SOILS IN DRY AREAS 65 



alike. The same is true of their chemical constituents. 

 The surface soils have more humus in them and more 

 of the mineral plant food is in an available form. But 

 the difference in these respects is oftentimes not very 

 marked. That it so is very fortunate, as deep storage is 

 thus made for moisture and much opportunity is given 

 for that soil moisture movement which carries plant food 

 in solution up to the surface soil. It also furnishes 

 deep feeding ground for the roots of plants. 



Should the subsoil be dense clay, the downward 

 movement of water would be hindered. Should it be 

 hard-pan it would be more effectively hindered. Should 

 it be coarse sand or gravel, the upward movement would 

 be entirely cut off, or virtually so. But when the subsoil 

 is much like the surface soil, none of these evils follow. 

 The most objectionable subsoils in dry areas include the 

 following: (1) soils that are underlaid with hard-pan 

 that is not distant from the surface ; (2) those that have 

 gravel seams not far below the surface or that are under- 

 laid with sand coarse in character, and with but little 

 clay interspersed between the soil grains; (3) subsoils 

 that are so compact that they are not easy of penetration 

 by air or by the roots of plants, and (4) subsoils that are 

 saturated frequently with seepage water that rises to the 

 surface. 



A hard-pan condition is usually brought about by the 

 action of lime, which is so abundant in the soil of semi- 

 arid areas, and water. Water carries down the lime in 

 solution as far as it goes, but, owing to the light precipi- 

 tation and the dry and hard character of the subsoil, it 

 does not go very far, and it goes down to about the same 

 distance from year to year. When the lowest limit of 

 water penetration is reached, it combines with other soil 

 ingredients and forms a layer of calcareous material so 

 dense and hard that it cannot be penetrated easily by 

 the roots of plants. Even where but little lime is pres- 



