Depth of Eoot Penetration 227 



where the aim is to bring the soil moisture to the upper limit 

 of productiveness without causing percolation below 4 or 5 feet, 

 will range from about 2.5 inches on the most open soils to 4.5 

 inches on soils of average texture. But when excessive drying 

 of the soil has taken place, then the amount of water applied 

 may range from 3.75 inches on the most open soils to as high as 

 even 11 inches on that which is of medium or fine texture. It 

 should be understood that many soils, when they become very 

 dry, develop shrinkage cracks, which permit very rapid and ab- 

 normally large percolation if excessive amounts of water are 

 applied at one time, and this without saturating the soil, the 

 water simply draining through the large open channels. In such 

 cases repeated smaller applications of water will ensure less loss 

 by percolation, permitting the soil to expand and close up the 

 shrinkage cracks. 



THE DEPTH OF ROOT PENETRATION 



The greater the depth to which the roots of a 

 crop may feed to advantage in the soil, the larger 

 may be the amount of water applied to the field at a 

 single irrigation without any passing beyond the zone 

 of root action, simply because 2 feet of soil will store 

 more water than 1 foot, and 10 feet more than 5. But, 

 further than this, where the roots of a plant penetrate 

 the soil deeply and spread widely, a much smaller per 

 cent of water in the soil will enable the plant to ob- 

 tain enough to carry on its functions to good advan- 

 tage. This is so because the roots go to the moisture, 

 and do not, therefore, need to wait for the moisture to 

 come to them at the extremely slow rate it is known 

 to travel in a relatively dry soil. Then, too, when a crop, 

 by reason of its great spread of root, is able to meet 



