54 



2. Organic Carbon. 



The organic carbon was determined by combustion with copper 

 oxide in a current of oxygen, the sample of soil having first been 

 digested with phosphoric acid solution and evaporated to dryness. 



The amounts of organic carbon (Table 53) on both types of the 

 series differ little from drift to drift. The Carrington loam carries 

 much less than the silt loam. The Marshall silt loam closely resembles 

 the former. 



Tabic jS. — Organic Curbon in the different scctio)is. 



Carrington silt loam Fargo silt loam Carrington loam Marshall 



Depth Kansan Late Wis. Kansan Late Wis. Kansan Late Wis. silt loam 



Inches per cent per cent per cent per cent jier cent per cent per cent 



1—6 4.76 4.48 9.47 5.71 2.83 3.06 2.83 



7—12 3.77 3.19 5.38 3.91 1.34 1.46 1.91 



13—24 1.83 1.78 1.66 2.10 0.79 0.81 1.00 



25—36 0.75 0.77 0.62 1.00 0.50 0.50 0.60 



Average 



1—36 2.28 2.29 3.23 2.64 1.12 1.19 1.32 



With the Fargo silt loam the upper two sections are much richer 

 in organic carbon on the Kansan than on the Late Wisconsin. This 

 is to be attributed to the Late Wisconsin glaciation obstructing the 

 drainage on the Kansan with resulting increased accumulation of 

 eroded surface soil developed during pre-Wisconsin time. The 

 organic carbon in the second and third foot sections on this type shows 

 much less difference, it being slightlv the higher on the I^ate Wisconsin. 



3. Nitrogen. 



Nitrogen was determined on both sets of samples from each field, 

 using the Gunning modification of the Kjeldahl method. The normal 

 variation in upland fields shows a decrease from the surface downward, 

 this being especially regular in the case of prairie fields (2, p. 219). 



Carrington Silt Loam. Considerable variation within the same 

 field and from field to field is shown on both drifts (Table 54). The 

 average amounts on the two drifts are very similar. 



It is to be observed that in the second and third foot-sections the 

 dift"erences between the two sets from the same field and between the 

 field samples are as great as, if not greater than, in the sections of the 

 surface foot where a difference in compactness and accordingly a dif- 

 ference in density (2, p. 219) might account for it. 



Fargo Silt Loam. As with the Carrington silt loam, the nitrogen 

 varies considerably from field to field and within a few of the individual 

 fields (Table 55). An extraordinary range is shown in Field IV on the 

 Kansan, where it varies from slightly over 1.0 per cent in the surface 

 to .028 per cent in the subsoil. 



