THE FARM 333 



ily. Clay consists largely of yet finer particles that rub 

 between the fingers when dry much as does flour, and when 

 wet form a soft plastic mass which hardens into lumps on 

 becoming dry. A loam is characterized by no particular 

 size of particles, but is loose and friable even when quite wet. 



Running water as an agent in erosion has been discussed in 

 a preceding lesson. It is to be noted here, however, that in 

 the transportation of eroded soils there is accomplished a 

 sorting of material according to fineness which results in 

 beds of sand in some places, a mud deposit as silt in another, 

 and clay banks elsewhere. From an agricultural point of 

 view this sorting is an unfortunate one when these lands come 

 under cultivation. It is one of the problems of scientific 

 farming to know how best to modify any particular soil by 

 the addition at least possible outlay in time, labor, and 

 capital of those materials which improve its fertility, struc- 

 ture, and conditions for tillage. 



Any sample of soil gathered from field, garden, or roadside 

 commonly has in it vegetable matter in different stages of 

 decay. This may be burned out by keeping a weighed 

 sample of the soil at a red heat for some time in an open iron 

 or porcelain dish. Any loss of weight due to this treatment 

 of a thoroughly dry soil represents the amount of organic 

 matter of both animal and vegetable nature that was in it. 

 The very large per cent of the soil sample remaining is the 

 mineral portion. Rock material in a finely divided state 

 constitutes very largely the "soil" in which plants root 

 themselves. From it they obtain the water and all other 

 food material necessary to their life and growth except carbon 

 dioxide. 



Soils have been formed by disintegration of the rocky crust 

 of the earth, a process that has been going on for all the ages. 

 When their fertility (crop productiveness) has once been 

 wasted it is restored naturally only after long periods. The 



