52 THE ESSENTIALS OF AGRICULTURE 



particles to measure an inch. As soon as the rock mass is 

 broken fine enough to support the lower forms of plants, organic 

 or vegetable matter l begins to accumulate. The lower plants, 

 such as lichens and mosses, are followed by higher forms, 

 whose decaying substance is the main source of the organic 

 matter of new soils. 



53. Kinds of soils. Soils are classified according to the way 

 in which they are formed. There are two general classes : 

 sedentary soils, or those which have not been moved ; and 

 transported soils, or those which have been moved by water, 

 wind, or gravity. Sedentary soils may be divided into residual 

 soils and cumulose soils. 



Residual soils are those which remain where they were formed 

 from the breaking down of the rocks. If an excavation be made 

 in the soil material until bed rock is reached, all stages of the 

 process of weathering can be observed. A large part of the soils 

 of the United States is of residual character. Some of these are 

 very good agricultural soils, some are poor, and between these 

 there are all possible gradations. They vary also from fine clays 

 to coarse sands and gravels. 



A limestone soil is a good example of a residual soil. Lime- 

 stone rock is composed largely of calcium and magnesium 

 carbonates, with some other substances, such as silica, iron 

 compounds, and clay. Limestone weathers by the dissolving 

 and washing away of a large part of the carbonate, under the 

 influence of water containing carbonic acid. The substances 

 remaining from the rock form the soil. 



Sandstone is nothing more than sand which, during geologic 

 ages, was cemented into a solid mass. Sandstone weathers by 

 the dissolving away of the cementing material which holds the 

 particles together and by the action of freezing water in the 

 pores of the rock. The soil formed consists largely of sand, 

 and is known as sandy soil. 



1 The term humus is often used in speaking of the organic matter in soils. 

 Strictly speaking, humus is that part of organic matter which has reached an 

 advanced stage of decay. 



