SOIL MOISTURE 



61 



approach sufficiently near the saturation point to check root formation. 

 When roots find an abundance of water close to the surface they branch 

 freely through the surface soil and show Httle tendency to go deeper, 

 particularly if conditions are more and more unfavorable for root develop- 

 ment at greater depths. These two 

 factors together probably explain the 

 comparatively shallow rooting of 

 most tree, bush and vine fruits in a 

 large portion of the humid region. 

 Compact, water-logged subsoils or 

 a high water table prevent the roots 

 from penetrating deeply. " The sur- 

 face inch or so is too dry during a 

 major portion of the growing season 

 to encourage root growth ; the result is 

 a distribution of most of the roots be- 

 tween the depths of 3 to 10 or 15 

 inches. 



When, however, moisture and 

 aeration conditions are favorable for 

 root development at considerable 

 depths, deep penetration occurs. 

 Thus Hilgard and Loughridge^^ state 

 that on some of the silty "low mesa" 

 soils of California the roots of cherry 

 and prune trees are frequently found 

 at depths of 20 to 25 feet. Such deep 

 rooting is also characteristic of fruit 

 trees in the loose soils along the 

 Mississippi and Missouri rivers. 

 "These are soils, however, with no 

 hardpan or plowsole and with the Sandy 

 water table many feet below the sur- '^''^S 

 face. In them soil grades insensibly 

 into subsoil. Indeed, subsoil in the 

 sense in which the term is generally 

 used, does not exist except below the 

 region of this exceptional root pene- 

 tration. It hardly need be pointed out that trees in such soils seldom 

 suffer from drought, even though there may be a series of dry years. 



When plants, accustomed to growing in a soil where shallow rooting is 

 necessary, are transplanted to one in which deep penetration is possible, 

 they first send out shallow lateral roots, their distribution being much 

 like that of the same plant in the region or soil from which it came. They 



Cloy 



bandy 

 Clay 



Pure 

 Sand 



Clay 



Sand 



Clay 



Fig. 7. — Influence of soil moisture 

 upon root distribution of Kuhnia gluli- 

 nosa. {After Weaver}^^) 



