354 



FUNDAMENTALS OF FRUIT PRODUCTION 



Table 62. — Surface Minimum Temperatures on Dry and on Wet Sanded Bogs 

 {Adapted from Cox*^) 

 (Degrees Fahrenheit) 



Date 



Sept. 14 



Sept. 24 



Sept. 27 



Sept. 28 



Sept. 30 



Oct. 1 



September mean 



by the selection of the coldest nights, shows that at the very time when 

 these differences are most important they are greatest. It might be 

 argued that the wet sand was coldest because it had given up more 

 heat; however it is stated that on Oct. 1 cranberries in this bog were 

 frozen, except in the dry sanded section. The lower minimum on the 

 wet sand is attributed to the heat lost in evaporation at the surface. 

 It should be remarked that irrigation with relatively warm water 

 at the time of frost apparently has proved of considerable value occa- 

 sionally but irrigation that merely wets the soil and keeps it cold is 

 probably injurious. An experimental investigation in Wisconsin showed 

 very little difference in temperature over irrigated and over unirrigated 

 blocks. 



King,io2 commenting on the results, stated: "Not only did frost form after 

 the water was brought to the areas but some of the rape leaves became stiff 

 with streams of water flowing both sides of the row. It is true, however, that a 

 very perceptible difference could be noted in the degree of stiffness which foliage 

 took on above and close to the water, and that which was more distant. For 

 close to the water the leaves did not become so rigid as to break in the hand while 

 at a distance from the water they did. 



"It is quite possible that were broad areas irrigated at such times the pro- 

 tection would be more marked, but it does not look very hopeful for the protec- 

 tion against night frosts by this method, especially where the temperature falls 

 3 or 4° below freezing." 



It seems evident from the data above that evaporation does not 

 interfere with radiation sufficiently to offset its cooling effect and that 

 unless the water actually imparts heat it is deleterious. A thoroughly 

 saturated soil is, however, likely to retard frost formation. 



Cox"^ states: "The explanation is found in the high specific heat of water. 

 A certain quantity of heat lost during the night time from relatively dry ground 

 and its vegetable cover cools the exposed portions of these poor heat-conducting 



