180 now CHOPS FiiED. 



fully one-third of the I'esi-'tancc of a clay or loam to the 

 plow thus disappearing, according to Schiibler's experi- 

 ments. 



Tillage, when carried on with the soil in a wet condi- 

 tion, to some extent neutralizes the effects of frost, espe- 

 cially in tenacious soils. 



Fall-plowing of stiff soils has been recommended, in 

 order to expose them to the disintegrating effects of frost. 



RELATIONS OF THE SOIL TO HEAT. 



The relations of the soil to heat are of tlie utmost im- 

 portance in affecting its fertility. The distribution of 

 plants is, hi general, determined by differences of mean 

 temperature. In the same climate and locality, however, 

 Ave find the farmer distinguishing between cold and warm 

 soils. 



The Temperature of the Soil varies to a certain depth 

 with that of the air ; yet its changes occur more slowly, 

 are confined to a considerably nairower range, and dimin- 

 ish downward in rapidity and amount, until at a certain 

 depth a point is reached where the temperature is invari- 

 able. 



In summer the temperature of the soil is higher in day- 

 time than that of the air; at night the temperature of the 

 surface rapidly fills, especially when the sky is clear. 



In temperate climates, at a depth of three feet, the tem- 

 perature remains unchanged from day to night ; at a depth 

 of 20 feet the annual temperature varies but a degree or 

 two ; at 75 feet below the surface, the thermometer re- 

 mains perfectly stationary. In the vaults of the Paris 

 Observatory, 80 feet deep, the tem})erature is 50° Fahren- 

 heit. In tropical regions the point of nearly unvarying 

 temperature is reached at a depth of one foot. 



