22 FIELD OPERATIONS OF THE BUREAU OF SOILS, 1919. 



It is now Itflitved that tlio characteristics of the soil in its pres- 

 ent stji«je of (k'velopiiunt have i)een (iclermined to a greater extent 

 by the soil- form iiii; })rocesses, which have resulted in the accumii- 

 Intion of or^^anif matter and in the weathei'in^, leaching, and oxi- 

 dation of tlie matciial iieai* the surface, than by original differences 

 in the character of the material. 



The soils of Wrigiit County were developed under the influence 

 <d' a native vegetation consisting of a heavy growth of grasses. 

 This vegetation was favored ])V a smootii but not flat topography 

 and a rather high moisture supply. The original constructional 

 surface of the region, with its poorly drained areas unfavorable to 

 tree growth, and its heavy grasses that spread fires, prevented the 

 encroacliment of forest. 



The heavy growth of grasses, flourishing upon soils generally re- 

 tentive of moisture, resulted in tlie accumulation of large quantities 

 of organic matter. This was in the form of finely tlivided carbona- 

 ceous material, chiefly derived from decomposed grass roots and in- 

 timately mixed with the mineral constituents of the surface soil. 

 The percentage of the organic matter and the tlepth to which it 

 affected the color and physical structure was determined very largely 

 by aAeragc drainage conditions. On flats and in sloughs the black 

 organic matter extends to a depth of "20 to 30 inches, while on the 

 high, well-drained ridges it does not affect the soil below a depth 

 of a few inches. The accumulation is also shallow in sandy spots 

 where the drainage is excessive. Muck and Peat represent extreme 

 conditions with respect to accumulation of organic matter. The de- 

 velopment of the upland soils is determined partly by the topographic 

 position. On the flat or undulating areas the average moisture con- 

 tent was formerly high, and the ground-water level was within 2 or 

 3 feet of the surface and in places higher. Consequently there was 

 a large accumulation of carbonaceous material in the surface soil 

 and upper subsoil, while the lower subsoil almost escaped leaching 

 and oxidation, resulting in a deep black soil filled with organic mat- 

 ter over a light-colored or mottled calcareous subsoil. These are the 

 characteristic features of the soils which have been classed with the 

 "Webster series. Similar conditions in the sloughs and dei)ressions 

 and on the poorly drained terraces give rise to the soils of the Fargo 

 series. 



Where the topograpliy ranges from gently to sharply rolling the 

 better surface drainage, the more vigorous movement of the soil 

 water, and deeper aeration have resulted in leaching and oxidation 

 to a depth of 3 or 4 feet. Soils of this group are represented by the 

 Carrington series and occur along the better drained stream valleys 

 and over the morainic hills. The typical profile has a dark-brown 

 surface horizon with a fine granular structure, ranging in depth from 



