958 EXPERIMENT STATION RECORD. 



falling temperature. This difference may be explained b^ the fact that 

 by the compacting of the soil its conductivity is increased, and to such 

 an extent that the accompanying increased absorption of heat due to 

 evaporation is for the most part eliminated. The presence of stones in 

 the soil results in a greater capacity for heat in consequence of increased 

 conductivity. At the same time a stony soil cools more rapidly than 

 one free from stones when the temperature falls. 



The heat conditions of the soil are dependent to quite a remarkable 

 extent on the quantity of water present. During the warm season the 

 temperature of the soil is generally lower when much water is present, 

 because as the absorption of heat by evaporation increases the specific 

 heat also increases. These differences in temperature are the smaller 

 the more the evaporation diminishes and the increased conductivity 

 due to greater humidity can exert its influence. They are, therefore, 

 during the cold season smallest when the exposure to the sun's rays is 

 slight, the air cool, and still the humidity of the air high and the top 

 layer of soil dry. The effect of cooling due to evaporation, external 

 Conditions being the same, is the more easily eliminated and counter- 

 balanced the less water the soil contains, the smaller its water capacity, 

 and the more slowly it replaces by capillarity the loss of water from the 

 surface. With regard to differences in temperature of wet and moist 

 soils (maximum 1 to 1.5° C), it seems unjustifiable to attribute the low 

 productiveness of wet soils to their lower temperature. 



In the freezing of water in the soil the phenomenon of supercooling 

 may be noticed. The soil water, under the influence of capillary ten- 

 sion, becomes cooled by frost below its real freezing point without 

 solidifying, but as soon as any impulse is given to its solidification it 

 passes at once into the form of ice. The heat liberated by this change 

 is sufficient to keep the soil at 0° C. until this heat is used up and the 

 temperature of the soil gradually comes into equilibrium with the lower 

 surrounding temperature. The supercooling temperature of soil water 

 seems to be lower as the humidity decreases and as the energy with 

 which the water is held by the soil increases. During continued frost 

 the temperature of the soil falls faster and lower the less the humidity 

 of the soil. Afterwards the differences in the temperature tend to 

 disapi^ear, or are, under certain circumstances, reversed. 



The i)enetratlon of frost takes place most quickly in quartz sand, 

 more slowly in clay, and most slowly of all in humus. When the soil 

 thaws out its temperature first rises to 0°, and remains constant for 

 some time before it undergoes a further rise in temperature. Thawing 

 takes place most quickly in quartz sand and most slowly in humus, 

 clay being, as usual, intermediate. Frost disappears most quickly 

 when there is least moisture in the soil.^ 



lE.Wollny, Landw. Jahrb.,5, p. 441; Forsch. Geb. agr. Phys., 1, p. 43; Ibid., 2, p. 

 133 ; 3, p. 325 ; 4, pp. 147, 327, and 5, pp. 34, 167. Ebermayer, Ibid., 14, p. 195. A. Petit, 

 Ibid., 16, p. 285. 



