78 SOILS: PROPERTIES AND MANAGEMENT 



This being true, the thicker the film, the greater is the 

 intensity of the color. The same quantity of iron, there- 

 fore, would make a greater showing in a sandy soil than 

 in clay, as the amount of internal surface of the former is 

 comparatively low and the film of iron oxide would there- 

 fore be thicker. 



It is evident from the data already presented that 

 the intensity of color arising from iron in the soil is due 

 to several conditions. Without a doubt the oxidation 

 that occurs is of primary importance, but the hydration 

 that very often takes place is a powerful modifying agent. 

 The thickness of the film, as determined by the amount 

 of iron present or by the texture of the soil, is probably 

 a factor having to do particularly with the intensity of 

 the coloration, although the color or tint itself may be 

 modified to a certain extent thereby. 



59. Agricultural significance of color. — The white or 

 the black color of a soil indicates the lack or the presence 

 of an important constituent, namely, organic matter. 

 This matter not only tends to keep the soil in good 

 physical condition, but also acts both as a plant-food 

 and as a source of energy for bacteria and other soil 

 organisms. A _ dark soil," provided its drainage and 

 climatic conditions are favorable, is usually a rich soil. 

 The dark color is no mean factor in temperature relation- 

 ships, since not only does a dark soil absorb heat faster 

 than a light soil, but the tendency of the former toward 

 reflection and radiation is much restricted. This is 

 important with crops which must go into the soil early 

 in the season, or which need to be pushed rapidly to 

 maturity. A dark color, with virgin soils especially, is 

 an excellent guide to fertility and general agricultural 

 value. 



