28 IRRIGATION FARMING. 



It has long been recognized by pra(5lical 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 

 development of crops. Years ago Johnson went so far 

 as to say, in ' ' How Crops Feed " : * ' It is a well-recog- 

 nized fadl that next to temperature the water-supply is 

 the most influential fa(5lor in the produ(5l of the crop. 

 Poor soils give good crops in seasons of plentiful and 

 well-distributed rain or when skilfully irrigated, but 

 insufi&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 determines its 

 fertility more than the chemical properties it may pos- 

 sess. 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 

 irrigation the interstices become filled with water, and 

 by too copious or too prolonged an irrigation the soil 

 becomes saturated, which excludes the air from the 

 soil — air so necessary to plant growth. A porous 

 subsoil removes the water of saturation and assists in 

 preserving the moisture adhering to the particles of 

 soil. The latter is the most favorable to the growth of 

 crops. In determining the condition of moisture in 

 the soil in the pradlical application of water, it is only 



