WINTER INJURY TO THE ROOTS 307 



in March when freezing was at its greatest depth for the season; these 

 are shown in Table 44. These figures are of special interest since they 

 show the protective effect of increased cover-crop growth induced by 

 fertilizer applications. The "cultivated with cover crop" plot had a 

 scanty growth. 



Table 44. — Depth of Freezing as Affected by Soil Covering 



Clean cultivated (no cover crop) 16 inches 



Cultivated, with cover crop 15 inches 



Sod 12 inches 



Fertilizer, cultivation and cover crop 10 inches 



Fertilizer (excess nitrogen) cultivation and cover crop. . . 7 inches 



Sandsten^^^ made measurements of the depth of frost penetration in 

 early February under different crops in Wisconsin. Table 45 shows his 



Table 45. — Frost Penetration under Different Cover Cropsi^^ 



Bluegrass sod 18.0 inches 



Clean cultivation (no cover crop) 16.0 inches 



Rape 15.0 inches 



Oats 8.0 inches 



Hairy vetch 7.5 inches 



data. He interprets these observations to emphasize the protective 

 value of an uncompacted cover, the bluegrass sod offering httle insulation 

 because of the lack of dead air spaces. He also considers the lower 

 amount of moisture in sod land to have an important bearing. In 

 connection with the data here cited from Gourley and from Sandsten it 

 should be recalled that the soil does not freeze until its temperature is 

 several degrees below 32°F. 



Oskamp'^2 reports soil temperatures observed in Indiana with differ- 

 ent soil covers. Table 46 is arranged from his data. It should be noted 



Table 46. — Monthly Minimum Soil Temperatures"* 

 (Degrees Fahrenheit) 



that this comparison is between straw mulch and land growing a cover 

 crop, which has been shown to have higher minimum temperatures than 



