SOIL AND SUBSOIL. 185 



promptly disintegrate it. The presence of humus acids in the 

 cements, if not apparent to the eye, is readily demonstrated by 

 immersing the hardpan fragment in ammonia water or a weak 

 solution of caustic soda ; when if humus acids are the main 

 cementing substance the fragment will fall to crumbs, or be 

 softened to an extent corresponding to the amount of the 

 humus present. Calcareous hardpan is, of course, readily 

 recognized by its quick disintegration by dilute acid, with 

 evolution of carbonic gas. 



In "alkali" soils containing sodic carbonate ("black- 

 alkali ") there is commonly found at the depth of two or three 

 feet an exceedingly refractory hardpan resulting from the 

 accumulation of puddled clay (see above chapt. 4, p. 62) in 

 the subsoil, or sometimes even on the surface of depressed 

 spots. This hardpan, easily destroyed by the use of gypsum 

 and water, is described more in detail in chapter 22, on alkali 

 soils ; it blues red litmus paper instantly. 



The Causes of Hardpan. The recognition of the cause of 

 hardpan is of considerable importance to the farmer, because 

 of the influence of the nature of the cement and the causes of 

 its formation upon the possibility and methods of its destruc- 

 tion, for the improvement of the land. 



It may be said in general that inasmuch as the cause of the 

 formation of hardpan is a stoppage of the water in its down- 

 ward penetration, the re-establishment of that penetration will 

 tend to prevent additional induration; moreover, experience 

 proves that whenever this is accomplished even locally, as 

 around a fruit tree in an orchard, the hardpan gradually 

 softens and disappears before the frequent changes in moisture- 

 conditions and the attack of roots. The use of dynamite for 

 this purpose in California has already been referred to; it seems 

 to be the only resort when the hardpan lies at a considerable 

 depth. When it is within reach of the plow, it may be 

 turned up on the surface by the aid of a subsoiler and will 

 then gradually disintegrate under the influence of air. rain and 

 sun. But when the hardpan is of the nature of moorbedpan, 

 containing much humic acid and perhaps underlaid by lx>g 

 iron ore. the use of lime on the land is indicated, and will in the 

 course of time destroy the hardpan layer. This is the more 

 desirable as in such cases the surface soil is usually completely 



