SOIL AND SUBSOIL. 131 



It is not always, then, that the commonly recognized distinc- 

 tion between surface soil and subsoil based upon humus con- 

 tent can be maintained. But the observation of everything 

 bearing upon this point is of the utmost importance in deter- 

 mining both the agricultural value and the mode of treatment 

 of the land. 



Losses of Humus from Cultivation and Fallowing. The 

 fact that humus accumulates in woodlands and meadows, 

 where no cultivation is given, would naturally lead to the con- 

 verse conclusion, viz., that cultivation causes loss of humus and 

 of its constituents. That this is actually the case is recognized 

 and widely acted upon in practice, and there is no question 

 that the general acceptance of stable manure as the most widely 

 useful fertilizer, despite its usually low content of plant-food 

 ingredients, is based upon the fact that it supplies vegetable 

 matter, in a condition highly favorable to its conversion into 

 humus. The most direct and cogent proof of the depletion of 

 the soil of both humus and nitrogen by continuous cultivation 

 of cereal grains has been given by Snyder, 1 who determined 

 the loss both of humus and of nitrogen suffered by a Minnesota 

 soil during eight years' continuous cultivation of wheat. The 

 total loss of nitrogen was 1700 pounds per acre, while only 

 350 pounds were utilized by the crop; about 1400 pounds being 

 dissipated as gas or leached out as nitrates. A conservative 

 estimate of the loss of humus suffered during the same period 

 was about a ton per acre annually, and this loss seriously de- 

 creased not only the nitrogen-content, but rendered the soil 

 more compact and less retentive of moisture. But by rotation 

 of the wheat with clover in alternate years, very nearly an 

 equilibrium of both humus and nitrogen-content was obtained. 

 In addition, the amount of available mineral plant-food was de- 

 creased by continuous grain culture. Ladd has made similar 

 observations in North Dakota, with similar results. 



That excessive aeration results in serious losses of humus 

 as well as of nitrogen, is very obvious in the arid region, where 

 it is the habit to maintain on the surface of orchards and vine- 

 yards during the dry, hot summers, a thick mulch of well- 

 tilled soil, thus preventing loss of water by evaporation. In 

 the course of years this surface soil becomes so badly depleted 



1 Bull. No. 70 Minn. Exp't Station, 1905. 



