24 IRRIGATION" FARMING. 



the production of fruits, cereals and vegetables. Inves- 

 tigation shows that one acre foot in depth of a fairly 

 good agricultural soil contains four thousand pounds of 

 phosphoric acid, eight thousand pounds of potash, six- 

 teen thousand pounds of nitrogen and lime, magnesia, 

 soda, chlorine, sulphur and silica, all of which are more 

 fully rendered available in maturing plant life when irri- 

 gation is brought into practice upon them. 



It has long been recognized by practical men, as 

 well as by many of our scientific investigators, that the 

 texture of the soil and the physical relation to moisture 

 and heat have much to do with the distribution and de- 

 velopment of crops. Years ago Johnson went so far :.s 

 to say, in How Crops Feed, "It is a well-recognized 

 fact that next to temperature the water supply is the 

 most influential factor in the product of the crop. Poor 

 soils give good crops in seasons of plentiful and well- 

 distributed rain or when skillfully irrigated, but insuffi- 

 cient moisture in the soil is an evil that no supplies of 

 plant food can neutralize." 



Recent investigations point to the conclusion that 

 the mechanical arrangement of the soil grains deter- 

 mines its fertility more than the chemical properties it 

 may possess. Experiments show that the greater the 

 number of soil grains in a given space the greater the 

 amount of air space, because the small grains, being 

 light, arrange themselves more loosely than the larger or 

 heavier ones. In a good wheat soil, when dry, there is 

 at least fifty per cent of air space ; that is, in a cubic 

 foot of soil one-half of the space is occupied by the soil 

 and one-half by the air. But during the process of irri- 

 gation the interstices become filled with water ; and by 

 too copious or too prolonged an irrigation the soil be- 

 comes saturated, which excludes the air from the soil 

 air so necessary to plant growth. A porous subsoil re- 

 moves the water of saturation and assists in preserving 



