810 EXPERIMENT STATION RECORD. 



rock. The percentage of humus i.s quite ^iiriaVjk^, but in general 

 declines quite uuifornily from the center toward each edge of the cher- 

 nozem zone. This variation is so uniform that it has been utilized by 

 Dokouchayev in the establishment of so-called isohumic bands. 



On the basis of humus content the chernozem may be divided into 

 four genetic subtypes: (1) The humus or rich chernozem of the eastern 

 central belt, which contains more than 10 per cent of humus: (2) the 

 medium or ordinary chernozem, which occupies the larger part of this 

 soil zone and contains 6 to 10 per cent of hunuis: (3) the southern 

 chocolate-colored chernozem, which merges into the chestnut soils of 

 the dry steppes, containing 4 to 6 per cent of huuuis; and (1) the 

 northern cinnamon-colored chernozem of central Russia, which occurs 

 in strips and spots, alternating with forest and light loess soils, and 

 which contains 'A to Q per cent of humus. 



The chernozems also show wide variations in the composition of 

 their mineral constituents, being clayey, sandy, calcareous, peatv, 

 alkaline, etc., according to the sources from which they are derived or 

 the conditions of their formation. The humus is but slightly soluble 

 in water. The total nitrogen content varies from 0.2 to 0.7 per cent 

 in the soil or from 5 to 8 per cent in humus. The clay content varies 

 from 20 to 40 per cent, zeolites from 15 to 35 per cent. Cold 1 per 

 cent hydrochloric acid dissolves from 3 to 5 per cent of matter from 

 the soil, excluding carbonates. The absorptive power varies from 20 

 to 43 per cent. The silicates of chernozem have undergone a high 

 degree of weathering and decomposition. Thus, of the 2 to 2.4 per 

 cent of potash, from one-fifth to one-half dissolves in 10 per cent hydro- 

 chloric acid. Of the 8 to 10 per cent of alumina from one-half to four- 

 tifths dissolves in the same reagent. The phosphoric acid varies from 

 0.12 to 0.3 per cent. In the upper horizon of the soil the carbonates, 

 mainly calcium carbonate, do not usually exceed 1 to 3 per cent, but in 

 chernozems derived from limestones the carl)onates sometimes reach 

 10 to 15 per cent. The sandy portion of the chernozem is very fine, 

 consisting of quartz, with an admixture of mica, feldspar, and other 

 silicates. According to Kostichev, the mijieral portion of chernozem, 

 excluding the carbonates, is very similar in composition to the loess 

 from which it is derived, there being a slight increase of phosphoric 

 acid, due to the accumulation of hunuis. In the foothills of the south- 

 ern Ural Mountains there occurs a variety of chernozem which contains 

 as much as 2 per cent of phosphoric acid. 



It may be said in general that the chemical properties of chernozem 

 are more favora))le than the physical. The particles are as a rule too 

 tine, from 60 to 80 per cent of the particles being ordinarily less than 

 0.05 mm, in diameter, and the proportion of silt (particles less than 

 o.ol mm. in diameter) sometimes reaches 58 ])ei- cent. Particles larger 

 than 0.5 mm. in diameter are either entirely absent or present in very 



