308 



FUNDAMENTALS OF FRUIT PRODUCTION 



uncultivated or cultivated bare land or sod. A direct comparison of the 

 extremes is not available, but by comparing minimum temperatures in 

 bare land with those in sod (Tables 43 and 44), then sod with cover crop 

 (Tables 44 and 45) and finally cover crop with straw mulch, some idea of 

 the superior protective qualities of the straw mulch can be formed. As 

 will appear later, the difference between safe and killing temperatures for 

 roots is slight and a few degrees are apparently more important below 

 ground than above. 



Soil Type. — Increased injury in sandy soil has been reported so 

 frequently that the precise temperature conditions existing in the lighter 

 soils should be examined carefully. Table 47 shows absolute minimum 

 temperatures for certain months, recorded at a depth of six inches, in 

 soils of different types. 



Table 47. — Absolute Minimum Temperatures in Different Soils 



{After Bouyoucos'^^) 

 (Degrees Fahrenheit) 



These figures show a sufficient difference to indicate a possible cause 

 for increased root killing in sandy soils. It should be stated, however, 

 that Bouyoucos records a very marked tendency for all soils to assume 

 a uniform temperature if air temperatures remain stable long. The 

 lower minima in sand are due probably to more rapid conductivity so 

 that a cold spell of short duration, as most cold waves are, would take 

 effect here but be over before it would affect some of the other soils to 

 the same extent. Thus, Bouyoucos states, ''The 12-inch depth of 

 gravel and sand froze Feb. 3, that of loam, clay and peat on Feb. 5, or 

 2 days later; while the 18-inch depth of the various soils froze as follows: 

 gravel, Feb. 6, sand, Feb. 8, clay, Feb. 10, loam, Feb. 1 1 , that of peat did not 

 completely freeze, its temperature remaining a few tenths of a degree 

 above 32°F. throughout the rest of the winter." As to the effect of 

 organic matter in soil, he comments on his investigations as follows: 

 "The minimum temperature attained was highest in peat, slightly less 

 and about the same in the various soils treated with peat and lowest in 



