No. 6. DEPARTMENT OF AGRICULTURE. 455 



uted more uuiformly throughout its mass than at present. In the 

 harder rocks, the sandy particles were cemented together by ma- 

 terials more easily disintegrated and the separation thus made in 

 the course of time enabled the movement of the particles so sepa- 

 rated in a dilTerent way, the coarser materials were distributed 

 and deposited as gravel and sand in one place, and the finer carried 

 and deposited in another, making the clay; the lime entered partly 

 in solution, and was distributed and finally deposited in another 

 place, thus giving us sandy, clayey and limy soils, all diilering from 

 each other in their amount and proportion of both the purely me- 

 chanical substances, which serve no other purpose in soils than 

 the support of the plants and in contributing to physical character, 

 and of the chemical substances which contain the essential fertil- 

 izing constituents, and which in their decay provide the food in an 

 available form. 



In addition to these kinds of soils, there are others of more recent 

 origin, made up largely of vegetable matter, due to its accumulation 

 in a partially decayed state; these are frequently rich in nitrogen 

 and poor in all of the essential mineral constituents. These con- 

 siderations as to the origin of soils are valuable in indicating their 

 chemical composition and possible potential value. 



Chemical Differences in Soils. 



In the next place, fertility depends not altogether on the amount 

 and proportion of the essential constituents contained in soils, but 

 also upon the character of the mineral substances which contain 

 them, that is, whether they are of such a character as to be readily 

 disintegrated or broken down, or whether they are hard and dense, 

 and thus resist those agencies which are active in causing such 

 disintegration. It is quite possible for a soil to be rich in all of 

 these constituents, and still be infertile, because the character of 

 the substances making the soil are such as to prevent their ready 

 attack by those agencies which cause the constituents to become 

 active. 



Physical Differences in Soils. 



The second valuable characteristic of soils is their physical char- 

 acter. This characteristic is not altogether separate and distinct 

 from the chemical, nevertheless it has to do more particularly with 

 the purely mechanical substances contained in soils and their fine- 

 ness of division, and has an important bearing upon the actual 

 fertility'. In order that we may understand this matter more clearly, 

 it is well perhaps to point out that the substances which constitute 

 the bulk of soils are in an insoluble condition, while the plants 



