52 FARM DEVELOPMENT 



a saturated peaty soil, and some species thrive with 

 their roots in standing or running water. Yet other 

 plants have become accustomed to very dry soils, in 

 which there is but little capillary water; and some even 

 live by securing water mainly from the air. 



Soil formation. The body of most soils consists of 

 rocky particles resulting from the action of changes in 

 temperature, and of sunlight, water, winds, ice, plants, 

 animals, man and other agencies on the rocky covering 

 of the earth's surface. In most places there has gradu- 

 ally accumulated above the rocky crust of the earth a 

 layer, many feet in depth, of more or less finely divided 

 mineral materials, and our present soils have been a long 

 time in reaching a condition suitable for growing crops. 

 In other cases the layer of pulverized material, coarse 

 or fine, or coarse and fine mixed, but thinly covers the 

 underlying rocks, or the solid rock remains uncovered. 

 Minute plants, as bacteria, lichens and mosses, and 

 finally grasses and larger plants, develop in or on ex- 

 posed masses of mixed minerals, and a soil with decay- 

 ing vegetable matter is formed which provides a home 

 hospitable to most of the flowering plants. 



Soils differ as to the size of the particles of which they 

 are composed and as to the arrangement or mixture of 

 these particles. The finest soils are clays made up of 

 very fine particles which are closely knit together. Silt 

 soils are not so fine nor are the particles of a nature 

 to adhere so closely to each other. Fine sandy soils, 

 medium sandy soils, coarse sandy soils and gravelly 

 soils, are grades in which the particles become coarser 

 and coarser, with less and less power to adhere 

 closely to each other. Clay, silt, sand, gravel and stones 

 may be mixed together in all kinds of proportions. Any 

 mixture of clay or silt and sand w r hich combines the 

 .adhesiveness of clay or silt with some of the open 

 crumbly character of sand makes a good medium soil, 



