WINTER INJURY TO THE ROOTS 



309 



the untreated sand." A continued turning under in the spring of cover 

 crops tends to raise the soil content of organic matter. The cover crop 

 protects, then, while above ground by blanketing the soil and when 

 turned under it affords some protection in the following winter through 

 the increased amount of organic matter it has supplied. 



Soil Moisture. — Another factor, possibly of equal importance, affect- 

 ing root-killing in sandy soils, is the amount of moisture present. No 

 evidence need be introduced here as to the comparatively low moisture 

 content of the average sandy soil. Emerson^^ made some very interest- 

 ing studies of the effects of moisture on killing, in which lots of 25 

 young trees each were exposed to a Nebraska winter, in boxes of loam 

 soil with varying degrees of moisture. His tabular statement of results 

 is reproduced here as Table 48. 



Emerson comments on his results in part as follows: "That the great injury 

 to the seedling roots in the drier soils is not due directly to the dryness alone but 

 to dryness and cold combined, is evident from the fact that the roots were 

 absolutely unhurt in equally dry soil kept in a cool dry cave. . . . That dry- 

 ness alone was not responsible is shown by the comparatively slight injury to 

 roots in rather dry soil which was protected by a 4-inch mulch of straw, while 

 roots in bare soil of almost the same moisture content were very badly hurt. 



"Just why severe freezing should injure roots worse in rather dry than in 

 moist soil is not shown by the test reported above. On further investigation it 

 may be found that roots are simply unable to withstand severe freezing or to 

 recover from it unless surrounded by an abundance of moisture. Be this as it 

 may, it is quite probable that one cause of the great injury in rather dry soil is 

 alternate freezing and thawing . . . the more water a soil contains the less 

 subject it is to frequent alternate freezing and thawing. 



"The fact that the apple seedlings were much less seriously injured where 

 protected by a mulch of straw than they were in bare ground is to be explained 

 by the effect of mulches on freezing and thawing of the ground. The latter was 

 tested during the winter of 1901-1902. The mulch protected the soil not only 



