22 VERTICAL FARMING 



small amounts and must be added artificially. Considerable 

 amounts are found in natural manures, but the great supply is 

 imported into this country from Germany. 



Phosphorus is also present in most normal soils, but the per- 

 centage is small in some of the most valuable soil provinces of 

 the country so that it must be added artificially. Large amounts 

 of phosphate rock are mined in this country. The rock may be 

 ground and added to the soil in the form of a fine powder, pro- 

 vided the soil has a good supply of organic matter. If this 

 organic matter is deficient and the soils are thin, the best results 

 are reported from the use of acid phosphate which is the phos- 

 phate rock after it has been treated with sulphuric acid. Large 

 amounts of phosphorus are used in the form of slag and bone 

 products. The United States is more than self-sustaining in 

 the supply of phosphates and export large amounts annually. 



Another of the important elements is nitrogen, which is found 

 in the form of nitrates in some of the desert regions. In this 

 form it is very soluble and is washed out of the soil by rains. It 

 is found in normal agricultural soils in varying amounts, but is 

 often in too small quantities. Commercially it is obtained from 

 fish and packing house scrap, from cotton seed, from nitrate of 

 soda imported from Chili, and from the air. Nitrogen is a gas 

 and makes up the larger part of the air, from which it may be 

 taken in large amounts by certain bacteria growing on the roots 

 of legumes and by other microscopic plants working alone. 

 Further mention will be made of this later. 



How Plants Feed. Plants feed by absorption through the 

 roots and by inhaling the air. When a plant is burned, most 

 of its weight is lost in the form of gases and but a small part 

 is left as ash. The ash contains the mineral matter which came 

 originally from the breaking up of the soil minerals, or was 

 added as a fertilizer. The rest, or the part that was lost in the 

 gas is made up of carbon, hydrogen, oxygen, and nitrogen. The 

 carbon is taken in through the leaves from the carbonic acid gas 

 of the air. The hydrogen and the rest of the oxygen are taken 

 in through the roots in the form of water, and the nitrogen is 



