310 



FUNDAMENTALS OF FRUIT PRODUCTION 



against severe freezing during cold nights, but also against alternate freezing 

 and thawing. The temperature changes observed on February 2, 3 and 4, 

 1902 — a very cold period — are especially interesting. The surface of the bare 

 ground thawed during the middle of the day and froze severely each night. Two 

 inches lower, however, the soil did not thaw out during this very cold weather, 

 though the temperature changes between day and night were great. The 

 temperature of the mulched ground, both at the surface and 2 inches beneath 

 it, remained constantly below the freezing point and, moreover, varied but little 

 during the period." 



Recent studies by Bouyoucos^* suggest an interesting possibility in 

 this connection. He shows that in practically all agricultural soils some 

 of the moisture remains unfrozen at ordi- 

 nary temperatures and, indeed, even at 



— 78°C. The amount of unfrozen water 

 varies with the kind of soil, becoming in 

 general greater as the soils vary from the 

 simple and non-colloidal to the complex 

 and colloidal. The amount freezing at 



— 78°C. is very little, if any, greater than 

 that freezing at — 4°C. It seems possible, 

 then, that the increased amount of root 

 injury in sandy soils may be due, in ad- 

 dition to the lower amount of moisture in 

 such soils, as mentioned above, to the ex- 

 tremely small amount of water remaining 

 unfrozen at temperatures only slightly 

 below 0°C. while the finer soils have a 

 reserve of capillary adsorbed unfrozen 

 water under such circumstances. 



It should also be recognized that temperatures in the different soils 

 may have been different. In any case, however, the result is the same; 

 damage is greater in soils that are dry at the time of freezing. 



Relation of Cover Crops to Root Killing. — The effects of single factors 

 on soil temperatures, and therefore on root killing, have been set forth. 

 The value of a snow cover has been shown; the increase of soil tempera- 

 tures with transition from bare ground through sod to cover crops has 

 been reviewed ; minima varying with the character of the soil have been 

 indicated and finally dryness of soil has been shown to be associated 

 with root killing. In orchard practice, however, these factors are rarely 

 operative singly and some rather complicated interactions may be 

 expected. 



Emerson's^^ studies on depth of freezing under two sets of conditions 

 are of great importance since they show the interactions referred to above. 

 Figures 30 and 31, reproduced from his studies, indicate depth of freezing 



Fig. 30. — Depth of freezing 

 under various covers, in absence 

 of snow. (After Emerson^^) 



