CONDITIONS AFFECTING SOIL FERTILITY 43 



the sun in any way are cold ; hence the saying, " There is 

 a difference of 100 miles of latitude between the north and 

 south sides of a tight board fence." The fence not only shuts 

 off the heat rays from one side of it, but it reflects back much 

 heat to the soil on the other. The best place for the early 

 vegetables is on the sunny side of such a fence. We see other 

 effects of the reflection of heat in our greenhouses and hot- 

 beds, where the air under the glass is warmed in large measure 

 by the heat reflected back from the soil. Dry soils are warmer 

 than wet ones, and well-aerated soils are warmer than those 

 in which the air does not circulate freely. Color also influences 

 the amount of heat absorbed by soils. Black soils absorb most, 

 red next, yellow next, and light soils least of all. In an experi- 

 ment with two samples of the same soil, one of which was 

 whitened with magnesia and the other blackened with lamp- 

 black, there was found to be a difference of more than 12 de- 

 grees in favor of the black specimen. A soil containing much 

 humus is warmer than one that lacks it. The decay of or- 

 ganic matter adds as much warmth to the soil as would be 

 given off if the matter were burned more rapidly in the air. 



The Fahrenheit and centigrade scales. There are two scales 

 in common use by which temperature is measured. In both, 

 the freezing and boiling points of water are cardinal points. 

 The Fahrenheit scale is the one most used in the United States 

 at present. On this scale zero is placed 32 degrees below the 

 freezing point of water, and the boiling point 180 degrees 

 above it, making 212 degrees between zero and the boiling 

 point. A much better arrangement is found in the centigrade 

 scale, where the freezing point of water is called zero and 

 the distance between it and the boiling point is divided into 

 100 degrees. A degree in the centigrade scale is therefore 

 larger than a degree Fahrenheit. To change a given number, 

 of degrees centigrade to Fahrenheit one must multiply by 

 | and add 32 (C. x f + 32 = F.). To change Fahrenheit to 



