112 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



utilized in preparing hot beds with manure and earth. The decay 

 of the manure raises the temperature of the hot bed. 



The quantity of organic matter in the soil is seldom sufficient 

 to affect its temperature to a practical extent. An application of 

 10 tons of partly decayed manure per acre may raise the tempera- 

 ture of the soil 2 F. for the first five days, iF. for the second 

 five days, and 0.6 F. the third. This is according to the experi- 

 ments of -Georgeson. 1 This increase in temperature might aid in 

 hastening the germination of seeds and in protecting a spring crop 

 from frost. 



Wagner found an average increase of temperature 0.7 to 0.2 

 F. during four to twelve weeks, due to heavy applications of 

 manure under field conditions. 



Control of Temperature. Artificial regulation of temperature 

 is practiced comparatively little in agriculture, though of con- 

 siderable importance in horticulture, in which hot beds, cold 

 frames, and green houses are used. The growth of tomato plants, 

 sweet potato vines, and other plants under glass or in protected 

 places, for transplanting when the soil becomes sufficiently warm 

 or danger of frost is past, is a kind of regulation of temperature. 

 In some regions, smoke fogs are produced to prevent frost and 

 thereby protect tender plants, or plants at critical stages of 

 growth. 



Color of Soils. Soils range in color from almost pure white, 

 through yellow, red, or gray, to black. The yellow or red colors 

 are due usually to hydrated oxides of iron. Organic matter gives 

 a soil a black color when wet, or a gray or brown or black color 

 when dry. The color thus affords some indication as to the char- 

 acter of the soil. Black or red soils are generally preferred by 

 practical farmers. The intensity of the color is not always an 

 indication as to the quantity of organic matter or oxide of iron 

 present. A coarse sand, by having a smaller surface to be colored, 

 requires much less coloring material than a clay soil which has a 

 large surface. 



1 Agr. Science i, p. 251. 



