24 VERTICAL FARMING 



The roots do not reach down to all of the water they use. 

 Some of it is pumped up to them as oil rises in a lamp wick, by 

 capillary action. This rise is much faster in well granulated 

 soils than in hardpan or tight clay. It is evident, therefore, 

 that no method of cultivation can reach down deep enough to 

 overcome the difficulties of feeding roots except blasting the 

 subsoil with explosives. 



Unavailable Plant Foods. Attention has already been 

 called to the large amounts of mineral plant food bound up in 

 insoluble minerals, and to the enormous amount of the highest 

 priced plant food (nitrogen) that is present in the air but not 

 directly available to the field crops as food. The changes that 

 some of these must undergo in order that they can nourish the 

 roots are chemically very complex, but, in the practice of the 

 art of farming, can be well controlled. The nitrogen must be 

 combined with oxygen. This change is most effectively brought 

 about by a certain group of bacteria which grows in knots on 

 the roots of peas, beans, clovers, alfalfa, and kindred plants. 

 They breathe in the free nitrogen gas and combine it with other 

 elements in such a way that large amounts are fixed and held 

 in the soil in a combined form that is very nourishing to suc- 

 ceeding crops and also to the crop with which they grow. Other 

 forms of organisms accomplish the same purpose, working 

 without the assistance of the leguminous associates. Both forms 

 require certain well defined conditions in which to work. Each 

 of these is so essential that it would be hard to name the more 

 important one. The soil must be well drained so that there is 

 no clogging up of the soil pores with water, but at the same time 

 the soil must be moist. The soils must also be warm, for the 

 activities of these wonderful little farmers' aids are retarded if 

 stopped by frost. Large supplies of air are equally essential. 

 As most of these conditions attend a deep tilled soil, it might be 

 said that the beneficial bacteria of the soil are all deep tillage 

 enthusiasts. They are found at considerable depths in the 

 porous types of soil, but cannot live much below the surface in 

 tight clays and hardpan. They also keep busy on insoluble 



