SOIL AND SUBSOIL 69 



tate or flocculate the suspended clay. These tight clay soils are 

 found in Southern Illinois, Missouri, Arkansas and many other 

 places. 



The tight clay layer becomes very hard when dry, but when 

 saturated with water it is very soft and posts may be driven into 

 it easily. 



The tight stratum prevents underdrainage and the topography 

 is almost invariably too flat for surface drainage. Damage to crops 

 by water is very liable to occur. To remove the excess, plowing is 

 done in small lands, the dead furrows are left open and by this 

 means water may be removed, especially since these furrows are 

 usually connected with a ditch at the end of the field. This tight 

 stratum very seriously interferes with the capillary movement. The 

 tight layer limits the storage of water to that part of the soil above 

 it. Even if water is abundant below, it is cut off because the roots 

 cannot penetrate this stratum and capillary movement through it is 

 so extremely slow as to furnish but a scanty supply, with the result 

 that crops are seriously affected by drouth. The effect of tight clay 

 is very difficult to overcome. For the permanent improvement of 

 soils of this kind, large applications of ground limestone, four to 

 six tons per acre, with the growing of deep rooting crops, such as 

 red, mammoth or sweet clover, are recommended. The puncturing 

 of the tight clay by these roots will without doubt produce better 

 conditions of drainage and aeration. Dynamite is sometimes used 

 to break up the tight clay, but this method is too expensive for gen- 

 eral farm use and besides the subsoil runs together again when 

 .saturated. The loess beneath this tight clay, which is from eight 

 to twelve inches thick, is ideal in physical composition. 



Hard Pan. Hard pan proper is formed by the deposition of 

 substances from solution around soil particles cementing them to- 

 gether into a more or less stony mass. The deposition of ttiis 

 cementing substance is due, possibly, to the stoppage of percolation 

 by an impervious stratum, evaporation brought about by some cause, 

 or loss of carbon dioxide, causing precipitation, as in the case of 

 lime carbonate. The cementing material is usually derived from 

 the decomposition of rocks and may consist of such substances as 

 iron, magnesia, lime or sodium carbonate and sodium chloride. 



Since the cause of hard pan is the stoppage of water in its 

 movement downward the renewal of percolation will be sufficient 

 frequently to destroy the hard stratum. If not too deep it may be 

 broken with plow or subsoil plow, but if beyond the reach of these 



