CHAPTER XV. 



THE COLORS OF SOILS. 



THE natural coloration of soils forms a prominent part of 

 the characters upon which farmers are wont to base their judg- 

 ment of land quality; hence the origin and value of soil-colors 

 deserve consideration. 



Black Soils. From the oldest times down to the present a 

 " rich, black soil " has commanded attention and approval. 

 The black and brown-black colors being almost invariably due 

 to the presence of much humus (very rarely to an admixture 

 of carbon (graphite), of magnetic oxid of iron, or sesquioxid 

 of manganese), it is obvious that the farmers' judgment coin- 

 cides with a high estimate of the agricultural value of humus. 

 A discussion of this point will be found in another place; but 

 the popular judgment is based quite as much upon the experi- 

 ence had in the advantages that usually accompany the pres- 

 ence of humus. It largely characterizes low grounds, and 

 therefore alluvial lands, whose richness is due to far more gen- 

 eral causes. But the shade of the blackness seen in the soil 

 deserves and usually receives close consideration. If tending 

 toward brown, acid humus or " sour " land is indicated ; unless 

 indeed the surface soil should be bodily derived from decayed 

 wood, as in the primeval forests. Forest soils in general are 

 usually dark-tinted for some inches near the surface, owing 

 to the presence of leaf mold, and mostly have an acid reaction. 



But the black tint is equally welcome to the land-seeker when 

 seen outside of alluvial and forest areas. Belts of "black 

 lands" appear on hillsides and plateaus; and these lands, 

 though clearly not alluvial, are also found to be preeminently 

 productive; witness the upland prairies of the western and 

 southern United States. These black soils are always charac- 

 terized by the presence of a full supply of lime in the form of 

 carbonate, under the influence of which the most deeply black- 

 humus is formed. In other words, the jet black tint is indica- 



28} 



