Factovi^ in Whiterkilling of Grahi Crops. 131 



heat and thermal conductivity of ice and water. When water 

 freezes the specific heat is reduced approximately one-half, 

 while the thermal conductivity is increased more than three 

 times. As a result the difference in apparent specific heat of a 

 wet and a dry soil is much less when they are frozen than when 

 they are unfrozen, while the difference in thermal conductivity 

 is greatly increased. The increase of the latter appears to be 

 suflficient to overbalance the difference in specific heat, per- 

 mitting the temperature fluctuations of the air to be more 

 quickly recorded in the soil. 



The survival of the plants followed very closely the daily 

 minimum temperatures. The highest percentage of survival 

 was obtained on the wet plots in one season and on the dry 

 plots another season. In the first case the soil was seldom 

 frozen during the winter, while in the latter it was frozen most 

 of the time. 



The relation of moisture to temperature and winterkilling in 

 the sand plots was contrary to its effects on clay or silt loam 

 when the soil was frozen, but corresponded qualitatively with 

 the other types when they were not frozen. In other words, 

 the temperature of the dry sand was lower and the survival 

 less than for the wet sand, regardless of whether the soil was 

 frozen. This was probably due to sand being a better con- 

 ductor of heat than other types, and hence the addition of 

 water has less effect on the thermal conductivity. Also sand 

 has a low water capacity, and consequently less change in the 

 ;ipparent specific heat takes place on freezing. 



The results may be taken to indicate that winter annuals 

 are more likely to suffer injury on wet soils than on dry ones, 

 except for very easily injured plants which may be killed with- 

 out the soil being frozen much of the time ; and more on sandy 

 soils than those of heaviest type, except when due to heaving 

 of the soil. 



Agricultural Experiment Station, Manhattan. 



