50 Agricultural Chemistry. 



present in the soil, is the worst conductor. A dry soil is thus 

 a very poor conductor of heat. Consolidation improves the con- 

 ductivity. Wetting the soil doubles the conductivity of sand, 

 limestone, or clay by displacing the air. We see then, that a 

 dry, loose soil will get very hot at the surface when exposed to 

 the sun, but the heat will penetrate to a slight depth. This ex- 

 plains why gravelly soils are best suited for early spring crops. 



Presence or absence of much water is the important factor 

 which chiefly determines the cold or warm character of a soil. 

 A still more potent reason for the coldness of wet soils, is, how- 

 ever, the loss of heat during evaporation. When a pint of water 

 is removed by evaporation from 97 pints, the 96 remaining pints 

 will have fallen 10 Fahr. in temperature unless this amount of 

 heat has been supplied from some external source. Undrained 

 meadows and heavy clays consequently are cold soils because 

 much of the sun's heat is, in these cases, consumed in evaporating 

 water. Parks found that an undrained peat bog 30 feet deep, 

 had a temperature of 46 when measured below a distance of 

 1 foot from the surface. In the middle of June he found the 

 temperature 47 at 7 inches below the surface, while the drained 

 portion had a temperature of 66 at this depth, and a tempera- 

 ture of 50 at 2 feet below the surface. Draining is the only 

 cure for a cold, wet soil. 



The temperature of the subsoil is practically constant through- 

 out the year at a certain distance from the surface. Observations 

 at Greenwich Observatory, England, in a well drained gravel, 

 showed that the variations of day and night are slightly felt at 

 3 feet from the surface. At 25!/2 feet the maximum temperature 

 usually occurs in the latter part of November and the minimum 

 in the first week in June. The difference between the two is 

 about 3. These observations make it clear that the soil and 

 subsoil are generally warmer than the air in autumn and cooler 

 than the air in spring. 



Tenacity of soil. The tenacity of a heavy soil is due to the 

 fine silt and clay. The coarser elements of a soil, such as a fine 



