COMPOSITION OF SOILS. 37 



The nature of the subsoil is also an important consid- 

 eration. If the soil rests immediately upon a rock, or 

 upon sand, it will be found to dry out much more rapidly 

 than if it rests upon a clay subsoil. In the first place, the 

 water passes rapidly beyond the reach of the roots, and 

 cannot readily get back ; in the second place, it does not 

 percolate so rapidly, while at the same time it retains its 

 connection with the surface. 



This point is very apparent to one who has observed 

 the red shale soils in central New Jersey. They are 

 reasonably rich in all forms of plant-food, yet those which 

 lie directly upon the shale or rock are much less pro- 

 ductive than those which lie upon a clayey subsoil. The 

 shale permits a too free escape of water, and crops suffer 

 more severely from drouth than those which lie upon a 

 clayey subsoil. It is essential that water be freely mova- 

 ble in soils, in order to properly prepare the food, as well 

 as to carry it to the roots of plants, but it must be freely 

 movable in all directions. 



Climate. — The climate is also a matter of importance : 

 rainfall, temperature, location, all exert an influence in 

 determining the value of a soil, and should be taken into 

 consideration in connection with its chemical composition. 



The average rainfall may be sufficient ; but if it is not 

 properly distributed throughout the growing season, nor- 

 mal growth is impossible. The same is true of tempera- 

 ture ; cold in harvest time is ruinous, though the normal 

 temperature for the year may have been attained. 



Loss and Gain to Soil. — A soil, whether cultivated 

 or not, is continually changing, the various causes which 

 combine to form soils being ever at work to make them 



