SOIL AND SUBSOIL. 121 



position of organic matter (mostly of the cellulose group) 

 derived partly from the roots, partly from the leaves and 

 stems of plants growing and dying on the soil, its accumulation 

 near the surface is natural. But since the depth to which roots 

 penetrate varies greatly not only with different plants, but very 

 essentially in conformity with the greater or less penetrability 

 of the soil and susoil, the depth to which the dark humus tint 

 may reach vertically varies correspondingly, from two or three 

 inches to several feet. In the case of soils that have been 

 formed by the gradual filling-up of swamps or marshes, the 

 humus-tint may reach to several yards depth. 



Surface Soil, and Subsoil. It is thus apparent that the term 

 " surface soil," while commonly confined by the farmer to the 

 portion turned by the plow or usually reached in cultivation by 

 any implements, may or may not belong, functionally, to layers 

 of greatly varying thickness. Similarly the term subsoil may 

 or may not refer, in individual cases, to parts of the soil mass 

 materially different from the surface soil. Yet this distinction 

 is of no mean practical importance, because the efficacy of one 

 of the most common measures of soil improvement, viz., sub- 

 soil plowing or " subsoiling," depends materially upon the 

 differences between soil and subsoil in each particular case. 

 Most of the diversity of opinion regarding the merits of this 

 operation is simply the result of a corresponding diversity in 

 the natural facts and cultural practice of each case. 



Causes of the Differentiation of Soil and Subsoil. One of 

 the prominent points of difference between surface soils and 

 subsoils has already been mentioned in the usual predominance 

 of root-mass in the upper layers; to which is added a part at 

 least of the substance of fallen leaves and stems of its vegeta- 

 tion. How much of this vegetable mass ultimately becomes 

 converted into humus, as well as the nature of the product 

 formed, depends upon a great variety of circumstances; some 

 of which have already been mentioned in connection with the 

 general discussion of humification ( chapt. 2, p. 20). Briefly 

 stated, the main controlling conditions are: the amount of 

 water or moisture present, the access of air ( oxygen), a proper 

 temperature, and the presence of the several organisms which 

 in the course of time take part in the process of soil-formation. 



