16 VERTICAL FARMING 



ments are of great importance and must be taken into consider- 

 ation. Many chemical elements are needed for the production 

 of a plant, but it is seldom, with the exception of potash, lime, 

 phosphorus, or nitrogen, that any of these is not present in 

 sufficient amounts. All of these except the last named occur in 

 many rocks, and are therefore found in the soils in varying 

 amounts. When, on a particular soil, one or more of these is 

 absent or deficient, it is necessary to add it in some form of 

 fertilizer or manure. 



The soil particles may not be weathered enough to make these 

 minerals available, or there may be little in the surface soil and 

 more in the subsoil. In either case the soil is improved and 

 the plant food brought within the reach of the crop by breaking 

 and stirring up the land with explosives. 



Fertilization and Chemical Correction. In addition to the 

 proper physical condition, it is necessary to have the chemical 

 condition of a soil well regulated in order that we may get the 

 proper returns from our labor. Some foods may need to be 

 added to the soil, or it may be essential that a harmful sub- 

 stance be removed or neutralized. This work will be considered 

 in a following chapter. 



