Introduction to the Principles of Soil Fertility 1759 



constitution of the soil 



The most evident fact to a person examining any soil is that it is a mass 

 of more or less pulverized rock thrown together either in layers or pro- 

 miscuously. Mixed with this rock material is partly decayed plant and 

 animal material, which gradually takes on a uniform dark color and pul- 

 verized condition and is known as humus. In the spaces between the 

 particles of this mass of pulverized rock and organic material, water 

 and air are held, and usually there is present a numerous flora of micro- 

 scopic plants. The mass has a temperature that depends on the physical 

 nature of the soil as well as on the prevailing climate. 



The food for plants is derived from the soil particles, from the soil 

 atmosphere, and from water, by chemical, physical, and biological proc- 

 esses. 



formation of soils, and classification 



We may first consider the ways by which soils are formed and the 

 chief properties that result therefrom. While the farmer can materially 

 modify any soil, the extent of such change is usually limited by economic 

 considerations. In practice the agricultural development of any region 

 that has been settled for a generation or more is a very reliable index 

 to the soil resources of that region. The variation in agricultural develop- 

 ment of different sections is largely a reflection of the inherent differences 

 in soil conditions. 



In the study of soil conditions in the United States up to the present 

 date, more than sixteen hundred different soils have been recognized 

 and many others remain to be identified. In New York State, which 

 has an unusual variety of soils, more than one hundred types have been 

 identified and described and many others will doubtless be recognized as 

 the investigation proceeds. 



The source of this information is the soil survey reports published 

 by the United States Department of Agriculture and by many States. 

 These are the most reliable general guides to the soil conditions and possi- 

 bilities of any region. The soils are classified into types that include 

 all material of essentially the same crop relation. This takes account of 

 the material to a depth of three or more feet, since the subsoil, quite as 

 much as the topsoil, influences plant growth, and in the subsoil the roots 

 of plants are usually distributed. The occurrence of the types of soil is 

 represented on maps; and in the accompanying report the types are 

 described, together with the general agricultural situation of the region ; 

 and its agricultural history. 



The usual unit of these surveys is the county. In New York twenty 

 areas have been surveyed or are in process of survey, and these have an 



