 



Fig. 5. Soil with several layers 

 of hardpan with soil between the 

 layers. 



