VERTICAL FARMING 11 



have been busy, not for days but for centuries untold, picking 

 up soils in one place and dropping them in another sorting 

 and arranging them until it is probable that any given section 

 of land anywhere contains particles contributed by every other 

 section in that district. 



Residual Soils. Not all soils are moved in these ways. In 

 places, sometimes large, sometimes small, the original rocks of 

 the locality have weathered down into soils that remain just 

 where they were formed. These are known as " residual soils," 

 and embrace a great variety, some of which are fertile, while 

 others are not so well favored. The proportion of residual soils 

 to transported soils varies greatly in the different parts of the 

 world. 



Some of the Physical Characteristics of Soils. Soils have 

 a number of marked physical characteristics, some of which are 

 of interest only to the exact scientist, but many of these char- 

 acteristics are of the greatest interest to the poorest farmer. 

 The greatest advances made in the Science of the New Agri- 

 culture have been due to the study of these physical character- 

 istics of soils, and the application of the discoveries along this 

 line have tended toward a better and more profitable agriculture. 



Soil Texture. One of the most noticeable differences in 

 soils is the variations in the size of the grains of rock powder of 

 which they are made. The fineness of a soil is spoken of as its 

 " texture." The sizes of grains most discussed and best under- 

 stood both by the student and farmer are : clay, silt, sand, 

 and gravel. It is well known that sand, loam, and clay soils 

 will not raise the same crops equally well. There are good 

 reasons for this. In a sandy soil the particles are relatively 

 large and do not pack so closely together. No matter how 

 tightly packed a soil may be, there are always small openings 

 and cavities between the particles. These are called " pores." 

 The sandy soils do not pack so closely together as do the clays, 

 and the pores are therefore larger and permit a much easier 

 movement of water and air in the soil. The clay soils pack 

 more closely together and reduce the size of the pores so that 



