1764 The Cornell Reading-Courses 



(/) Gravity. — A further agency in soil formation, which is of small 

 importance in this State, is gravity, which accumulates a mass of rock 

 debris at the foot of steep slopes. 



2. Source oj soil material 



Soils are made up chiefly of pulverized rock, and the kind or kinds of 

 rock from which a particular soil was formed has a considerable effect on 

 both its physical and its chemical nature. A large number of kinds of 

 rock are recognized by geologists. A few of the groups that cause con- 

 siderable differences in soils are: (a) original crystalline rocks, such as 

 granite; (b) sandstone, shales, rocks, and slates; and (c) limestones and 

 marbles. The proportion of lime in a soil is determined to a large extent 

 by the kind of rock from which it was formed, as well as the way in which 

 it was formed. Where the rock is pulverized without much washing, this 

 effect is especially marked. In New York the most calcareous soils occur 

 in the neighborhood of the limestone formations, but not necessarily on 

 them, due to the movement and mixing by glacial ice and water. They 

 are especially well developed in a strip of country ten to twenty miles wide 

 and extending southward from a line through Utica, Syracuse, Rochester, 

 and Niagara Falls. They are also found in detached areas in the St. Law- 

 rence valley, the Mohawk valley, and the Hudson valley region. 



J. The series properties 



The color, drainage, content of organic matter, and lime, have important 

 influence on the productive power of soil, and are therefore recognized 

 in their classification. This is called the series division. 



4. The type properties 



Finally, and perhaps most important in practice of any single propert^^ 

 there is the fineness, or texture, of the material — whether gravel, sand, 

 silt, clay, or some mixture of these. As a result of mixture, there are 

 produced loams, clay loams, sandy loams, and silt loams. The composition 

 of these will be described in a subsequent lesson. 



FACTORS THAT MAKE A SOIL FERTILE 



The soil must be viewed as a factory, in which the various materials 

 essential to plants are contained and in which many processes that con- 

 tribute to fertility are carried out. The efficiency of the soil depends 

 very largely on the nature and openness of the soil structure, and this in 

 turn is largely determined by the fineness of the soil particles and their 

 arrangement. If the soil is too open and porous, sufficient water is not 

 retained, the soil is inclined to be warm and droughty, and crops do not 

 thrive. On the other hand, if the soil is too compact and impervious. 



