124 RELATION TO PHYSICAL CONDITIONS 



usually completely leached of its lime content, and is con- 

 sequently extremely unthrifty." 



Cameron (5) gives the following explanation of the origin 

 of hardpans: "The application of the present views re- 

 garding solutions to the study of hardpan phenomena gives 

 promise of valuable as well as interesting results. A hard- 

 pan may be defined as a layer of the soil, usually near the 

 surface, having the texture of the soil just above and below 

 it, but more or less closely cemented by some material. 

 In general, hardpan is a characteristic of soils where drain- 

 age is very poor or where standing soil waters may ac- 

 cumulate. The cementing material is often lime carbonate, 

 but may be other material, as the hydrates of iron and 

 alumina or silica. Hardpans vary much in their physical 

 properties. They are sometimes as dense and close- 

 grained as a well-characterized rock, requiring blasting or 

 similar methods to break them up. In other cases they 

 may be partly porous, and when brought to the surface 

 disintegrated with ease, and there are all grades between 

 these extremes. 



"The objections to their presence in the soil are evident. 

 They prevent the penetration of plant roots, and, more 

 important, they prevent the moisture from rain, irriga- 

 tion, etc., sinking into the soil and thus being conserved for 

 future use. They also prevent the water that may be 

 beneath them from being drawn to the surface and made 

 available for the plants. 



"The formation of a calcium carbonate hardpan is the 

 most readily understood, and this has been dwelt upon at 

 some length in a paper by Gardner and Stewart (8). It 

 is there pointed out that resolution and reprecipitation 

 are important factors. But when the calcium carbonate 

 does not exist, as such, in the soil or in the vicinity, so as 



