CLIMATIC AND GEOCHEMICAL RELATIONSHIPS 75 



A soil low in humus, and with the iron either absent or 

 unoxidized, will be of a light color. Sea sands are good 

 illustrations of this condition. A well-drained soil con- 

 taining large quantities of organic matter will present a 

 deep black color in spite of the oxidized iron, as the latter 

 will be masked to a large extent. If humus is low or 

 lacking and the iron is oxidized, a red or a yellow color 

 may characterize the soil. As might be expected, there 

 are blendings of these three primary colors, and grays, 

 browns, and yellows of varying intensities are common. 



57. White and black soils. — The light colors in soils 

 are not due to the agencies of weathering, but rather to 

 a lack of such action. The cause of such coloration is 

 therefore not hard to explain. The development of the 

 black or dark colors and tints, being due to the accumula- 

 tion of organic matter, indicates the operation of two 

 favoring conditions: first, climatic agencies that stimu- 

 late the luxuriant development of plants ; and, secondly, 

 sufficient aeration to promote a favorable decay of such 

 tissue. It is a well-recognized fact that in order to\ 

 develop a black color from decaying vegetable matterj 

 fairly good aeration must be provided. If such a coi/ 

 dition does not prevail, the decayed material has a lighter 

 hue and may exhibit toxic properties which will check 

 or inhibit plant growth. The development of the black 

 color, therefore, in a normal well-drained soil, is an in- 

 dication of good soil sanitation. 



58. Red and yellow soils. — The presence of iron, as 

 already noted, is a very important factor in rock weather- 

 ing, and the discoloration due to its presence is an unfailing 

 indication of chemical decay. The iron in minerals 

 occurs usually as ferrous oxide, which is soluble, especially 

 if the water circulating among the rock fragments carries 



